National Academies Press: OpenBook

Impact of Shoulder Width and Median Width on Safety (2009)

Chapter: Chapter 4 - Design Elements Recommendations

« Previous: Chapter 3 - Data Analysis
Page 27
Suggested Citation:"Chapter 4 - Design Elements Recommendations." National Academies of Sciences, Engineering, and Medicine. 2009. Impact of Shoulder Width and Median Width on Safety. Washington, DC: The National Academies Press. doi: 10.17226/14252.
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Suggested Citation:"Chapter 4 - Design Elements Recommendations." National Academies of Sciences, Engineering, and Medicine. 2009. Impact of Shoulder Width and Median Width on Safety. Washington, DC: The National Academies Press. doi: 10.17226/14252.
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Page 28
Page 29
Suggested Citation:"Chapter 4 - Design Elements Recommendations." National Academies of Sciences, Engineering, and Medicine. 2009. Impact of Shoulder Width and Median Width on Safety. Washington, DC: The National Academies Press. doi: 10.17226/14252.
×
Page 29
Page 30
Suggested Citation:"Chapter 4 - Design Elements Recommendations." National Academies of Sciences, Engineering, and Medicine. 2009. Impact of Shoulder Width and Median Width on Safety. Washington, DC: The National Academies Press. doi: 10.17226/14252.
×
Page 30
Page 31
Suggested Citation:"Chapter 4 - Design Elements Recommendations." National Academies of Sciences, Engineering, and Medicine. 2009. Impact of Shoulder Width and Median Width on Safety. Washington, DC: The National Academies Press. doi: 10.17226/14252.
×
Page 31

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27 Chapter 3 described the development of prediction equa- tions from which a set of recommended AMFs could be deter- mined for practicing engineers to use when evaluating safety trade-offs for selected design elements. In this chapter, each of the design elements that were found as statistically signifi- cant in the prediction models is discussed with a view toward developing these practical recommendations. A general dis- cussion of the literature is first presented (review Chapter 2 for more detail). The values from the models developed in this research are then presented with a brief discussion demon- strating the appropriate AMF values. Finally, a recommended set of values is presented along with discussion of the justifi- cation for the proposed AMFs. The justification and associated discussion is considered essential, especially for cases in which the results seemed to be inconclusive. The first step in establishing the proposed recommended values for each design element was the circulation of a draft set of recommendations among project team members. The team then met to discuss (1) the proposed values, (2) the justification of the recommendations, and (3) the identification of any issues that might result in diminishing the practicality of the proposed values. The team meeting represented an expert panel since it included three safety engineers, two highway designers, and a highway safety analyst (see Table 15). The team debated the values presented, discussed the existing work (both past and that of NCHRP Project 15-27), and prepared a recommended set of AMF values through a consensus-building process. The team determined that the current recommendations for shoulder width and median width were reasonable (i.e., the values were similar to those in past research, and any differences in magnitude could be explained). The team was not able to make a final recommendation for median barrier presence because it was not included in all models for divided highways. Finally, the AMFs developed for the presence of paved right shoulders and left-turn lanes produced counterintuitive results, and the research team concluded that neither AMF should be included as a design element with a guideline. Average Shoulder Width Recommendation The research team reviewed past literature, the recom- mended values for the HSM, and the AMFs from NCHRP Project 15-27 and agreed that there is an influence on crash occurrence from the presence of shoulders. Using this back- ground information, the team determined that the values noted for all crashes for undivided highways are reasonable and in accordance with current rends and literature. The team fur- ther recommended the use of only the AMF for all crashes for undivided highways since the shoulder width was not a sig- nificant variable in the single-vehicle models. The team considered the values provided for all three models for divided highways, and it recommended using the values from the single-vehicle crashes as those for divided roadways. The team determined that the values for multi-vehicles and all crashes were high and probably reflective of other influences such as volume. This adjustment is considered justifiable based on previous work by Harwood et al. (26) and the recommended values in the HSM (22). The recommended values are sum- marized in Table 16. The modification factors in Table 16 are for all crashes and not for specific types of crashes that could relate to shoulder width issues. The recommended values are similar to those proposed in the HSM as noted above, and those of the divided highways are comparable for almost all categories with the only exception being that of the 8-ft shoulder AMF. For undivided highways, the differences between the NCHRP Project 15-27 and the HSM-recommended AMFs were larger. These dif- ferences are attributed to the fact that the HSM factors were developed for shoulder-related crashes while the AMFs for NCHRP Project 15-27 were developed for all crashes. Even though a comparison with the HSM values is not wholly appropriate due to the difference in crash types used in each model, the comparison is supported by the observed similar- ities in trends and agreement of findings. Future research C H A P T E R 4 Design Elements Recommendations

should address the lack of AMFs for shoulder width greater than 8 ft since the literature indicates that the safety effects for such shoulder widths are unknown. Supportive Background In general, shoulder width has an influence on crashes, with increasing shoulder width having a positive (i.e., reducing) effect on crashes. There is also some evidence that wider shoulders may encourage higher operating speeds since they may communicate to the driver the presence of a wider space for correcting errors. Finally, number of lanes, lane width, and shoulder width are all interrelated to some degree, and the geometric value choice for each of these elements typically has an effect on the other elements. Most research completed to date focused on two-lane, two-way rural roads. An additional problem is that most recent studies have analyzed urban or suburban multilane highways (rather than rural roads), result- ing in an even smaller number of available references for this design element. Two recent studies examined the effect of shoulder width on crashes (22, 27). Both studies focused on paved shoulders and determined AMFs for shoulder-related crashes and for divided and undivided roadways. The models developed based on the data from this research demonstrated that there is a relationship between shoulder width and crashes. The general trends observed from previous studies as well as those for two-lane, two-way rural roads were also supported by the models developed. The current study distinguished between divided and undivided highways as well as between single- and multi-vehicle crashes. This classi- fication permitted development of four distinct models to address issues particular to crash types and the influence of the presence of the median. Aggregate models were also developed for all crashes to allow for a comprehensive approach and 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 (i.e., the sum of right and left shoulders divided by two) in the same direction. For undivided four-lane highways, the shoulder width was a significant predictive variable for multi-vehicle and all crashes. The coefficient in the model for multi-vehicle crashes is −0.11 (1− exp(− 0.11) = 0.10) and for all crashes is −0.07 (1− exp(− 0.07) = 0.07). The negative sign indicates the ben- eficial influence of the shoulder width. These values could be used as an indication of the relative safety gains from the increase of the shoulder by 1 ft. However, their magnitude seems relatively high, and it is likely that such large reductions may not be feasible. For divided highways, the shoulder width was included in all three models. The coefficients were −0.05 (1− exp(− 0.05) = 0.05) for single–vehicle; −0.14(1− exp(− 0.14) = 0.13) for multi-vehicle; and −0.12 (1− exp(− 0.12) = 0.11) for all crashes. Again, the negative sign demonstrates the reduction of crashes associated with an increase in shoulder width. The magnitude of the coefficients for the multi-vehicle and all crashes again seems high. The similar analysis for injury-only crashes did not produce any significant changes in the coefficients noted here. The variable was significant only for divided highways, and the coefficients were practically the same as those noted for all crashes. The AMFs for each condition obtained from the models developed in this research are summarized in Table 17. 28 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 Table 15. Team “expert” panel. 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. Table 16. Recommend AMFs for average shoulder width (ft).1

Median Width Recommendation The research team reviewed past literature, the recommended values for the HSM, and the AMFs from NCHRP Project 15-27 and agreed that there is an influence on crash occurrence from the median width. The team determined from available background data that the values noted for the only model with median width influence are reasonable and in accor- dance with current trends and literature. The only available AMF based on the models developed in this research is for multi-vehicle crashes, and it yields a 1% reduction for every added foot of median width. The values obtained from the models for multi-vehicle crashes are reasonable and agree with the previous research. The recommended values are summa- rized in Table 18. These modification factors are for all crashes and not for specific types of crashes that could relate to median width issues. The recommended values are greater than those pro- posed in the HSM. This difference may be derived from the fact that the HSM values specifically account for median- related crashes while determining all crashes. This level of data refinement was not possible for the research reported here, and an adjustment consistent with the HSM could affect the values recommended in Table 17. Another possible relationship that could exist and could have an influence on these values is the presence of a median barrier. Roadway segments with a barrier have typically narrower medians; this could influence the AMFs as shown in the HSM values. However, the available data were not large enough to examine this interaction. To determine the AMFs for all crashes, one could assume that the median width has “no effect” on single-vehicle crashes and, therefore, the AMF for single-vehicle crashes could be considered as 1.00. In this case, a weighted AMF can be esti- mated using as weights the relative percentages of single- and multi-vehicle crashes for the roadway of concern. Supportive Background The key objective for the presence of medians is traffic separation. Median design issues typically address the pres- ence of medians, along with type and width. There has been some research completed on these issues and their implications on safety. However, past research indicated three safety trends: (1) cross median crashes (i.e., between opposing vehicles) are reduced with wider medians; (2) median-related crashes increase as the median width increases with a peak at about 30 ft and then decrease as the median width increases beyond 30 ft; and (3) the effect of median width on total crashes is questionable (32). The section in the HSM on multilane rural roads proposed AMF values for rural multilane highways based on whether a median barrier was present (22). These values accounted for the total number of crashes while considering median-related crashes. This research distinguished between divided and undivided highways as well as between single- and multi-vehicle crashes. The effect of median width was only evaluated for divided highways. This classification allowed for the development of two distinct models to address the particular issues relative to crash types. Aggregate models were also developed for all crashes to allow for a comprehensive approach and the deter- mination of the overall effects of the median barrier presence. The only model where median width was significant was for multi-vehicle crashes, and it had a positive effect (i.e., crashes are reduced with wider medians). This trend is supported by 29 Average shoulder width (ft)1 Category 0 3 4 5 6 7 8 10 Undivided, multi-vehicle 1.39 1.00 0.90 0.80 0.72 0.64 0.58 0.46 Undivided, all crashes 1.22 1.00 0.94 0.87 0.82 0.76 0.71 0.63 Divided, single-vehicle 1.17 1.00 0.95 0.90 0.85 0.81 0.77 0.69 Divided, multi-vehicle 1.51 1.00 0.87 0.76 0.66 0.58 0.50 0.38 Divided, all crashes 1.43 1.00 0.89 0.79 0.70 0.62 0.55 0.44 1 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. Table 17. AMFs based on prediction models for average shoulder width. 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 Table 18. Recommended AMFs for median width, divided roadways.

the general observation that roadways with wider medians will exhibit lower crash rates than will roads with narrower medians. The model developed showed that the coefficient was −0.010 (1−exp(−0.010) = 0.01). The analysis of the injury-only crashes included this variable again only in multi-vehicle crash models with a similar coefficient (−0.009). The AMFs developed for median width based on the model developed are summarized in Table 19. Median Barrier Recommendation The research team reviewed past literature, the recommended values in the HSM, and the AMFs from NCHRP Project 15-27 and agreed that there is an influence on crash occurrence from the presence of median barrier. However, the values obtained from this research are based on a small sample (200 segments, less than 5% of the data) and therefore no recommendations were made. The research team also determined that there are several other factors that may also be influential such as barrier type (which was not available for this study), volumes, use of barriers (presumably roads with higher ADT and narrower median are likely to have barriers), and distance between barrier and travel lanes (potential for avoiding colliding with barrier). Therefore, a properly supported recommendation is not possible. It should be noted that although no recommendation is made for this design element, other factors should be consid- ered in determining the impact of the median barrier presence. Median barriers are typically placed to reduce crossover crashes. As such, cross-sectional studies (i.e., studies that compare segments with and without median barriers) may not be best suited for this evaluation. Before and after studies may be more appropriate since they generally compare the same roadway environment and population of users and allow for a better estimate of the effect of changes. The increase in crashes noted in the models in this research is also considered reasonable if one considers that the median barrier is an obstacle within the roadway environment and, as such, the potential for more crashes exists. For roadways with median barriers, one can assume that an errant vehicle will not simply rest in the median avoiding a crash but rather will hit the median, resulting in a crash. Other issues that were not examined and could have an influence are the placement of the median barrier and its dis- tance from the travel lanes. These both could have a positive influence in avoiding the obstacle and, thus, not resulting in a crash. Finally, the severity and type of the crash with and without the median barrier should be also considered. Median barriers have the potential to reduce crossover crashes, which often result in serious injuries or fatalities. Therefore, the pres- ence of the barrier has the potential to impact severity levels. Supportive Background The literature review has identified conflicting results for the presence of median barriers. Some have noted that the effectiveness of the presence of medians on safety cannot be conclusively identified but noted that there is potential for the median to impact safety (33). Others have shown that median barriers have a positive effect—they reduce crashes (34)— and others have indicated that there is a relationship between median barrier presence and left shoulder width (38). Another trend noted in the literature is the overall increase in number of crashes with median presence but a reduction of the level of severity for these crashes (39). In general, the fact that an obstacle is placed within the roadway environment that pro- vides a target for collisions can lead to an increased number of crashes. The type of median barrier is also important: studies have shown that different types (especially concrete) have the potential to increase crashes (36). The issue to be considered here is whether the placement of a median barrier will act positively or negatively on the safety of the roadway segment considered. The presence of a barrier will result in a reduction of cross-median type crashes, but it also has the potential to increase median-related crashes since its absence may allow drivers opportunities to stop their vehicles in the median. The models developed in this research identified that the presence of median barrier had an effect on crashes for divided highways. As noted above, the values obtained here are based on a small sample (200 segments, less than 5% of the data) and should be viewed cautiously. This research distinguished between divided and undivided highways as well as between single- and multi-vehicle crashes. This classification allowed for the development of two distinct models to address the partic- ular issues relative to crash types. Aggregate models were also developed for all crashes to allow for a comprehensive approach and the determination of potential overall effects of median barrier presence. 30 Median width (ft) Category 10 20 30 40 50 60 70 80 Multi-vehicle AMF 1.00 0.91 0.83 0.75 0.68 0.62 0.57 0.51 Table 19. AMFs for median width on divided roadways.

For all three models (single-vehicle, multi-vehicle, and all crashes), the presence of median barrier had a negative effect (i.e., crashes increased). This trend is supported by the general observation that roadways with median barriers exhibit higher crash rates than do roads without them. The models developed in this research yielded coefficients of 0.999 (1−exp(0.999) = 1.71) for single-vehicle; 0.523 (1−exp(0.523) = 0.69) for multi- vehicle; and 0.781 (1−exp(0.781) = 1.18) for all crashes. The analysis of the injury-only crashes included this variable only in the single-vehicle and all-crashes models with similar trends and magnitudes. The AMFs developed for each condition from the models developed in this research are summarized in Table 20. Applications These AMFs can be used to estimate the relative impact of the choice of the value of a design element for a rural four- lane roadway segment. The process described herein can also be applied to determine the safety implications using differ- 31 Category AMF Single-vehicle 2.71 Multi-vehicle 1.69 All crashes 2.18 Table 20. AMFs for median barrier presence, divided roadways. ent values for a single or combination of design elements. The ratio of AMFs for two different conditions can be used to establish the relative change in crashes anticipated from the change in the values of the design element. The use of this approach was noted as a method for estimating change in crashes by using where ΔN is the change in crashes and AMFi are the AMFs for the designs to be evaluated. This equation was modified from the form presented by Lord and Bonneson since no base models or base estimates are available in the method presented here (49). A positive value of ΔN denotes an increase in crash frequency. The following example demonstrates the use of the AMFs for estimating the safety implications from design choices: An agency is evaluating the effects of widening the shoulder of a four-lane undivided highway from 4 ft to 8 ft. The AMFs for divided roads obtained from Table 17 are 0.94 for 4-ft shoulders and 0.71 for 8-ft. Using Equation 15, the expected crash change will be Therefore, increasing shoulder width from 4 ft to 8 ft will result in a 24% reduction in crashes per year per mile. ΔN = − = −0 71 0 94 1 0 24 . . . ΔN AMF AMF = − 1 2 1 15( )

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 633: Impact of Shoulder Width and Median Width on Safety explores crash prediction models and accident modification factors for shoulder width and median width on rural four-lane roads.

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