Research to Better Inform the Safety Performance of Local and Urban Roads Related to Design Alternatives and CSS

Focus Area

Context Sensitive Solutions


Community & Cultural Concerns, Environmental Process







Research Idea Scope

The design of local roads, including issues of width and the question of planting trees in medians, is often made based on a widely held, but not always proven, belief about the safety of certain design approaches. For example, the width of local roads has been established in many areas by the flawed use of studies on recommendations of road widths based on state highway manuals, or the unfounded misconception that narrow local roads are unsafe (because of auto accidents or hindered access to fire emergency vehicles). In addition, local stakeholder values that favor trees planted in urban conventional highway medians are often in conflict with state traffic engineering guidelines.  These guidelines have been established using accident data that is debated by designers familiar with the Context Sensitive Solutions process.  Some argue that many guidelines for trees in urban roadsides have been extrapolated from rural accident data. 
Many local jurisdictions fail to appreciate distinctions between design speed and typical operating speeds on local roads. Many local jurisdictions have ignored transportation agency recommendations for less than 12-foot travel lanes for local roads. Also, Most traffic studies have focused on highway and connector road use.  Short local roads are often overlooked in these types of studies.  This is unfortunate, because in many suburban communities, most new road pavement that is created (sometimes up to 70%) is in local roads (like those in subdivisions).  Ironically, it is quite common now to have a 6-lot subdivision with 1 acre lots required to have a 30-foot wide road, even though it might be connecting into a historically old 18-ft connector road with considerable traffic.
To evaluate the real impact of connector road width and the safety implications of trees/vegetations in median strips, a detailed analysis of many years of all fires and auto accidents would need to be evaluated in several communities.  This analysis would include interviews about needs and perception by fire chiefs (often the biggest advocates of wide roads).  Such a study would also require traffic counts on local roads, and perhaps even traffic experiments with obstacles (parked cars) etc.) to determine how traffic changes, and to determine what is the practical narrowest width that ensures public safety.
TERI Administrator Note (June 2007): Related Research
TRB 86th Annual Meeting Presentation; The Safety Effects of Urban Principle Arterial Streetscape Redevelopment Projects Including Street Trees: A Context-Sensitive Case Study (2007)

This report documents safety performance of landscaped medians and other streetscape improvements on State Route (SR) 99 in Washington State. Findings contained in this report are based on a before-and-after study of streetscape improvements made in SeaTac, Washington. Findings from the first two phases of SeaTac’s redevelopment project analyses are shown and indicate no significant change in frequency or severity of crashes. Crashes in the combined study area decreased slightly, while SeaTac Phase 2 showed a slight increase. A shift in crash locations occurred, with fewer mid-block and increased intersection crashes. U-turn crashes increased after construction, from four to 35 within three years. These changes relate directly to the access control effects of the medians The number of crashes reporting striking a fixed-object decreased in the Phase 1 roadway segment but increased in the Phase 2 segment. When trees were involved, the small size of the trees appears to have limited the severity of the resulting crash, increasing the likelihood that they would be classified as “property damage only.” Concern related to the future growth of trees and the corresponding increased severity is identified. The findings also shows that placing trees in narrow medians and near road segments with turning movements resulted in high levels of tree strikes.

Urgency and Payoff

Documenting the relationship between local road with (e.g. subdivision roads) and public safety will help justify smart growth techniques that will benefit the environment, and will also save both developers and the taxpayer considerable costs associated with road construction, maintenance, and the deployment of public services and utilities.

Suggested By

Scott Bradley, TRB CSS Taskforce Minnesota DOT Dr. Joe Costa, Buzzards Bay National Estuary Program, June 15, 2007 Doug Brown, California Department of Transportation

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