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29 Table 17. AMFs based on prediction models for average shoulder width. Average shoulder width (ft)1 Category 0 3 4 5 6 7 8 10 Undivided, multi-vehicle 1.39 1.00 0.90 0.80 0.72 0.64 0.58 0.46 Undivided, all crashes 1.22 1.00 0.94 0.87 0.82 0.76 0.71 0.63 Divided, single-vehicle 1.17 1.00 0.95 0.90 0.85 0.81 0.77 0.69 Divided, multi-vehicle 1.51 1.00 0.87 0.76 0.66 0.58 0.50 0.38 Divided, all crashes 1.43 1.00 0.89 0.79 0.70 0.62 0.55 0.44 1 The average shoulder width for undivided highways is the average of the right shoulders; for divided, it is the average of left and right shoulder in the same direction. Median Width and, therefore, the AMF for single-vehicle crashes could be considered as 1.00. In this case, a weighted AMF can be esti- Recommendation mated using as weights the relative percentages of single- and The research team reviewed past literature, the recommended multi-vehicle crashes for the roadway of concern. values for the HSM, and the AMFs from NCHRP Project 15-27 and agreed that there is an influence on crash occurrence Supportive Background from the median width. The team determined from available background data that the values noted for the only model The key objective for the presence of medians is traffic with median width influence are reasonable and in accor- separation. Median design issues typically address the pres- dance with current trends and literature. The only available ence of medians, along with type and width. There has been AMF based on the models developed in this research is for some research completed on these issues and their implications multi-vehicle crashes, and it yields a 1% reduction for every on safety. However, past research indicated three safety trends: added foot of median width. The values obtained from the (1) cross median crashes (i.e., between opposing vehicles) models for multi-vehicle crashes are reasonable and agree with are reduced with wider medians; (2) median-related crashes the previous research. The recommended values are summa- increase as the median width increases with a peak at about rized in Table 18. 30 ft and then decrease as the median width increases beyond These modification factors are for all crashes and not for 30 ft; and (3) the effect of median width on total crashes is specific types of crashes that could relate to median width questionable (32). The section in the HSM on multilane rural issues. The recommended values are greater than those pro- roads proposed AMF values for rural multilane highways based posed in the HSM. This difference may be derived from the on whether a median barrier was present (22). These values fact that the HSM values specifically account for median- accounted for the total number of crashes while considering related crashes while determining all crashes. This level of median-related crashes. data refinement was not possible for the research reported This research distinguished between divided and undivided here, and an adjustment consistent with the HSM could highways as well as between single- and multi-vehicle crashes. affect the values recommended in Table 17. Another possible The effect of median width was only evaluated for divided relationship that could exist and could have an influence on highways. This classification allowed for the development of these values is the presence of a median barrier. Roadway two distinct models to address the particular issues relative segments with a barrier have typically narrower medians; to crash types. Aggregate models were also developed for all this could influence the AMFs as shown in the HSM values. crashes to allow for a comprehensive approach and the deter- However, the available data were not large enough to examine mination of the overall effects of the median barrier presence. this interaction. The only model where median width was significant was for To determine the AMFs for all crashes, one could assume multi-vehicle crashes, and it had a positive effect (i.e., crashes that the median width has "no effect" on single-vehicle crashes are reduced with wider medians). This trend is supported by Table 18. Recommended AMFs for median width, divided roadways. Median width (ft) Category 10 20 30 40 50 60 70 80 Multi-vehicle 1.00 0.91 0.83 0.75 0.68 0.62 0.57 0.51