Effects on Safety

More than four decades of research conducted throughout the United States have shown that access management improves roadway safety. These safety benefits are attributable to improved access design, fewer traffic conflict locations, and higher driver response time to potential conflicts. Some key findings on the impacts of arterial access management on safety are summarized below.
  • As access density increases, crash rates increase.
    Relative increases in crash rates are remarkably consistent among the various studies. Figure 1-2 shows composite crash rate indices derived from the analysis of 37,500 crashes, as compared with a synthesis of previous studies 1. The indices were developed by correlating crash rates with access density - using the crash rates for 10 access points per mile as a base and then averaging crash rates for each access density. For example, these indices suggest that an increase from 10 access points to 20 access points per mile would increase crash rates by roughly 30 percent.
    Composite Crash Rates (click in image to see full-size image)
    Figure 1-2. Composite Crash Rates
  • Roadways with nontraversable medians are safer at higher speeds and at higher traffic volumes than undivided roadways or those with continuous two-way left-turn lanes (TWLTL).
     Numerous studies from across the nation have been conducted relating to undivided, TWLTL, and divided roadways with a nontraversable median. Based on studies, it can be concluded that roadways with a nontraversable median have an average crash rate about 30 percent less than roadways with a TWLTL. Table 1-1 summarizes the representative crash rates by median type for urbanized areas. Additionally, where ADT exceeds 20,000 vehicles per day and the demand for mid-block turns is high, a raised median should be considered.2
    Table 1-1: Accident Rates
    Representative Accident Rates (Crashes Per Million VMT)
    by Type of Median - Urban and Suburban Areas
    Median Type
    Total Access Points Per Mile (1)
    Undivided
    Two-Way Left-Turn Lane
    Non-Traversable Median
    < 20
    3.8
    3.4
    2.9
    20.01 - 40
    7.3
    5.9
    5.1
    40.01 - 60
    9.4
    7.9
    6.8
    > 60
    10.6
    9.2
    8.2
    Average Rate
    9.0
    6.9
    5.6
    (1) Includes both signalized and unsignalized access points.
1. Gluck, J., H.S. Levinson and V.G. Stover, NCHRP Report 420: Impacts of Access Management Techniques, National Cooperative Highway Research Program, Transportation Research Board, Washington, D.C., National Academy Press, 1999.
2. Texas Department of Transportation (TxDOT), Roadway Design Manual, 2002.