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32 CHAPTER 5 Conclusions and Suggested Research Conclusions use. The effect of median width on crashes is approximately an 8% reduction with every 10-ft increase in median width. This research aimed to develop a set of recommendations An AMF for all crashes could be developed by assuming that for evaluating the safety implications of selected design element the AMF for single-vehicles is 1.00 and estimating a weighted trade-offs. The research team used an expert-panel approach average using the percentages of single- and multi-vehicle where prior research was reviewed and discussed along with crashes as weights. the models developed herein. The team discussed and com- pared past work with that completed here and recommended a set of AMFs that could be used in determining the safety Suggested Research effects from the change in the values of a design element. This research identified the following areas in which addi- Their final recommendations were for shoulder width and tional research is needed to address areas where the available median width for four-lane roads with 12-ft lanes. The available data are too limited to support meaningful conclusions: data did not allow for the development of additional recom- mendations even though the presence of median barrier was 1. The effect of median barrier was identified in this research also considered. The values recommended here are higher than and in the literature. However, the small number of seg- those proposed in the HSM mainly because they address all ments with barriers did not allow for evaluation of the crashes rather than only crashes related to the specific element. effects of barrier type or of the interaction between barrier This fact explains the larger magnitude of these AMFs since presence and barrier width or barrier proximity to the they capture the effect of a larger number of crashes. travel lanes. These issues should be addressed in the future Two sets of recommended AMFs for shoulder width were to determine the effectiveness of median barriers and to developed that could be used based on whether the roadway review existing guidelines for their placement. The literature is divided. Each set addresses the effect of the shoulder width suggests that barrier type can influence crashes; this is on the potential crash occurrence for the total number of another area of potential future work. crashes and represents the relative change from using the 2. An original goal of this research was to determine the specific value. Through the expert panel approach, the research effect of the number of lanes and lane width on crashes team concluded that these AMFs were appropriate and rea- for multilane rural highways. The available data did not sonable to use for estimating the effect of the shoulder width allow for estimating this effect. The effect of lane width has on crash occurrence. Increasing the shoulder width by 1 ft been documented in past research, and it was demonstrated for undivided highways effects an approximately 6% crash to have an effect on crashes. This is a design element that reduction, while for divided highways the reduction is 5%. could influence driver behavior and operating speeds and, These values are in accordance with past work and demonstrate therefore, additional attention should be paid to determin- the positive effect of shoulder width on crash occurrence. ing the safety implications from lane width trade-offs. A single set of AMFs is recommended for the median width, 3. The effects of paved shoulders and the presence of for multi-vehicle crashes for divided roadways, since this vari- left-turn lanes were identified in this research, but pro- able was only present in the model for multi-vehicle crashes. vided seemingly counter-intuitive results. The models The research team through its expert-panel approach deter- showed that crashes increase on segments with paved mined that this factor was reasonable and recommended its shoulders and left-turn lanes. These features are generally
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33 considered to be safety improvements; as such, additional to provide datasets that could be used in research to research is needed to determine their effectiveness and establish and evaluate nationwide trends, several variables to determine whether conditions exist where their presence are not common to all states. Further areas of concern may indeed contribute to crash occurrence. It is possi- are the differences in the level of detail provided by each ble that the presence of paved shoulders may encourage state and the inconsistencies in the coding within common higher speeds, while the presence of left-turn lanes may variables. For example, values of common variables are create an obstacle in the road that has an impact on spe- not coordinated; this often leads to aggregation of data to cific crash types. fewer detailed categories or even to binary (yes/no) values. 4. The research team discovered that there is a lack of Some effort is recommended to normalize these entries uniformity among the various state databases that are and to develop a truly uniform data set that will facilitate available in the HSIS. Although the HSIS was developed improved nationwide research evaluations.