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28 Table 15. Team "expert" panel. Name and Agency Expertise Ken Agent, University of Kentucky Safety engineer Dominique Lord, Texas Transportation Institute Highway safety analyst Jerry Pigman, University of Kentucky Safety engineer Wendel Ruff, ABMB Engineers, Inc. Design engineer John Sacksteder, HMB Professional Engineers, Inc. Design engineer Nikiforos Stamatiadis, University of Kentucky Safety engineer should address the lack of AMFs for shoulder width greater presence of the median. Aggregate models were also developed than 8 ft since the literature indicates that the safety effects for for all crashes to allow for a comprehensive approach and such shoulder widths are unknown. potential determination of the overall effects of the shoulder width. It should be noted that the shoulder width used here is the average total width for the left and right shoulders Supportive Background (i.e., the sum of right and left shoulders divided by two) in In general, shoulder width has an influence on crashes, with the same direction. increasing shoulder width having a positive (i.e., reducing) For undivided four-lane highways, the shoulder width effect on crashes. There is also some evidence that wider was a significant predictive variable for multi-vehicle and all shoulders may encourage higher operating speeds since they crashes. The coefficient in the model for multi-vehicle crashes may communicate to the driver the presence of a wider space is -0.11 (1- exp(- 0.11) = 0.10) and for all crashes is -0.07 for correcting errors. Finally, number of lanes, lane width, and (1- exp(- 0.07) = 0.07). The negative sign indicates the ben- shoulder width are all interrelated to some degree, and the eficial influence of the shoulder width. These values could geometric value choice for each of these elements typically be used as an indication of the relative safety gains from the has an effect on the other elements. Most research completed increase of the shoulder by 1 ft. However, their magnitude to date focused on two-lane, two-way rural roads. An additional seems relatively high, and it is likely that such large reductions problem is that most recent studies have analyzed urban or may not be feasible. suburban multilane highways (rather than rural roads), result- For divided highways, the shoulder width was included in ing in an even smaller number of available references for this all three models. The coefficients were -0.05 (1- exp(- 0.05) = design element. Two recent studies examined the effect of 0.05) for singlevehicle; -0.14(1- exp(- 0.14) = 0.13) for shoulder width on crashes (22, 27). Both studies focused on multi-vehicle; and -0.12 (1- exp(- 0.12) = 0.11) for all crashes. paved shoulders and determined AMFs for shoulder-related Again, the negative sign demonstrates the reduction of crashes crashes and for divided and undivided roadways. associated with an increase in shoulder width. The magnitude The models developed based on the data from this research of the coefficients for the multi-vehicle and all crashes again demonstrated that there is a relationship between shoulder seems high. width and crashes. The general trends observed from previous The similar analysis for injury-only crashes did not studies as well as those for two-lane, two-way rural roads were produce any significant changes in the coefficients noted also supported by the models developed. The current study here. The variable was significant only for divided highways, distinguished between divided and undivided highways as and the coefficients were practically the same as those noted well as between single- and multi-vehicle crashes. This classi- for all crashes. The AMFs for each condition obtained from fication permitted development of four distinct models to the models developed in this research are summarized in address issues particular to crash types and the influence of the Table 17. Table 16. Recommend AMFs for average shoulder width (ft).1 Average shoulder width (ft)2 Category 0 3 4 5 6 7 8 Undivided 1.22 1.00 0.94 0.87 0.82 0.76 0.71 Divided 1.17 1.00 0.95 0.90 0.85 0.81 0.77 1 The AMFs are for all crashes and all severities. 2 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.