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32 Table 16 Recommended CMFs for Average Median Width on Divided Roadways Average 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 Source: Stamatiadis et al. 2009. assumption that median width had no effect on single-vehicle vious studies on encroachment data: Hutchinson and Ken- crashes, so their recommended CMFs for average median nedy (1966), Cooper (1980), and Calcote et al. (1985). The width, shown in Table 16, are for multi-vehicle crashes. Rec- Hutchinson and Kennedy study involved observation of ommendations for shoulder width CMFs are provided in the wheel tracks on medians of rural Illinois Interstate highways section on shoulder width elsewhere in this chapter. in the mid-1960s. Cooper conducted a similar encroachment study in Canada in the late 1970s. This research involved Tarko et al. (2007) investigated the impact of median weekly observations of wheel tracks on grass-covered road- designs on crash frequency. They analyzed data collected sides of rural highways of various functional classes. The data in eight participating states using negative binomial regres- collection periods were during summer months on highways sion and before-and-after studies, and they examined crash with speed limits between 80 and 100 km/h. Calcote et al. severity using a logit model. The results of their analyses attempted to overcome the major problems with both the quantified the separate effects of changes in median geom- Cooper and the Hutchinson and Kennedy studies, but they etry for single-vehicle, multiple-vehicle same-direction, and "still did not offer an effective method to distinguish between multiple-vehicle opposite-direction crashes. They concluded controlled and uncontrolled encroachments." that results were significantly different for the various classes of crash types, indicating that reducing the median width An overwhelming majority of the encroachments recorded without adding barriers (even if the remaining median width involved vehicles "moving slowly off the roadway for some is still reasonably wide) increases the severity of crashes, distance and then returning into the traffic stream without any particularly opposite-direction crashes. Further, they found sudden changes in trajectory," which could be caused by "a that reducing the median width and installing concrete bar- fatigued or distracted driver drifting off the roadway, or a con- riers eliminated opposite-direction crashes but doubled the trolled driver responding to roadway or traffic conditions." frequency of single-vehicle crashes, increased crash severity, and tended to lessen the frequency of same-direction crashes. Roadside Horizontal Clearance The developers of the Roadside Safety Analysis Program included encroachment frequency curves (shown in Fig- ure 6) and adjustment factors to increase encroachment rates on horizontal curves and vertical grades (shown in Figure 7) (Mak and Sicking 2003). The developers found three pre- FIGURE 6 Encroachment rates used in Roadside Safety FIGURE 7 Encroachment frequency adjustment factors for Analysis Program (Mak and Sicking 2003). curvature (Wright and Robertson 1976).