Potential Safety Effects of Lane Width and Shoulder Width on Two-Lane Rural State Highways in Idaho
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Potential Safety Effects of Lane Width and Shoulder Width on Two-Lane Rural State Highways in Idaho

  • Published Date:

    2012-07-01

  • Language:
    English
Filetype[PDF-1.20 MB]


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  • Edition:
    ; 08/14/09-07/30/12.
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  • NTL Classification:
    NTL-OPERATIONS AND TRAFFIC CONTROLS-Traffic Control Devices ; NTL-OPERATIONS AND TRAFFIC CONTROLS-Traffic Flow ; NTL-SAFETY AND SECURITY-Accidents ; NTL-SAFETY AND SECURITY-Highway Safety ;
  • Abstract:
    This study provides a comprehensive evaluation of the relationship between crash rates and shoulder width and lane width for two-lane rural state highways in Idaho. Crash Modification Factors (CMFs) for shoulder width and lane width were developed using Idaho crash data covering the period from 1993 to 2010. The CMFs developed as part of this project will allow ITD to assess the potential safety benefits of shoulder widening projects. In addition to all crashes, models for single-vehicle and multiple-vehicle crashes were also developed. The CMFs presented in this study follow the general trends of prior knowledge and research. The results of the analysis showed that there is no significant difference between 12 ft lanes and 11 ft lanes in terms of safety for all types of crashes. The CMF for highways with 11 ft lanes was 1.02 indicating a marginal 2 percent increase in all crashes in comparison to highways with standard 12 ft lanes. The CMFs for highways with very small shoulders (less than 1 ft) were 1.16, 1.17, and 1.15 for all crashes, single-vehicle crashes, and multiple-vehicle crashes, respectively. This corresponds to an average increase in crashes of 16 percent when compared to highways with a 3-ft shoulder width. For highway sections with a shoulder width of 8 ft or more, the CMFs were 0.87, 0.90, and 0.83 for all crashes, single-vehicle crashes, and multiple-vehicle crashes, respectively, indicating an average reduction in crashes of approximately 13 percent when compared to highways with a 3-ft shoulder width. Idaho’s crash data was also used to investigate the characteristics of pedestrian and bicycle crashes on two-lane rural highways. The results show that roadway sections with a right paved shoulder width of 4 ft to 6 ft had the lowest number of pedestrian and bicycle crashes. The probability for a pedestrian/bicycle crash increases significantly for roadway sections with shoulder widths less than 3 ft. The likelihood of a crash also increases for roadway sections with shoulder widths of 8 ft or more.
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