The minimization of salt related impacts should be one objective of any management strategy formulated for roadway drainage systems. Efficient employment of anti-icing programs and other management systems minimizes the introduction of salt alternatives into the environment.

The Transportation Association of Canada [N], Oregon DOT [N], and NYSDOT [N] make the following overarching stewardship practice recommendations for reducing salt usage, included below in addition to practices suggested by recent research. A number of these practices are expanded upon in subsequent sections.

• Practice anti-icing by promoting a timely response to snow and ice events in order to prevent a bond from forming between the frozen precipitation and the pavement. This strategy consumes much less material than a de-icing strategy. Anti-icing solution must be applied before snow cover occurs, as otherwise the application may become diluted and brine will not be established between pavement and snow.
• Evaluate road and weather conditions and trends to ensure that the proper type and timing of treatment is made.
• Snow and ice control decision-making should be based on ongoing monitoring of pavement temperaturesrather than air temperatures. Pavement surface temperatures can fluctuate significantly depending upon the time of day, degree of cloud cover, sub-surface conditions (i.e. frost penetration, moisture presence, thermal retention properties, etc.) and type of pavement. Therefore ongoing monitoring of pavement temperatures is important to good decision-making.
• Plow off snow or slush prior to applying materials to decrease dilution and increase effectiveness of the materials.
• Do not overload the material spreader, to avoid spillage.
• Control spreading speeds to reduce bounce and scatter.
• Control spread patterns to concentrate material where it is most effective on the road. Solid road salt is usually placed on the crown or high side of the driving surface where a good crossfall and traffic will distribute the resulting brine over the road. When re-applying material, consider the possibility of partial vs. full and spot vs. blanket treatments where appropriate. Wider spread patterns are called for when spreading on deteriorated pavements where an undulating surface or poor crossfall will not ensure adequate chemical migration across the entire road, or when rapid distribution is required to address frost or black ice conditions.
• Consider alternative treatments (e.g., plow only, use of snow fencing) which do not involve materials usage where applicable. Non-chemical deicers have the potential to introduce less salt and environmental contamination. Innovative techniques in debonding were explored in SHRP Report H-644, Ice-Pavement Bond Disbonding―Surface Modification and Disbonding, including noncontact and contact methods, additives to alter surface texture, electromagnetic radiation, and abrasive air and liquid jets applied directly to ice pavement interface.[N] In terms of contact debonding technology, SHRP Report H-673, An Improved Displacement Snowplow, describes the research on improving the design of snowplows, as well as design, fabrication, and testing of plows incorporating improvements, toward the effort of decreasing energy consumption during plowing by twenty percent.[N] Improved Cutting Edges for Ice Removal presents an evaluation of snow plow blade geometry and its effects on the force required to remove ice from a highway pavement surface including prototypes and testing of three different cutting edges.[N]
• Alter application methods and rates in sensitive areas: [N]
  • Use CMA on bridges and roads where permitted and during freezing fog in lieu of sanding, when optimum conditions exist, where adjacent water bodies support a 100:1 dilution factor or there is a vegetative buffer between the road and water body and where there is no standing, shallow water.
  • Place barriers in site specific locations where appropriate and practical, along streams or direct drainages to route sanding/anti-icing material away from watercourses.
  • Reduce plowing speed in sensitive areas.
  • Stop sidecast sweeping within 50 feet of structures over water, where structurally possible.
  • Identify and creating facilities to capture sanding material where appropriate.
  • Reduce quantity of sand applied where appropriate.
  • Clean inlets prior to first rain as feasible.
  • Modify blade angles or blower hoppers in sensitive areas.
  • Educate DOT maintenance staff on water quality and fishery resource issues.
• Return unused materials to stockpiles and avoid heavy “end of beat” applications that empty the load
• Keep accurate records of materials usage to allow monitoring and improvement of operations. While it is not practical to monitor all runoff from roadways for chloride levels, transportation agencies should consider monitoring salt vulnerable areas. One municipality worked with their local conservation authority to add chloride monitors to their stream monitoring network.

Anti-icing is the proactive use of any melting agent to assist melting and resist the formation of a bond between snow and ice and the pavement surface.  Highway anti-icing is the snow and ice control practice of preventing the formation or development of bonded snow and ice by timely applications of a chemical freezing-point depressant. It provides a maintenance manager with two major capabilities: the capability for maintaining roads in the best conditions possible during a winter storm, and the capability to do so in an efficient manner with the fewest chemicals and environmental impacts possible.

Anti-icing can involve application to the roadway of liquids, pre-wetted solid granular materials or dry granular material. Thus, anti-icing is not confined to using liquids. Direct liquid applications are efficient since they provide melt action immediately and do not take time to dissolve and form brine. Furthermore, liquids do not depend on the presence of heat from the ground, sunlight or traffic to dissolve (endothermic reaction). The timing of the application is not as critical as with granular materials; the principle is that traffic will help the liquid migrate across the road cross-section and yet not develop into road spray. Liquids can be applied in advance of the start of a storm. If the application is earlier than the onset of a storm, a NaCl brine will evaporate leaving a salt crystal residue in the surface pores/texture of the pavement (and which will redissolve and reform a brine with precipitation); conversely, hygroscopic brines (such as CaCl2 and MgCl2) will attract moisture and continually wet the road until they are dissipated. The approach to resisting the bond is not to wet the road, but simply to provide enough chemical to enhance early-storm safety with an application of chemical that stays on the road. The intention is not to “wash” or even fully wet the road with an equivalent chemical loading as that of a granular application. Generally, an equivalent weight of salt applied as a liquid (e.g. dissolved in water) performs better than the same weight of dry granular salt because the liquid is fully retained on the road surface. The cost on a dollar-per-gram basis may be greater for liquid only applications (depending on the liquid used), however the offsetting safety benefits have to be considered.[N]

European and Scandinavian experience has shown that as little as 5 to 10 g/m 2 (65 to 130 lb/lane-mile) of salt is needed for preventing salting treatment for frost, black ice, and light snow. There appears to be no consensus among European countries regarding the rate of salt spreading during continuous snowfalls. Estimates of application rates under these conditions range from 10 to 60 g/m 2 (130 to 780 lb/lane-mile). Highway agencies in the United States have found that reducing the conventional application rate to quantities on the order of 4 or 5 g/m 2 (42 or 65 lb/lane-mile) is not generally possible with current equipment that is designed for deicing application rates of 23 to 38 g/m 2 (300 to 500 lb/lane-mile) and higher. Liquid freezing-point depressants offer the advantage of precise and uniform application over a wide range of rates.[N]

According to a study by the Strategic Highway Research Program, experiments at nine state highway agencies, anti-icing treatment requires less chemical use than most deicing procedures and makes it easier to achieve bare pavement conditions.[N]As a result, anti-icing can provide cost savings as well as environmental benefits. For example, the Iowa, Missouri, Oregon, and Washington DOTs realized cost savings and the following benefits in test programs: [N][N][N][N]

Fewer snowplow trips were made. The anti-icing truck only had to make one trip for every three trips made by the larger conventional snow-removal truck.

Crews experienced less wear on equipment due to fewer snowplow runs.

When required, plowing was easier and faster. Less time was spent clearing roads. Crews were able to complete snow removal on roads that received anti-icing treatment up to three hours sooner than on conventionally treated roads.

Fewer chemicals were needed by applying the treatment prior to snowfall. With fewer chemical applications needed, the anti-icing method was better for the environment.

With reduced costs for labor and chemical use.

Since Boulder began using a liquid solution comprising 29 percent magnesium chloride and 71 percent water in 1993, sand use has decreased by 55 percent. When all costs are considered, using the liquid chemical costs $2,500 per lane mile, as compared to $5,200 for deicing and sanding operations.[N] The Center for Geotechnical Engineering Science (CGES) at the University of Colorado at Denver completed a CDOT-sponsored study on ”Environmentally Sensitive Sanding and Deicing Practices” in 1994. The study recommended the formulation of an optimal practice that minimizes the use of sand and increases the use of environmentally friendly chemicals for the purpose of enhancing winter highway traction and maintaining both environmental health and human respiratory health. Implemented since 1994, the shift has had a direct beneficial impact on all issues related to safety, cost, environment, and human health and has improved the Colorado air quality, allowing Colorado to avoid exceedance of the EPA PM 10 standard over subsequent winters. [N] The Idaho Department of Transportation (IDT)’s anti-icing retrofits showed reductions in annual averages of abrasive quantities, labor hours, and winter crashes over five years. [N]

Stewardship practices to minimize materials application and release to the environment include the following:

• Since anti-icing is preventive in nature it is desirable to have the first application completed two hours prior to the anticipated event, or at a minimum prior to bond forming on the road surface. The anti-icing chemical solution concentration will decrease as it is diluted with water from either the melting of the snow/ice or falling rain/freezing rain and becomes less effective. 
• Pavement should be cleared of as much snow, ice, or slush as possible before reapplying a liquid anti-icing material. Application rates for liquid anti-icing operations are based on local experience as documented through logs.

FHWA’s Manual of Practice for an Effective Anti-Icing Program: A Guide For Highway Winter Maintenance Personnel describes program factors, practice recommendations, and guidance for conducting anti-icing operations during specific precipitation and weather events. [N] Recognizing that the development of the program must be based on the specific needs of the site or region within its reach, FHWA provides the caveat that no short discussion or list of recommendations can completely cover the range of conditions facing agencies continent wide; instead the guide is to be used as a starting point for developing its own anti-icing program, and to modify the recommendations when necessary in order to accommodate local experience, specific site concerns, and agency objectives. The report does present specific recommendations for anti-icing operations for five weather events, from light snow storms to heavy ones, frost, freezing rain, and sleet. Guidance on maintenance actions for each event is provided for several pavement temperature ranges and for initial and subsequent operations. Temperature trend, an important factor, is also indicated. Solid, liquid, and prewetted solid chemical application rates are suggested where appropriate―rates not to be considered as fixed values but rather the middle of a range to be selected by an agency according to its local conditions and experience. Traffic volumes were not found to have a consistent or dominant influence on pavement condition or traction to suggest varying chemical application rates except in the case of frost and black ice, and that category is the only one incorporating traffic as an operational consideration. The guidance presented was based upon the results of four years of anti-icing field testing conducted by 15 State highway agencies and supported by the Strategic Highway Research Program (SHRP) and FHWA, and then was augmented with practices developed outside the U.S., where necessary, for completeness. Steps in initial operation of an effective anti-icing program include: [N]

• Information assembly upon first notice that a winter storm or frost/black ice event may affect the maintenance area, including weather forecasts, weather radar data, satellite data, local road condition and RWIS data, pavement temperature forecasts, and any RWIS data from areas outside the immediate maintenance jurisdiction that might have already have been affected by the approaching storm. The information must be reviewed to estimate when and where the event will begin, its extent, and severity.
• Decision on whether or not to initiate a treatment, when to start it and what type of treatment to apply can be made after the review is made of the information assembled. The decision is based on when precipitation is expected to start, what form it will be, the probable air and pavement temperatures, the anticipated trend of the temperatures, the expected sky conditions, the wind speed and direction, and the intended timing of the treatment.
• Chemical application. Either dry solid chemicals, liquid chemicals, or prewetted solid chemicals can be used as an initial anti-icing treatment. Whichever is used, the timing of the application should be consistent with the underlying objective of preventing the formation or development of bonded snow or ice, and should reflect an underlying readiness consistent with a preventive strategy. That is, it should be made in anticipation of or in prompt response to worsening pavement conditions. Applications in advance of snowfall are not necessary for preventing bonded snowpack, but early applications when the pavement condition is no worse than wet, slushy, or lightly snow covered are for the most part necessary for anti-icing success. As this may not always be possible, for example because of a limited fleet or heavy traffic, pretreating the road before a snowstorm may be the only way to ensure that all areas are treated before conditions deteriorate. Chemical application at the right time can reduce chemical usage and environmental effects.
  
  For snowstorms, initial liquid applications can be made either as a “pretreatment” in advance of the storm or as an “early-storm treatment,” i.e., soon after snowfall has begun and/or when the pavement temperature is dropping toward freezing. A pretreatment can be made well ahead of a storm as long as the storm does not start out with above freezing temperatures and rain, washing the chemical away. In the case of early-storm treatment, the application may be made onto dry, wet, light slush, or lightly snow covered pavement. Late applications onto pavements with more than a light covering of slush or snow can result in excessive dilution of the chemical, and risks failure. These should always be coordinated with plowing. Recommendations for use of solid and prewetted solid chemicals, plowing, and time when doing nothing are most appropriate are also discussed.

Direct liquid applications can be applied over multiple lanes by trucks traveling at higher speeds (than conventional salt spreading) with due regard for traffic. Trucks used for straight liquid applications can range in size, to accommodate frame-mounted or slide-in tanks. Truck configurations may include small trucks with tanks ranging from those used as patrol vehicles (pickups to two-tons) to vehicles used for vegetation spraying or bridge washing in the off-season; larger trucks used for water applications or calcium dust suppression applications in the off-season; and/or full-size, larger capacity tractor trailer tanker units used for long distance hauling in the off season. The application of liquids can be triggered by sensors and sprayed on a road or, more commonly, a bridge deck surface via Fixed Automated Spray Technology (FAST).

A Guide to Selecting Anti-Icing Chemicals and Considering Environmental Impact is available on-line.[N] The purpose of the guide is to specify the key performance measures that are required from an anti-icing chemical, and suggest ways of grading chemicals according to those performance measures. It also provides a method whereby an agency can weight these measures according to the specific needs of that agency, including Freezing Point Depression, Consistency, Environmental Impact, Stability, Corrosion, Handling, and Documentation. SHRP Report H-683, Anti-Icing Study: Controlled Chemical Treatments, developed correlations between meteorologic parameters and chemical effectiveness that can indicate the optimum conditions for a particular anti-icing chemical application. [N]

An anti-icing program is only as good as an agency’s ability to predict the onset of winter weather events accurately. Understanding and interpreting weather information can be critical to the success of any winter snow and ice removal operation. Knowing when, where and what type of deicing material to use for a particular winter weather event can be a challenge. Knowing where to find the weather information needed to make decisions and what information to use can be difficult.

National Weather Service forecasts are not sufficiently site-specific and do not include all the data necessary to provide the accurate, real-time storm prediction and road temperatures that make anti-icing strategies effective. Thus, Road Weather Information Systems (RWIS) are an essential tool in a successful anti-icing program. Using pavement and atmospheric sensors and communication systems, RWIS collect and deliver roadway and weather condition data to decision maters in the maintenance garage and even behind the wheel of the snowplow. The data from a system of RWIS sensors along a highway network―especially along trouble spots—help maintenance personnel know when and how fast a winter weather event is approaching. The RWIS data indicate the kind of precipitation likely, where the precipitation will freeze on the roadway, and other information that will help Maintenance forces decide when to apply the minimum amount of chemicals to be effective.[N]

Recent research has found that the use of an incentive-based compensation model built on RWIS results in reduced use of salt compared with a compensation model based on measures to such an extent that the Swedish National Road Association is making preparations for changing the whole compensation system before the next winter season 2004–2005.[N]

Road weather information systems (RWIS) are networks of weather data-gathering and road condition monitoring systems and their associated communications, processing, and display facilities which provide decision information to maintenance managers. The most visible components of RWIS are the roadside installations of system components. A single site, which may have many sensors, is referred to as a remote processing unit (RPU) station, typically consisting of atmospheric sensors mounted on a tower, sensors embedded in the pavement surface and beneath the surface, and an enclosure which contains data processing capability and communications equipment. Data from the sensors are formatted at the RPU and transmitted to a central processing unit (CPU) where they may be stored, retransmitted to other workstations or locations, or accessed directly. The CPU can be a separate computer or a workstation.

Another component of a RWIS is the data processing and display capability used by the maintenance personnel. The actual system configuration depends on the management structure of the maintenance organization. This component can be a computer workstation in a maintenance facility or at a District or Area headquarters. It can also be a portable computer a manager, supervisor or foreman takes home.

• For made from a central office, one workstation with the CPU may suffice.
• If decision making is decentralized, workstations and/or portable computers should be available to the local decision makers for them to access data.

Data from RWIS are used to determine when and where to apply salt and other materials-commonly called deicing chemicals-that either prevent ice from bonding to the pavement or break the ice-to-pavement bond. The technology helps maintain ice-free roadways, cuts down on labor costs, and reduces chemical use. [N]

Sensor-based RWIS has been in use for over 25 years by road and airport authorities around the world. Beyond giving road information and trends, RWIS sites and networks provide information required to develop specific forecasts as well as some service documentation. RWIS supports winter road operations in the following ways: [N]

• An understanding of pavement temperature forecasts and trends can improve the accuracy of decision- making.
• Sensors embedded flush in the pavement, as well as sub-surface, generate data that can be sent back to central locations allowing trends and forecasts to be developed.
• Pavement sensors can monitor pavement temperature, wet/dry status, freeze point of the solution on the road, presence of chemical and concentration (for some chemicals), as well as subsurface temperature.
• Tower-based sensors can also provide real-time information of typical atmospheric conditions such as precipitation, relative humidity, dew point, air temperature, and wind speed and direction.
• Weather forecasting services can use road-based information to provide “road weather” forecasts to help the road maintainer make better decisions regarding snow and ice control.
• Salt use optimization is achieved by more accurate deployment of equipment and application of chemicals.
• Other types of sensors and systems can be added to RWIS to further support road maintainers (e.g. road-imbedded device to measure road friction and snow cover, automated liquid deicer application system―Fixed Automated Spray Technology (FAST), etc.).
• The RWIS can be equipped to perform other beneficial functions. A camera can be attached to provide real-time weather information. A laser device can measure visibility. The intensity and accumulation rate of snow can be measured. And the station can activate changeable message signs to warn drivers of snow, high winds, and other hazardous conditions.

By doing a better job of predicting where and when crews and materials will be needed, agencies are able to reduce usage and expenditures while maintaining level of service. Pilot tests have indicated the potential for wider scale reductions; for example Mass Highway estimated that a complete RWIS could yield savings of $150,000 to $250,000 during a typical Boston winter.[N] NJDOT is equipping all crew supervisors with portable computers so that they can access RWIS and other data at any time and winter maintenance decisions are made by the people most familiar with the roads and weather in a particular area, estimating that the resulting savings in chemical, labor, and equipment costs could reduce snow and ice control expenses by 10 to 20 percent statewide. A fully implemented system was estimated to eliminate at least one chemical application pass per storm. [N] NYSDOT is stressing pavement temperature not air temperature and in-pavement sensors are beginning to provide this information. At NYSDOT, 10 percent of trucks have units, plus supervisors have hand-held units to estimate pavement temperature.

Strategically placed RWIS stations provide forecasts that are 90 to 95 percent accurate, a rate which is improving with addition of further stations and better technology. In sum, [N]

• Crew chiefs have a better idea of how much deicing chemicals to apply to the pavements and when, cutting costs and minimizing any environmental impacts.
• Maintenance activities can be better planned and executed. Labor, material, and energy costs are reduced. DOT operations have become more efficient, giving the agency a return on investment of 200 percent to 1,300 percent. [N]
• Road safety is enhanced and the public benefits from faster response to weather-related emergencies.

Additional information about RWIS, their selection, procurement, siting, use, maintenance, and calibration can be obtained in the two-volume SHRP report Road Weather Information Systems Volume 1: Research Report and Road Weather Information Systems Volume 2: Implementation Guide (SHRP-H-351).[N] [N]The National Cooperative Highway Research Program (NCHRP) recently completed Project 6-13, Guidelines for Snow and Ice Control Materials and Methods , to help maintenance managers select appropriate strategies and tactics for specific winter storm conditions. NCHRP has distributed the report to state departments of transportation. In combination with the results of NCHRP Project 6-16, Guidelines for the Selection of Snow and Ice Control Materials to Mitigate Environmental Impacts —now in progress—the report will provide a complete winter maintenance handbook for managers. Supplementing RWIS data with real-time friction measurements may be useful for managers allocating resources for snow removal as a storm is occurring, and NCHRP Web Document 53, Feasibility of Using Friction Indicators to Improve Winter Maintenance Operations and Mobility , provides practical insights. NCHRP Project 6-15, Testing and Calibration Methods for RWIS Sensors, in progress, will assemble best practices and produce practical guidelines to ensure the reliable operation of RWIS sensors in the field. [N] Together, these resources and the RWIS tools below can help maintenance managers optimize road safety and minimize chemical usage.

Maintenance decisions should not be based on a rigid, automatic basis but rather on the assessment of a need. In contrast to prescribing that chemicals be applied, or plow runs be made every hour or two or other fixed interval, decision on treatment need can be based on a number of information sources, including the visual observations of precipitation/weather and pavement conditions from patrols and from operators, an indication or the measurement of chemical concentration on the pavement, and the measurement of frictional resistance to sliding. [N]

Real-time knowledge of the pavement surface state is necessary for making an informed decision on treatment: the pavement temperature, whether it is wet or dry, and some indication of the concentration of a freezing-point depressant. The most important is pavement temperature, as the solubility of all chemicals varies with temperature. Lower temperatures bring about less solubility. An ice-control chemical must form a solution in water in order to depress the freezing-point. The pavement temperature will determine if it will form an ice-melting interface at the pavement surface. Air temperature is less important at the critical time of application and immediately following since there is usually a lag between air temperature change and the response of the pavement surface. Nonetheless, the air temperature trend is important to track because pavement temperature will usually follow the air temperature within a few hours depending on the difference in the air temperatures, the amount of solar radiation, wind, and the characteristics of the road. Remote measurement of amount and type of precipitation will guide the maintenance manager in deploying available resources most effectively. It is not unusual for part of a region to be receiving freezing rain, another part snow, and still another no precipitation. Using the most appropriate chemical and application rate for the condition, scheduling only plowing, or choosing to do nothing can all be informed decisions based on road and weather information.

Pavement sensors accomplish this monitoring and warning function. In addition to their real-time monitoring function, pavement temperature sensors can be used to generate a forecast of pavement temperature trend and warn when it will drop below freezing. This warning can occur several hours before the event, providing sufficient time to plan operations and avoid unnecessary costs.

In addition to measuring temperature most pavement sensors give a relative value of the chemical concentration on the sensor surface based on conductivity measurement. It will serve as a guide to whether some chemical remains on the road and help in making the decision whether or not to retreat. Another capability is available on some of the newest types of pavement sensors: measurement of the freezing-point of the solution on the detector. Its value lies in warning of the refreeze of a chemical treatment which has been diluted by melted snow or ice.

Thermal mapping, or thermography, is the process of determining thermal profiles of road surfaces using infrared sensors. Thermal mapping profiles can be used to infer pavement temperatures between sensor locations where the temperatures are known. An extension of this process is to forecast temperatures along the roadway based on the forecasts of temperatures at known points. The measurements are typically made in the early morning hours, when there is the least change in the pavement temperature during the measurement process. They are also made under different atmospheric conditions, since the radiation balance at the surface is related to the atmospheric conditions, including cloud cover, wind speed, and precipitation. A variation of thermal mapping is called road climatology. Additional data are acquired when measuring pavement temperature, including air temperature, relative humidity, and climatological characteristics of the pavement environment. The additional data are input to a short-range (up to 4 hour) forecasting model for pavement temperature.

Thermal mapping of highway segments has been conducted in several States, including Washington , Nevada , and Minnesota . The data from thermal mapping have assisted in siting RPU stations, forecasting pavement temperatures for locations where no RWIS sensors exist, and for developing snow- and ice-control strategies. Other potential locations for thermal mapping include those areas where anti-icing operations are used, where reduced chemical areas exist, or where a significant number of different microclimates exist in a given area. Thermal mapping may also point to representative RPU locations that can eliminate the need for one or more sites. Better routing or allocation of maintenance resources and personnel is possible based on thermal mapping. The data can allow staging of responses to only those road segments expected to be below freezing. It can also indicate certain areas or locations that may not need attention. Research has indicated that thermal information from the road environment can be obtained using relatively inexpensive hand-held radiometers. Vehicle-mounted instruments for measuring pavement temperatures are already used by some State highway agencies.

There is some thought that thermal mapping should be considered when variations of pavement temperature greater than 5 degrees C (9 degrees F) are possible, or when the road elevation changes more than about 200 m (650 ft) over the segment length of interest. These “rules of thumb” are for general guidance and have not been validated by research data.

Decisions about material application are improved when information about the current road surface temperature is available and the temperature trend is known. Infrared thermometers (IRTs) are portable devices that can be used to determine the current road surface temperatures while mobile along the road network. Both hand-held and truck-mounted versions are available; with the mounted versions measuring ambient air temperature as well. Truck-mounted versions allow continuous monitoring of the road surface while the vehicle is moving down the road. The data can be recorded and transmitted as part of the data stream of a GPS/AVL system (see Operational Support Equipment later in this document). IRTs need to be checked and calibrated to confirm their accuracy and to be confident in the reading.

Decision about material application can be improved by having better information about the current friction level of the road surface. Devices that measure the degree of friction on the road surface have the potential to eliminate the unnecessary use of salt on roads with adequate traction. In some cases friction sensors are mounted on the spreader vehicles and used in conjunction with on-board mounted pavement temperature measurement equipment to automatically control the application rate of snow and ice control chemicals. Several DOTs and suppliers are conducting research on affordable and convenient measurement methods.

Measurement of friction was used successfully in the SHRP and FHWA anti-icing projects. An agency may find it reasonable to establish this as a technique used during patrols. There are many devices for measuring friction. Skid trailers are commonly used for the measurement of the coefficient of friction, but for various reasons related to safety and equipment deterioration, they are not normally used on snow-covered pavements. Specialized vehicles incorporating a fifth wheel, which measures the increase in force when braked at a controlled slip rate, are available, but high cost has limited their use mainly to airports. A low-cost device was used in both the SHRP and FHWA test programs because it can be installed in most any vehicle and can produce reliable measurements. It gives a direct readout of friction coefficient when the vehicle is hard-braked from 65 km/h (40 mph). Its repeatability is acceptable for treatment analysis and decision support purposes, provided the device is calibrated and operated in accordance with the manufacturer’s specifications. Because it requires hard braking, however, it is not suitable for use in heavy traffic.

A 2004 TRB paper on the Feasibility of Using Friction Indicators to Improve Winter Maintenance Operations and Mobility presented the results of NCHRP Project 6-14, which evaluated the feasibility of using friction indicators as tools for improving winter maintenance operations and mobility. As part of the project, information was collected and reviewed regarding the use of friction indicators for winter maintenance operations decision-making, operations performance evaluation, and motorist information. In addition, short-term and long-term implementation scenarios were developed in which friction measurements could be used to improve winter maintenance safety, operation, and mobility. The study also found that analyzing information collected from low-cost and reliable friction measuring devices and other data, such as pavement temperature, traffic, and weather conditions, could be useful for allocating snow-fighting resources in real-time. The information gathered suggested that a traction-control system is the most promising technology for practically and safely measuring friction in winter conditions, followed closely by deceleration and slip devices. Forecasting surface friction based on models that relate data such as temperature and traffic was also identified as a promising technique for improving winter maintenance operations, but further research is needed in this area. [N]

The availability of chemical concentration indicators appears to enhance the timing of subsequent applications by providing indications of the dilution of the chemical. After a storm event has passed and the road has become bare and dry, there often is a residue of chemical on the road surface which can be activated with the next precipitation event. The concentration of salt contained in roadway slush is the determinant of the freeze point temperature of the slush. It is helpful for decision-makers to know the residual salt concentration on the road. An RWIS road sensor will provide this information, enabling a manager to time the reapplication of chemicals so that the operation is complete before the freezing-point of the brine on the pavement surface starts to climb and, especially, before it reaches 0 oC (32 oF). Where decision makers have confidence in these data, they can be used as a basis for establishing cycle times of the repeat applications for different conditions.

Portable salinity sensors are available, although their high cost makes widespread use unlikely.  Existing salt concentration meters permit only point-to-point measurement and are, therefore, not suitable for road management that relies on longitudinally continuous concentration measurement. These existing methods of measurement require that field personnel stop the vehicle and manually take measurements on the pavement. Consequently, this method is not convenient and is also dangerous for field personnel. The New England Transportation Consortium has been working on development of a method and prototype for the continuous measure of deicer concentration. [N] Another tool on the horizon is a “chemical presence” sensor that can measure the chloride concentration of road spray in a vehicle’s wheel well.

Nowcasting refers to the use of real-time data for short-term forecasting. It relies on the rapid transmittal of data from RWIS installations, radar, patrols, and any other information source for making a judgment of the probable weather and pavement condition/temperature over the next hour or two. Nowcasting is one important tool for making the decision of when to call in personnel. Mobilization timing may vary among sites, therefore the frequency of weather information updating required for a nowcast will also vary with the site. Nowcasts can be provided by a weather service or performed by the maintenance manager. Specially trained maintenance managers in some highway agencies already perform this duty using the necessary information available from a variety of sources.

Vehicles can affect the pavement surface in several ways: tires compact snow, abrade it, displace or disperse it; heat from tire friction, engine, and the exhaust system can add measurable heat to the pavement surface. Vehicle tires also bounce a proportion of applied chemicals off the pavement. These positive and negative impacts on the effectiveness of anti-icing treatments should be considered in the decision-making process. The traffic information most important for making operational decisions is the variation of traffic rate throughout a 24 hour period.

There is no substitute for visual observation of weather conditions and conditions of the pavement surface. Observations remain an important tool for making operational decisions even when an agency has access to and experience with new technology such as RWIS. Use of patrols for this purpose can be highly effective. Though the State or local highway patrol can fulfill this role, trained maintenance personnel are better prepared to judge the severity of conditions and to make or recommend corrective action.

An Introduction to Standards for Road Weather Information Systems (RWIS) describes three categories of standards (here as guidelines, recommended procedures, protocols, and other practices) that formalize some of the processes involved in deploying and maintaining RWIS sensors: siting standards, calibration standards, and communication standards.  While the standards are not mandated, agencies are encouraged to use the introduction as a starting point to learn about RWIS standards and to consider how they might use these standards to reinforce their own RWIS operations.

The ITS standards program has produced a number of weather-related standards, including the Environmental Sensor Station standard for road weather information systems (RWIS), weather elements in the Advanced Traveler Information Systems standards, as well as a number of other standards.[N] There are many examples of the use of ITS to improve transportation system operation under adverse weather conditions, including closed-circuit television (CCTV), RWIS, 511 (the national traveler information number), road closure notification/diversion coordination, dynamic message sign (DMS) advisories, variable speed limit (VSL) technologies and enforcement, in-vehicle devices, sensor/detection systems and other field devices, signal control systems, land closure/ direction change systems, smart work zones, and highway advisory radio (HAR). Recommended practices for ITS deployment include the following: [N]

• Make sure that area jurisdictions have compatible equipment, can share data, and have similar operating standards and procedures.
• Use technology to make sure the right equipment and the right people are at the right place at the right time and for the right reasons.
• Deploy systems so that they can prove their benefit in specific, quantifiable ways.
• Evaluate their effectiveness – from both a cost and benefit perspective – to demonstrate value to the traveling public and to relevant stakeholders, including elected/appointed officials. 
• Seek both short- and long-term wins from technology deployment.

Future projects, including the Vehicle Infrastructure Integration (VII) initiative, the Infostructure, and the Integrated Network of Transportation Information (INTI), hold great promise in providing better weather information via ITS applications in the not too distant future.[N]

As identified by the FHWA Road Weather Management Program two problems stand out in RWIS: 1) There are consistent complaints that weather information, and the road-condition predictions dependent on it, remains insufficiently timely, accurate, and relevant, and 2) RWIS remains a profusion of disparate environmental information sources, incompatible in communications protocols, and information formatting. [N] In 1999, FHWA sponsored the Surface Transportation Weather Decision Support Requirements (STWDSR) project, which defined the decision maker, not the information sources, as central.

More than 100 types of operational information needed for winter road maintenance decisions (at the fourth or lower level of a taxonomy) were defined, which could in turn be divided broadly into four types: Resource status, weather, weather-related road condition, and other road information. The “environmental” information on weather is just part of what is needed. For road-maintenance purposes, weather is usually a predictor of the road conditions that are the immediate interest, and there is a large inferential gap between the two, is primarily due to the fine-scale climactic differences of road versus atmosphere and to the different dynamics and time constants of the atmosphere versus road heat-energy and mass transfers. [N]

The basic decision problem is to choose an alternative with the best, but uncertain, impact on the goals. The uncertainty comes in part from the uncertain causal relationship between a control action that is chosen and its execution by resources in the transportation systems. The true transportation outcomes under winter weather threats are almost always the result of joint decisions among maintenance agencies, other road operating agencies (e.g., traffic management), and road users. All decision-support information acts causally on a decision at the central time, and all uncertainty comes from flawed observation of data in the past and flawed translation to the central time. As all decisions have risk, or uncertainty in the outcome measures of the alternatives, maintenance managers request reliability indicators or “worst case” values for their information. [N]

The development of a prototype winter Maintenance Decision Support System (MDSS) is part of FHWA’s Office of Transportation Operations (HOTO) Surface Transportation Weather Decision Support Requirements (STWDSR) initiative. The objective of the MDSS effort is to produce a prototype tool for decision support to winter road maintenance managers. The MDSS is based on leading diagnostic and prognostic weather research capabilities and road condition algorithms, which are being developed at national research centers. Several candidate road weather technologies currently exist at national laboratories, but the new technologies needed to be integrated, refined, and tailored to address road maintenance weather issues. The project will also identify new and focused research that must be conducted to address specific winter maintenance decision support needs not addressed by current technologies. The project began in 2001, with work with state DOTs on the development of a prototype MDSS, which is moving into demonstration and evaluation of selected prototype components in an operational environment. The MDSS project goal is to develop a prototype capability that: [N]

• Capitalizes on existing road and weather data sources.
• Augments data sources where they are weak or where improved accuracy could significantly improve the decision-making task.
• Fuses data to make an open, integrated and understandable presentation of current environmental and road conditions.
• Processes data to generate diagnostic and prognostic maps of road conditions along road corridors, with emphasis on the 1- to 48-hour horizon (historical information from the previous 48 hours will also be available).
• Provides a display capability on the state of the roadway.
• Provides a decision support tool, which provides recommendations on road maintenance courses of action.
• Provides all of the above on a single platform, with simple and intuitive operating requirements, and does so in a readily comprehensible display of results and recommended courses of action, together with anticipated consequences of action or inaction.

Acquisition of precision application equipment is a large cost center for winter maintenance operations, and often requires a business case or justification for the purchases. Thus, the objectives of applying new technology to winter maintenance operations are: [N]

• Reduction in accidents
• Return on investment
• Reduced chemical usage and improved environmental stewardship

Benefit-cost analysis performed by Iowa researchers demonstrated that integration of the newer emerging technologies in the concept vehicle met the business case, reducing accidents, increasing mobility, reducing adverse environmental impacts, and generating positive economic effects.[N] DOTs are also ordering and using multi-purpose equipment; for example, slide-in equipment, used for winter maintenance, gives plow trucks the capability to carry a variety of products to better match the environmental concerns within an area.[N]

Mechanical removal of ice and snow can be facilitated by preventively treating roadways with road salts. Such pre- or early-storm applications will often minimize the overall amount of road salts required to achieve the desired surface friction level. Reacting to a snow and ice event and applying road salts after a bond has formed requires additional salt to be used; proactively treating the road surface just prior to the event, or just as it commences, can prevent a bond, simplify the mechanical removal and expedite the achievement of bare pavement.

Some transportation agencies choose to leave a small amount of snow on the road before salt is applied in order to keep the salt from bouncing or being blown off the road surface by passing traffic or wind. This can increase the amount of salt required to “de-ice” or melt the snow packed on the road, and is not as efficient in retaining salt on the road as other methods (e.g. slower spreading speeds, pre-wetting, “zero-velocity” spreading, etc.)

General overviews of technology available and disadvantages and advantages of their use are summarized below from the Transportation Association of Canada and a TRB Report on Snow Removal and Ice Control Technology. [N][N]

• The total amount of salt used for winter maintenance is significantly influenced by the characteristics of the spreader equipment.
• Spreader controls should be capable of delivering several precise application rates.
• The application rate should be consistent whether the spreader is full or nearly empty, regardless of material variations, or temperature changes.
• When purchasing new equipment, transportation agencies should require test results from suppliers to confirm that the equipment will achieve precise application rates under all conditions.
• Spreaders must operate in a severe environment of low temperatures, high moisture, poor visibility, and corrosion, often with limited maintenance. Controllers must be easy to load, and simple to operate.
• Ideally, a spreader should be adaptable for other tasks, or the hopper should be easily removed so the trucks can be used for other operations during the summer.
• Hoppers must be constructed so that all sand and salt can be easily removed from the body.
• Spreaders should be fitted with screens to ensure that frozen clumps of material or other contaminating material that would jam the chain/conveyor mechanism are not loaded into the spreaders.
• Cab shields should be fitted to assist in loading the spreaders to ensure that all loaded salt enters the box, and material is not spilled over the truck.
• Spreaders should be manufactured from a material that will resist corrosion. Special chlorinated rubber primers and epoxy-based primers will increase coating life. Stainless and galvanized steel, and fiberglass bodies are available but can be relatively expensive. High strength, low alloy self-coating steel, used with good surface preparation and special primers has been proven to provide a cost effective body life of up to fifteen years. Manufacturers also supply spreader bodies constructed of fiberglass. These bodies are lighter and thus provide increased payload possibilities, but are also more expensive than steel.
• Electrical wiring for controls and lighting, and hydraulic components must be enclosed in vapor proof, or sealed systems.
• Neoprene spinners are frequently used to improve durability and spreading efficiency.

Salt and sand application methods can be modified to meet differing requirements.

• Salt use sometimes can be reduced by applying the salt in concentrated locations (e.g. windrowed on the crown), rather than being spread uniformly or broadcast across the entire road surface.
• In most cases solid or pre-wetted salt should be applied in a continuous narrow windrow along the centerline of the road. The concentrated mass of material minimizes the tendency of the material to bounce or be blown off the road by passing traffic. Salt going into solution drains down the crossfall of the road, and can migrate under packed ice and snow; a uniform section of road is then bared off initially along the center of the road to provide two-wheel stability for traffic. Application in a windrow is achieved without using the spinner, by dropping the material from a chute. Windrowing on the centerline will not work if the crown of the road is not consistently on the centerline, or the road surface is badly deteriorated which could cause the salt brine to pond in some areas. Centerline application is also not appropriate if the entire road surface is slippery and immediate de-icing is required. In these situations, higher salt application rates may be needed across all traffic lanes.
• Application ahead of the drive wheels can provide improved traction under the drive wheels of the spreader vehicle. Application close to the driver’s cab also enables the driver to monitor the application to ensure that material flow has not been impeded.

Hopper Spreaders

• Conventional hopper spreaders provide good control of material application and dependable service. However, they are the least versatile for other operations during the off-season. New hopper designs, including rear-discharge, slide-in units with a longitudinal agitator bar and belt conveyor, are gaining popularity, particularly for pre-wetted applications.

The primary limitation of tailgate spreaders is the inconvenience of raising the dump box and the possibility that the box will not be raised high enough to ensure that sufficient material is dumped in the hopper to provide consistent delivery. The rear discharge restricts the operator view of the operation and ability to ensure that the material is being discharged at the right location. The vertical clearance and the upward and rearward shift of the center of gravity when the box is raised can cause instability and is a safety concern in some areas.

Dual dump spreaders were developed to overcome problems identified for tailgate spreaders while still providing a multi-purpose spreader that could be used year round. They function as regular rear dumping bodies when not being used to apply winter maintenance materials. Disadvantages of this spreader are the high weight compared to a regular dump truck, and the need to raise the body while driving to move the material to the front of the truck. This reduces the truck’s stability and care is required by the operator to ensure that sufficient material covers the cross conveyor at the front to maintain a precise application rate. The pivots have been a source of failure and replacement is expensive.

Multipurpose spreaders incorporate most benefits of the other spreaders. A recent design makes use of a U-shaped box to ensure that no material hangs up in the box and that all material can be easily removed from the box at the end of the shift. Material is either discharged in a windrow using a chute for concentrated action, or spun across the lane using spinners. The spreader provides precise application rates and all the advantages of distribution in front of the rear wheels. Cross conveyors are easily removable during the summer so that there is no tare weight penalty. The units are lightweight and provide year round use, and the body can be easily switched to carrying construction materials (simply by installing a pan or tray across the floor conveyor). As these units can carry substantial loads, care must be exercised to ensure that adequate truck components, axles, springs, and wheels are specified to carry the load. This is particularly important on combination units that are also equipped with snow plows.

Based on the premise that no salt particle should be placed dry onto the road surface, and that fine salt is the gradation of choice for prompt dissolving and melting, certain spreader design characteristics cater better to liquid and fine salt use in prewetted applications. The salt must be of a fine gradation in order for it to retain the brine moisture content and fine salt does not travel as easily on certain chain-type conveyor systems. These spreaders allow a “high-ratio” salt application rates up to 255 liters per ton of salt, or at a ratio of 30:70 liquid-to-solid by weight. This requires a large capacity of liquid onboard and adequate pumping capability that may not be possible or practical on a conventional retro-fitted unit. They are either frame-mounted or slide-in, rear-discharge v-hoppers can stand on self-contained stilt legs in the maintenance yard, and remain tarped until needed. Pre-wetting liquid can be applied directly on the spinner, that is designed to spread the material across a given area of the road cross section. Areas that only have access to coarser salt may find that the liquid component must be reduced since saturation can be achieved with less liquid.

All spreaders require an accurate electronic controller to ensure that the appropriate application rate is achieved. Simple hydraulic circuits, used to maintain a steady application rate, are still in use in many transportation agencies. This equipment starts to exceed the desired application rate as soon as the truck speed drops below the design speed and an excessive salt application is then dumped on the road. Early models of the electronic controllers were not dependable and required extensive maintenance. The new models are improved but can still require  patience.

Modern spreaders use electronic groundspeed spreader controls to provide consistent, accurate application rates. The truck speed is monitored from the truck’s speedometer drive, and the spreader output is adjusted to maintain a steady output at the set rate per kilometer. Both open loop and closed loop systems are available to monitor material flow and provide increased accuracy of the spread rate (closed loop systems provide confirmation of the actual application rate). Electronic controllers automatically increase the output rate if a second spinner is actuated (if so equipped) to treat truck climbing and turning lanes. With some electronic units, calibration settings can be applied electronically using infrared controls.

Manufacturers can now provide units that record, for printing, information about the amount of salt used, the time it was used, and the associated application rate, for analysis and control by the transportation agency. Information that is captured and logged can include: amount and type of material applied, gate position, run time, blast information, average speed, spread width/symmetry, etc. Units are also available that incorporate global positioning systems (GPS) for automated vehicle location (AVL) and to identify where the material was discharged (either generating a passive history or a live transmission). There is currently no industry standard format in place for this information reporting; it is difficult to compare and combine the information from the units supplied by the various manufacturers.

With normal spreaders, a high percentage of the dry salt applied to the road bounces off the road due to the combination of the impact of the granules hitting the pavement, and the speed of the spreading vehicle. Most transportation agencies now theoretically constrain their spreading speed to avoid wasting salt due to the scatter effect at higher speeds. In practice however, speeds of 40 km/hr and more are not uncommon. If salt could be applied at higher speeds, combination units would be much more productive as the unit could apply salt at plowing speeds. This would allow for safer operating condition since trucks could move at the speed of traffic. Casting material rearward has shown potential for salt use reduction by increasing the percentage of applied salt that is retained on the road, and in the required location on the road. This is a concept by which the salt is discharged rearward at exactly the same speed as the spreading vehicle is traveling forward. The two velocity components cancel each other causing the salt to drop on the road as if the spreading vehicle was standing still.

To-date, the available equipment has experienced some operational problems such as material caking, uneven discharge and mechanical complications (fan/blower) under certain conditions. One manufacturer makes use of a shielded-spinner at the mid-chassis discharge location, discharging at a point just beyond the width of the rear wheels where the material is “flung” rearward. Another manufacturer used a high-speed blower to discharge the salt rearward. This results in a large cloud of salt that can be hard to control and may be affected by side winds. Also, the spreader units may not suitably handle pre-wetted material or finer sands. Though useful for salt applications, there is no good way to spread sand with these spreaders. Modifications are being developed and it is anticipated that further refinements will enable transportation agencies to reduce application rates and increase application speeds using this concept.

Ground-speed spreaders and prewetting are recommended to permit high-speed spreading of salt in a windrow patter on bare pavement. Equipment manufacturers and material suppliers have attempted to overcome these problems by using spreaders designed to place the material on the road at zero velocity or in a controlled location, and through the application of small quantities of deicing liquids to the dry material before it is applied. The specialized spreading equipment is referred to generically as ground-speed spreaders, and the application of liquid deicer is referred to as prewetting. Prewetting did not significantly improve material placement over dry salt at a spreader speed of 34km/h and is therefore not recommended as a means of reducing material loss during spreading at current operating speeds. At 60km/h, prewetting made a small but significant improvement over dry rock salt in material placement using a centerline chute and a ground-speed spreader, and it is therefore recommended as a means of reducing material loss during salt spreading at high speeds.[N]

Zero Velocity Spreaders can optimize the use of deicing material through the controlled distribution of the material. The material is dispensed at the same velocity of the forward motion of the equipment. This helps reduce bounce and whip off allowing more of the material to remain on the pavement, saving up to 40 percent in de-icing material and reducing salt runoff to the surrounding environment. The zero-velocity spreader applies material in such a way that the material lands at a velocity that is zero relative to the road surface. The spreaders, which mix and spread liquid and solid deicers, use technology that enables plow trucks to apply chemicals at speeds as fast as 35 miles per hour, which increases efficiency and safety in terms of the speed differential between plows and traffic. In 1994 and 1995, Iowa was the FHWA test site for the zero velocity spreader, at that time a new concept in roadway chemical spreaders. Mn/DOT tested eight zero-velocity spreaders that same season and discovered savings of 30 percent or more. Even in 1995 when the spreader was priced at around $10,900 compared to $2,000 to $2,500 for a common spreader, tests indicated that material savings compensated for the increased cost.[N]

At PENNDOT, during the 1995-1996 winter season, the use of 4 trucks equipped with the system resulted in average material savings of about 50 percent and a cost savings of about $2 per mile per truck. In 1997-1998, PENNDOT purchased 95 additional ZVS units and another 150 units in 1998-1999, equipping all of Pennsylvania’s Interstates and limited access highways with ZVS. The systems were expected to pay for themselves in about 1½ years. PENNDOT also equipped every new dump truck with a ground speed control salt spreader system known as the AS2 system, an on-board computer adjusts the discharge rate of salt and anti-skid material according to the speed of the truck. The truck’s operator inputs how wide the material needs to be spread and the desired tons of salt and anti-skid material to be used per lane mile. At intersections or other areas that may require a heavier application of salt, the operator may use a “blast button” for a preset number of seconds.[N]

Applying liquid melting agents or pre-wetted salt can prevent or clear frost more quickly than solid salt. Pre-wetting is a commonly used practice to improve retention and keep salt on the road by reducing the effects of bouncing, blowing and sliding of the salt or sand particles. This technique uses salt brine, liquid calcium chloride or other liquid chemical to wet the salt or salt as it is spread on the road. Pre-wetting also enhances the melt action of the chemical present by speeding the dissolving of salt and the formation of brine.

Spraying stockpiles and truck loads has also been termed pre-wetting or “pre-treating”, but this practice is not as practical since the granules are not uniformly coated, the liquid may drain out of the solid material and the performance on the road is not consistent throughout the route. Therefore, pre-wetting should be done by spraying the salt as it is discharged from the chute, or at the spinner. A straight liquid will avoid the endothermic cooling effect that solid salt can have on pavements. Practical considerations relate to the gradation of the salt being wetted, the maximum liquid to solid ratio that can be mixed, the amount of mixing action, caking/clumping concerns, etc.

Pre-wetting is commonly considered to have the following benefits:

• The deicing effect of the salt spread onto the highway surface is achieved more quickly, with time lag significantly reduced or even eliminated.
• A significant proportion of the salt spread by dry spreading techniques ends up o the channels of the highway or on the highway verge because of particle bound and the action of traffic. It is also claimed that this may well increase the longevity of the salt action on the highway surface, with a direct result of possible reductions in salting frequency.
• It is claimed that significant reductions (on average one-quarter, but up to one-third) in the overall amount of salt use can be realized.
• Because less salt can be used, and more of it stays o the road surface, pre-wetting techniques can lead to significant environmental benefits compared with traditional dry salting techniques.
• Damage to concrete structures is likely to decrease with high-purity prewetted salt although a calcium chloride wetting agent may cause more damage than a sodium chloride one.

While pre-wetting may provide significant potential for reductions in salt use, it can increase the complexity of the required equipment and controller. Pre-wetting requires additional equipment. Storage tanks for the liquid(s), or brine making equipment are required, along with pumps to load the spreaders. The on-board liquid capacity and loading time are factors to consider. Additional maintenance is required such as ensuring that the liquid filters, lines and nozzles are purged and the equipment cleaned at the end of the storm to prevent clogged lines and seized equipment. Prewetting is not universally acclaimed. A University of Iowa study found that prewetting at the stockpile had little effect on the ability of the abrasives to remain on the pave­ment surface when delivered and that prewetting while loading or final prewetting at the truck spinner was found to help keep salt and other chemicals on the road surface when first delivered but may do little to help material stay on the road.[N] Brine is a method that has mixed reviews. While brine has uses les salt, melts faster, and dries road surfaces faster than prewetted salt, maintenance areas must have equipment for spreading prewetted salt, as brine is not suitable for use during heavy snowfalls. If the road surface is very wet or precipitation is ongoing, there is a risk that the brine will be excessively diluted and the liquid will refreeze.[N]

The Transportation Association of Canada outlines the following recommended practices for pre-wetting: [N]

• Adjustment of the spray nozzles is critical. Tests by one state department of transportation showed that they never achieved more than 60 percent coverage of the salt. The remaining 40 percent of the pre-wetting liquid was effectively being applied directly on the road. Also, as the wetting agents are corrosive, it is important that corrosion resistant nozzles and non-contact pumps are used to ensure dependable performance.
• Utilize the latest research on optimum liquid application rates; extensive testing is currently being performed.
• The application pumps on the spreaders should be regulated by ground speed controllers to ensure the correct liquid application rate is maintained under all conditions.

A recent study on the possibility of decreasing the use of salt by changing the spreading method found that saturated brine (20 percent) is spread more evenly across the road than prewet salt, and more salt from the brine is still present on the road 2 hours after spreading as compared with prewet salt. Several statistical analyses were carried out, giving a useful picture of the amount of residual salt on the roadway and indicating that more salt from brine than from prewet salt is active on the roadway and that degradation of residual salt is crucially affected by high traffic intensity.[N]

Areas that experience a high number of frosting or black ice events each winter season have traditionally required a significant amount of labor and road salt to manage properly. Maintaining material on the road to deal with frosting events can be difficult and expensive on roads with higher traffic volumes. Applying the material just prior to an anticipated event is ideal. Fixed, automated liquid anti-icing spray systems, called FAST systems, have been developed to help organizations better manage these demands and place the right material, in the right amount, in the right place and at the right time. Fully automated FAST systems have been developed that use sensors embedded in the roadway and mounted on bridge towers, elevated ramps, or intersection approaches. Site mounted computer hardware and software and nozzles embedded in the roadway or the parapet wall automatically apply liquid anti-icing chemical to the road surface just prior to a forecasted icing event.

The information in the remainder of this section has been previously profiled by AASHTO, FHWA, and TRB in relation to “Smart Bridges.”[N]

The New York City Department of Transportation developed a fixed anti-icing system that is comprised of a control system, a chemical storage tank containing liquid potassium acetate, a pump, a network of PVC pipes installed in roadside barriers, check valves with an in-line filtration system, 50 barrier-mounted spray nozzles, and a Dynamic Message Sign (DMS). The DMS displays warnings to alert motorists during spray operations. A Closed Circuit Television (CCTV) camera allows operators to visually monitor the anti-icing system. Each self-cleaning nozzle delivers up to three gallons (11.4 liters) of chemical per minute at a 15-degree spray angle. This angle minimizes misting that could reduce visibility. Two nozzle configurations were implemented to investigate different spray characteristics. On both sides of one bridge section, nozzles were installed 20 feet (6.1 meters) apart for simultaneous spraying. On another section, sequential spray nozzles were mounted on only one side of the bridge. Due to concerns about bridge deck integrity, nozzles were barrier-mounted rather than embedded in the road surface.

System operators consult television and radio weather forecasts to make road treatment decisions. When anti-icing is deemed necessary, “ANTIICING SPRAY IN PROGRESS” is posted on the DMS and the system is manually activated to spray potassium acetate on the pavement for two to three seconds, delivering a half-gallon per 1,000 square feet (1.9 liters per 92.9 square meters). Operators then review forecasts and view CCTV video images to monitor weather and pavement conditions. If there is a 60 percent or greater chance of precipitation and pavement temperatures are predicted to be lower than the air temperature, maintenance crews are mobilized to supplement anti-icing operations with plowing to remove snow and ice.

An analysis of maintenance operations found that bridge sections treated with the anti-icing system had a higher level of service than segments treated by snowplows and truck-mounted chemical sprayers. Road segments treated by the anti-icing system have less snow accumulation than sections treated conventionally, improving roadway mobility and safety in inclement weather. The system was most effective when chemical applications were initiated at the beginning of weather events. If potassium acetate was sprayed more than an hour before a storm, vehicle tires dispersed the chemical necessitating subsequent applications. The system also improves productivity by extending the life of bridges and minimizing treatment costs associated with mobilizing maintenance crews, preparing equipment, and traveling to treatment sites on congested roads, in addition to minimizing salt runoff to the environment. The DOT would like to expand the anti-icing system by integrating a Road Weather Information System (RWIS) with the control system, the CCTV camera, and the DMS to improve treatment decision-making. A wireless or fiber optic cable communication network is envisioned for connectivity of these elements. Deployment of the system on the entire Brooklyn Bridge and on other local bridges is also anticipated.[N]

A 2003 report for the Mid-America Transportation Center and the Nebraska Department of Roads (NDOR) developed guidelines for prioritizing bridge deck anti-icing system installations. This research was undertaken with the objective of developing a decision-support tool that can aid NDOR with the prioritization of bridges for installation of automatic anti-icing systems. Based on a literature review on automatic bridge anti-icing systems was conducted, factors considered important in the installation of automatic anti-icing systems included accident history, bridge alignment, weather, traffic, and bridge distance from maintenance yard. The factors were included in a database and decision-support tool that assisted NDOR in narrowing the list of candidate bridges for NDOR. Some of the sources and GIS layers included were: NDOR bridge inventory, NDOR accident database, archived weather data from the High Plains Regional Climate Center and the National Weather Service, Nebraska streets database, Nebraska rivers and streams database, and NDOR maintenance yard data.[N]

Regardless of the spreader or Fixed Automated Spray Technology chosen, the service provider must have faith that the application rate settings are indeed accurate. Spreaders should be calibrated to avoid the over-application of de-icing agents or abrasives and use no more than is necessary for snow and ice control.

• A calibration policy should be established to assure the material settings are correct. Preferably, if application is by weight, then calibration should also be by weight. Calibration checks or recalibration should take place several times during the season:
  • Calibration should occur after repairs.
  • Calibration should occur when distribution calculations show a discrepancy between theoretical and actual.
  • Calibration spot-checks on units in the fleet should be scheduled throughout the season.
• Operators should be able to easily track fuel and material usage.
• In order to apply the proper amounts of anti-icing, de-icing and/or traction enhancing materials, spreading equipment should be calibrated for both solid (typically salt) and liquid (typically salt brine, calcium chloride, magnesium chloride or IceBan/MAGic) applications.
• To ensure proper placement of materials, equipment affecting the spread pattern should be adjusted to match the required use. Critical system components include the automatic ground speed controller, the flight chain or belt, the gate opening, the chute, the liquid nozzles (if applicable), the spinner and the deflectors.
• Maintenance districts should calibrate their equipment regularly and train their operators so they understand the reasons behind pre-treating application practices and quantity of materials to be applied under specified conditions. Training presentations should be available at each District. Presentations should be showing the exact details on how calibration is performed and should be reviewed each year before beginning calibration.
• Because of the adverse conditions under which snow and ice equipment operates, periodic checks should be made to confirm proper settings. Calibration is necessary to find out how much salt and/or abrasives are discharged at each auger setting.
• All truck and spreader combinations, both Department owned and rented, should be calibrated every year.  Calibration should be completed prior to the snow season. Calibration can be done using sand or other abrasive materials if the truck is used on routes where salt is not spread.  Department personnel should calibrate non-municipal rental trucks equipped with spreaders. 
• Those servicing state roads under lump sum agreements typically are responsible to calibrate their own equipment. Department personnel may assist with their calibration programs if requested. 
• As part of calibration all Department trucks should have their augers and spinners mechanically restricted. Augers are to be restricted to spread no more than the maximum amount of material approved for the route or routes to which that the truck is assigned. 
• Spinner speeds should be restricted so that no spinner will spread more than a ten-foot width of material when the truck is stationary. 
• Records should be kept for each piece of equipment. Calibration information for each spreader is stored electronically on a laptop computer. If any controller is replaced, calibration information can be downloaded to the new controller as a starting point to recalibrate.

Accurate records should be maintained of the locations of de-icing agents and abrasives application and the quantities of de-icing agents and abrasives used. Various types of equipment support the winter maintenance program either by helping manage the operations by generating useful data or by supporting the service delivery itself.

Equipment is available to assist with meeting the following necessary functions for environmental stewardship and effective minimization of materials application:

Material Usage Monitoring

Loader Mounted Electronic Weighing Equipment

Loading extra material onto a spreader can lead to overloading or the temptation to over apply the salt. In the past, operators tended to load a little extra salt as there was no exact method of determining the amount of material loaded, and they did not want to run out without completing the route. Overloaded trucks also contribute to contamination in the area of the salt storage facilities. Salt heaped above the side boards is thrown off the trucks as they negotiate curves to exit the yards.

• With electronic scale control systems operators can more precisely load the right amount of salt. This device is a relatively inexpensive, durable, and accurate weighing device consisting of a transducer load cell mounted to the loader bucket arm. These devices can measure a predetermined load size for the scheduled route (length of route X application rate + a limited contingency amount for bridge decks, intersections, etc.). Models are available that will record with the loader in motion so that the loader operation is not impeded.
• The units will record the amount loaded for future printing and analysis. Though the equipment can be overridden, it provides the operators with a mechanism to accurately measure and control the amount of material loaded on the spreaders.

Weighing the trucks as they enter and leave the maintenance yard is one way of determining the material loaded and the resulting spread rate for the serviced route. This function can be automated with a weigh-in-motion pad that tracks the equipment movement and can serve to reconcile the data from the spreader controller and other documentation.

Pump meters will likely be used to measure delivered brine, but not likely be on each pre-wet unit.

• A meter should be in place at the brine supply facility, whether the source is hauled brine or manufactured brine, in order to track loading times and quantities.
• A cross reference should be incorporated in the electronic log to identify the truck loaded for future reference.

• Tracking equipment movements along with the services provided is possible via proven GPS receivers/ transmitters and software.
• This electronic record can be actively followed real time or can be passively recorded for later analysis.
• AVL can support a route optimization exercise, to rationalize the number of trucks required and thus the expected salt to be used on the roads serviced.
• This equipment can provide operational support to greatly enhance the monitoring of salt usage, to demonstrate prudent usage and to correlate with the achievement of the required level of service.

Sand and chemicals should be stored and handled in a manner to minimize any contamination of surface or ground water. Care should be taken to prevent runoff from chemical tanks or chemical treated stockpiles. Covered storage for dry chemicals is preferred.

• As noted by Oregon DOT, chemicals and sanding materials should be free of contaminants known to cause water quality problems. Some of these include: Arsenic Barium Cadmium Chromium Fluoride Lead Mercury Nitrate Selenium Other heavy metals Hydrocarbons.

• Extensive environmental contamination has been identified in the area of salt storage yards. Much of this contamination results from poor salt handling practices.
• Conveyors are available which are designed to allow salt trailers to dump directly into the conveyor for movement into the storage facility.
• Loaders used to fill spreader vehicles are often fitted with buckets that are too large for the spreader hopper bodies, resulting in spillage. Though they have a slower production rate, smaller buckets are available for most loaders. Side dumping bucket attachments can also be used to provide quick precise loading.

• Whatever equipment is used for moving salt, it should provide a way of tracking the flow so the quantities can be reconciled.
  • Pre-loaded drop-hopper loaders meter salt into spreader trucks.
  • Overhead silos can be pre-filled with salt to similarly meter salt into spreader trucks.
  • Pneumatic handling equipment can handle fine material that is used for either direct application onto the road or for blending with sand.

• Ideally, blended winter sand stockpile are put up in favorable, dry conditions. Relatively dry sand stored indoors should not require more than 1-2 percent salt by weight; more moisture in the sand may require more blended salt (up to 5 percent), but the purpose still is to keep the sand free-flowing, and not to support melt action.
• Traditionally, blending took place on the apron to the storage shed, with several buckets of sand spread level, followed by one bucket of salt trickled on the surface; the resulting blend was loaded in the dome, and the process was repeated. Though highly inefficient, it was also highly inaccurate, and produced sporadic result on the pavement surface. Equipment to support high-production stacking and uniform, light blends now involves a form of dual-auger pugmill or a twin conveyor feed. In either case, two supply lines are metered to an accurate ratio and the final conveyor stacks the completed mixture.

• The concentration should be checked with a hygrometer to measure the specific gravity of the solution. The percent of saturation is determined by reference to specific gravity charts for the specific solution temperature.
• Water supply flow rates are a critical factor. Production sites may require cisterns to ensure adequate water supply where well production rates are poor.
• Manufactured salt brine can be pumped directly into tanks mounted on the spreaders or transferred to holding tanks at the maintenance yards.
• Stored brine will normally stay in solution as long as there is not evaporation or a drop in temperature below eutectic.
• Corrosion inhibition requirements can complicate the brine manufacturing process.
• Additives such as rust inhibitors may complicate long-term storage, in which case agitation or recirculation could be considered.

• Sampling containers and a refractometer or hygrometer should be available for sampling and testing the concentration.

The Wisconsin Department of Transportation (DOT), in cooperation with eight Wisconsin counties, embarked on a 4- to 5-year effort to implement advanced technologies in winter maintenance vehicles. The effort equipped winter maintenance vehicles are equipped with differential Global Positioning System (DGPS) receivers and numerous additional sensors that collect environmental data (e.g., pavement and air temperature), equipment status data (e.g., plow up/plow down), and material usage data (e.g., salt application rate). These data are telemetered to a dispatch center and recorded on magnetic media for later downloading. Data are transmitted and recorded as often as every 2 seconds. A geographic information system (GIS) application, dubbed “Wiscplow,” was developed and initially deployed for testing within participating counties, combining vehicle data with manually entered data (e.g., storm durations, vehicle configurations, and labor and equipment cost rates) and with spatial data representing roadway centerlines attributed with functional class, number of lanes, patrol sections, and route systems. Outputs include reports on computed performance measures (e.g., cycle time and hourly average salt application rate by patrol section and storm) and decision management tools (charts, graphs, and maps) showing relationships among performance measures (e.g., salt application rate versus pavement temperature by patrol section and storm).[N]

Winter operations performance measures and decision management tools were identified, defined, developed, and refined in an iterative process, with state and county transportation decision makers, that included a series of meetings, communications, and two workshops.[N] Concerning material usage, Wiscplow can generate up to 19 performance measures and chart relationships among them. Sample performance measures include average pounds of salt per lane mile for each operator and event, hourly average for each patrol section of gallons per lane mile of anti-ice liquid, tons of salt used for each event and patrol section, and cubic yards of sand used for all events for each patrol section. Sample charts include average pavement temperature, salt, and sand application rates by patrol section for a winter storm event and seasonal cumulative salt use on each patrol section. Concerning equipment usage, Wiscplow can generate performance measures such as cost for each attachment unit for each event and patrol section, cycle time for each patrol section and storm, and total operating distance, season-to-date, for each attachment unit. Charts of relationships among these measures include production rates for equipment units by roadway class and cumulative operating hours for units of an attachment class. Concerning labor, Wiscplow can generate labor hours per lane mile for each patrol section and storm and percentage of labor costs attributed to clean-up for each storm. The map display can be queried and attributes displayed for roadways, patrol sections, and each data point in a vehicle track, including operator name, time, air temperature, pavement temperature, vehicle speed, front plow status, right-wing status, left-wing status, and scraper status. The user can scroll down to see material application rates, on which patrol section and route the vehicle is traveling, and the route measure of the vehicle’s location. Ultimately, Wiscplow is intended to help transportation agencies at multiple levels (i.e., central office, districts, and counties) to measure performance of winter operations. [N]

In addition to evaluations of chemical residue, friction, and changing temperatures during a storm, it is beneficial for the personnel of each maintenance area to conduct a post-storm evaluation of the treatment effectiveness. This can help identify areas needing improvement and changes that can be made in the treatment strategy. A post-season review of treatment effectiveness is likewise helpful. It can help identify where changes are needed in equipment, material, and route configurations, and can begin a process of engineering an anti-icing program to fit the exact needs of a site or agency. It can also help identify where changes in personnel procedures and training are needed to improve the effectiveness of the winter maintenance program.

Advanced ITS technologies are expected to automate winter operations performance measures and provide them in real-time to snow-fighting supervisors. The idea is to measure outcomes like roadway friction rather than just outputs like the time and amount of salt applied. Field studies of roadway friction measurement have been done at the NASA Wallops flight facility, and in Iowa, Minnesota and Michigan. There has also been an ongoing, coordinated study in Norway. [N]

The data logging and reporting capabilities of loader scales, electronic controllers and GPS/AVL systems can assist transportation agencies in more accurately tracking their salt use. Progress in implementation of best salt management practices can be measured in improvements to the fleet. Monitoring and record keeping should include:

• Type and amount of winter materials being placed.
• Percentage of fleet equipped with electronic spreader controllers.
• Percentage of fleet equipped with pre-wetting.
• Percentage of fleet equipped with direct liquid application.
• Percentage of fleet calibrated annually.
• Percentage of staff trained in equipment use.