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25 Figure 1. AMFs for average shoulder width. Figure 3. AMF for undivided roadways, multi-vehicle crashes. Multi-Vehicle Crashes Undivided Highways Single value AMFs were obtained for two variables: median barrier and left turn lane. The AMF for median barrier was Single-Vehicle Crashes 1.69, indicating that median barriers increase crash potential by 69% in comparison to roads without barriers. The AMF A single value AMF was obtained for only one variable, paved for left turn lanes was 1.57, indicating that the presence of shoulders. The AMF for this is 1.46, indicating a 46% increase left turn lanes increases crash potential by 57% in compar- for roads with paved shoulders in comparison to roads without ison to roads without one. Two additional continuous vari- paved shoulders. ables entered the model; the AMFs for these are shown in Figures 1 and 2. Multi-Vehicle Crashes A paved right shoulder was a predictor variable in this case; All Crashes the AMF was 0.62 for paved shoulders, indicating a 38% reduction in comparison to unpaved shoulders. An AMF for Single value AMFs were obtained for three variables: paved the continuous variable shoulder width, the effect of this right shoulder, median barrier, and functional classification. variable can be estimated from Figure 3. As used here, the The AMF for the paved right shoulder was 1.26 indicating shoulder width is the average width for both the left and right that paved shoulders increase crashes by 26% in comparison shoulders. to unpaved shoulders. For median barrier, the AMF was 2.18, indicating that median barriers increase crash potential by 118% in comparison to roads without barriers. The AMF for All Crashes functional classification was 1.19, indicating that arterials The only significant variable was the shoulder width for increase crash potential by 19% in comparison to other roads. which the effect can be estimated from Figure 3. An additional continuous variable also entered the model; this AMF is shown in Figure 1. Injury Models In addition to all crashes, models were developed for injury- only crashes. These models followed the same data grouping as the all crashes (i.e., data were split into divided and undi- vided highways and single-, multi-, and all vehicle crashes). For undivided roadways, no variable was significant enough to be entered in the model other than the ADT. This indicates that none of the variables of concern had any significant influence on injury-only crashes on undivided roads. For divided roads, the models were very similar to those observed for all crashes. For the single-vehicle crashes, pres- ence of median barrier, functional class, shoulder width Figure 2. AMFs for median width. and presence of left turn lane had impacts. Most of these