Further Development of the Safety and Congestion Relationship for Urban Freeways (2014)

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21 C h a p t e r 4 This chapter presents conclusions of the research and recommendations for the implementation of the research results. Conclusions 1. The results of the research considering relationships between crash rate and traffic density for additional metropolitan areas (Sacramento and Kansas City) confirmed the findings of the original research for the Seattle and Minneapolis– St. Paul metropolitan areas (1, 4). 2. Crash rate on urban freeways varies with traffic density in a U-shaped relationship with higher crash rates at very low traffic densities (due primarily to single-vehicle crashes), higher crash rates at very high traffic densities (due to multiple-vehicle crashes), and the lowest crash rates at medium traffic densities. This result was found for both fatal-and-injury and property-damage-only crashes. 3. This finding implies that design treatments that are effective in reducing congestion levels on urban freeways (between approximately LOS C and LOS F) should also be effective in reducing crashes. Figure 3.1 and Equations 3.4 through 3.6 present relationships based on the combined data for three metropolitan areas (Seattle, Minneapolis–St. Paul, and Sacramento) that can be used to quantify the effect on crash rate of reducing congestion within the range from LOS C to LOS F. 4. Further analyses of data for Sacramento freeways dem- onstrated that the relationships shown in Figure 3.1 and Equations 3.4 through 3.6 are applicable to both recurrent and nonrecurrent congestion. recommendations It is recommended that the relationship between crash rate and traffic density shown in Figure 3.1 and Equations 3.4 through 3.6 be used to represent the safety-congestion rela- tionship in the spreadsheet Analysis Tool developed in Proj- ect L07 in place of the original relationships based on only two metropolitan areas (1, 4). Conclusions and Recommendations