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11 The review conducted by Hauer (24) of studies that attempted Table 2. AMFs for lane width (22). to model the effect of lane width on multilane rural highway crashes found no correlation. The same review indicated that Lane width (ft) there was only one study where lane width was included in Roadway 9 10 11 12 the models (25), but these were for freeway facilities. An AMF Undivided 1.13 1.08 1.02 1.00 represents the anticipated change in safety when a particular Divided 1.09 1.05 1.01 1.00 geometric design element value changes in size. An AMF greater than 1.0 represents the situation where the design change is associated with more crashes; an AMF less than 1.0 indicates Most available research has examined this relationship fewer crashes. Typically, AMFs are estimated directly from the for urban roadways, and some relationship has been found coefficients of models derived using crash data or expert panels between the lane width and crashes for these roadways. How- that review current literature and determine the magnitude ever, these relationships are not applicable for the roadways of the AMF. Estimation of AMFs from models assumes that considered in this research project (which examines multi- (1) each AMF is independent since the model parameters are lane rural roads only) and therefore are not discussed further. assumed independent and (2) the change in crash frequency In summary, there is limited past research documenting any is exponential. In practice, AMFs may not be completely effects of lane width on crashes for multilane rural roads. The independent since changes in geometric design characteris- only study with definitive factors is the new HSM work that tics on highways are not done independently (e.g., lane and is based on an expert-panel approach. shoulder width may be changed simultaneously) and the com- bination of these changes can influence crash risk. Nonetheless, Shoulders experience in deriving AMFs in this manner indicates that the assumptions are reasonable and, with thoughtful model Shoulders placed adjacent to travel lanes accomplish several development, the resulting AMFs can yield useful information functions including emergency stop and pull off, recovery about the first-order effect of a given variable on safety. area for driver error, and pavement edge support (1). The use A study by Harwood et al. (26) examined AMFs as part of of shoulders to provide an area where a vehicle could stop resurfacing, restoration, and rehabilitation (3R) projects. poses an additional hazard since past research has shown that An expert panel adjusted the AMFs developed for two-lane, 11% of fatal freeway crashes are related to vehicles stopped on two-way rural roads to allow for their use in multilane roads, shoulders (29). There is also evidence that wider shoulders specifically four-lane roads. The factors show no effect for 11-ft may encourage higher operating speeds because they may lanes and an 8% to 11% increase for 9-ft lanes. These AMFs communicate to the driver the presence of wider space for are summarized in Table 1. correcting errors. Finally, the number of lanes, lane width, The section of the HSM on multilane rural roads devel- and shoulder width are interrelated, and the choice of geo- oped as part of NCHRP Project 17-27 (21) also proposed metric value for each of these elements typically affects the AMF values for lane width on rural multilane highways (see other elements. Table 2) based on the work of Harwood et al. (26) and Harkey Most of the research completed to date focuses on two-lane, et al. (27) through the deliberations of the joint NCHRP two-way rural roads (30). An additional problem is that most Projects 17-25/17-29 Expert Panel Meeting. Two sets of values of the recent studies have analyzed urban or suburban multi- were developed from the studies of Miaou et al. (28) and lane highways (rather than rural roads), resulting in an even Harkey et al. (27), based on whether the roadway was divided smaller number of available references for this design element. in the presence of a median barrier. These values accounted Hadi et al. (25) examined the effect of shoulder width on for the total number of crashes while considering median- crashes on multilane rural highways. Their findings indicated related crashes. The recommended values were adjusted from that for four-lane rural divided roads, a small reduction in the normal baseline of 30-ft median presented in the report. crashes (1% to 3%) can be attained if the unpaved shoulder is widened by 1 ft. The authors also indicate that the roads with shoulder widths between 10 ft and 12 ft have the lowest Table 1. AMFs for lane width for four-lane crash rates. However, this relationship is present only for highways (21). unpaved shoulders, and the reduction factor should be used cautiously. Lane width (ft) Harwood et al. (26) also produced AMFs for multilane 9 10 11 12 highways, again using an expert panel to adjust the AMFs of Four-lane undivided 1.11 1.06 1.00 0.99 two-lane rural roads. In this instance, the panel determined Four lane divided 1.08 1.04 1.00 0.99 that the effect of shoulder width is similar for both multi- and