Resolving Critical Gaps in the Interface Between Travel Models and Emissions Models

Focus Area

Air Quality


Air Quality







Research Idea Scope

This research will identify promising analytical techniques that enable four-step planning models, used to produce region-wide estimates of vehicle activity data, to interface with modal emission rate models. The research will also establish guidance on determining an appropriate link classification system based on the degree of variation in the characteristics of different highway facilities and the quality of travel demand model inputs. Because the accuracy of such approaches depends on the accuracy of the estimated traffic volumes and speeds, sensitivity analysis and reasonableness checks will be needed to verify the accuracy of modal activity estimates. Data collection, field observation, and small area simulation may be needed to establish speed/acceleration conditions for signalized intersections that account for slowdowns and delays that do not involve complete stops. The research will identify and implement additional experiments to generate vehicle activity data and develop relationships between traffic conditions (volume/capacity), link characteristics, and control/management scenarios. On the microscale and corridor simulation side, the researchers will evaluate the accuracy of outputs from current simulation models with respect to speed and acceleration profiles. Based on the results, the researchers will propose modifications to the simulation models that will enhance their use in modal emissions modeling. Targeted research and validation efforts will be needed to refine the modal activity estimates.

EPA has been developing the next emission model, MOVES, for several years. This new model, which will replace the existing emissions model, will be required for a variety of emissions and air quality analysis for conformity and NEPA regulatory purposes. This multi-year research effort would include a number of tasks, including reviewing the MOVES model and identifying issues of concern for the transportation community and evaluating, testing, and validating the model to assure accurate and relevant results. Possible tasks may include:

  • Identify transportation data collection needs
  • Identify model calibration and validation issues related to transportation/traffic data, such as modal activities and traffic operations
  • Conduct sensitivity analysis
  • Develop and conduct training for transportation agencies

TERI Administrator Note (June 2007): Related Research
FHWA FY07 STEP Project: Testing, Evaluation and Validation of New Emission Model

TERI Administrator Note (June 2007): Related Research
North Carolina State University, Bridging the Gap Between Transportation and Air Quality Modeling (2004)

TERI Administrator Note (January 2009): Related Research
The MOVES from MOBILE: Preliminary Comparisons of EPA’s Current and Future Mobile Emissions Models, Sonntag, Darrell Bruce; Gao, H Oliver, Transportation Research Board 86th Annual Meeting, 2007

Urgency and Payoff

Considerable resources and effort have been devoted to the development of models that seek to simulate “real world” travel conditions and emissions on a second-by-second basis. These advancements are designed to better account for the deviations from standard driving cycles that produce dramatic increases in emissions. Studies suggest that estimated emission reduction benefits from signal coordination, for example, can more than double when modal activity effects are taken into account. To use emission rates produced by modal models, it becomes necessary to adapt travel demand models to provide compatible activity data output. Travel demand models currently output traffic volumes and speeds based upon internal volume delay functions. Currently, there is no general method for developing appropriate speed and acceleration data from these aggregate model outputs. Lookup tables established using small area simulation approaches are not well suited for evaluating a number of transportation control measure strategies (ramp metering and related intelligent transportation system measures), because they are based on volume-to-capacity ratio and basic link characteristics. Simulation models used in corridor and microscale analyses must be similarly adapted to link with modal emissions models. Directly linking to a microscopic simulation model does not currently appear advisable, given the evidence in the literature that current microsimulation models may fail to produce realistic acceleration and deceleration behavior. Many simulation models contain no provision for less than “emergency braking,” and car-following algorithms may not be accurate. There is a need to assess the algorithms used in standard practice simulation models for compatibility with modal emissions models. Research is needed to fill critical data gaps, and validate and enhance the body of knowledge with respect to speed post-processing techniques, mesoscopic regression models that predict the number of stops along arterials, and simulation in small scale networks to generate link level modal activity.

Suggested By

Transportation Research Board 2002 Environmental Research Needs Conference Notes

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