Impact of Shoulder Width and Median Width on Safety (2009) / Chapter Skim
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From page 17... ...
Methodology Over the past decades, interest has increased in estimating the safety implications from changes in various design elements. To be able to determine these changes, models were developed that could predict the crash-rate frequency or the number of crashes as a function of various traffic conditions and values of geometric elements.
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From page 18... ...
The project panel recommended that the research be concentrated on multilane rural roads and that it should be limited to specific design elements: lane width, shoulder width, and median type and width. The possibility of examining the contribution of clear zones was also discussed, but this decision was made contingent on a determination of data availability and potential feasibility.
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multilane rural roads; (2) geometric elements including lane and shoulder width and median type and width; (3)
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, while most median widths for Kentucky were narrower (more than one-half of the segments were less than 20 Divided Undivided Variable Categories CA KY MN CA KY MN All 16,951 8,035 5,106 3,495 1,037 1,068 Crashes Injury 4,045 2,765 681 995 405 133 Yes 571 539 615 164 73 84 Principal arterial No 183 71 46 125 8 31 Yes 95 3 6 NA NA NA Median barrier No 659 607 655 NA NA NA Yes 624 530 595 243 68 47 Paved right shoulder No 130 80 66 46 13 68 0 0 1 10 6 2 49 0–2 49 27 1 20 7 2 2–4 102 32 14 124 1 9 4–6 87 218 99 36 19 13 6–8 412 329 536 75 31 18 Average shoulder width (ft) 8+ 104 3 1 28 21 24 <5 65 61 91 103 4 34 5–10 116 172 268 80 31 38 10–15 181 239 178 53 23 32 15–20 131 89 92 34 12 10 20–25 89 30 26 12 8 ADT (vehicles/day; 000s)
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Data Analysis As noted above, predictive models were developed to evaluate trade-offs among selected design elements. The unit of analysis is a roadway segment with its associated crash history.
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. The SAS statistical software was used to develop the prediction models and to determine their coefficients (46)
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; MW = median width (ft) ; FC = functional class principal arterial (no/yes)
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shoulder width. 24 Divided Highways Crash Type MPB MAD MSPE SMD R2 Single 0.90 5.20 157.75 4261.49 0.8425 Multi 1.42 5.31 188.03 5196.85 0.6816 All 2.47 9.44 532.68 6548.12 0.8028 Undivided Highways Crash Type MPB MAD MSPE SMD R2 Single 0.69 4.49 93.21 1001.04 0.7837 Multi 1.07 5.66 202.48 1460.82 0.6598 All 1.78 8.51 469.45 1585.85 0.8055 Table 13.
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From page 25... ...
An additional continuous variable also entered the model; this AMF is shown in Figure 1. Undivided Highways Single-Vehicle Crashes A single value AMF was obtained for only one variable, paved shoulders.
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From page 26... ...
This issue was addressed by a meeting of the research team at which a reasonable magnitude was estimated by consensus for each selected design element. This approach allowed the research team to consider past research, weigh the findings of this research, and adjust the magnitude of the AMFs as needed to reflect practical and research experience.
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