Click for next page ( 13


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 12
12 Table 3. AMF for shoulder width Table 5. AMFs for paved shoulder for multilane highways with width (27). ADT > 2500 vehicles/day (21). Paved shoulder width (ft) Paved shoulder width (ft; one side) 0 2 4 6 8 3 4 5 6 7 8 1.18 1.11 1.05 1.00 0.95 1.0 0.97 0.95 0.93 0.91 0.90 two-lane rural roads, so the AMFs could remain the same. AMFs for shoulder width for rural multilane segments. These The proposed AMFs are presented in Table 3. AMFs are for paved shoulders and also include the Harkey Further research interest has been placed on shoulder et al. AMFs for undivided highways (see Table 6). type, which can impact crashes and therefore roadway safety. In general, the literature is silent on the relationship Again, the focus of work on this topic has concentrated on the between shoulder and safety for multilane rural roads with two-lane, two-way roads: almost no research has been directed the exception of the new HSM work. As was the case for the to multilane roads. Rogness et al. (31) used before-and-after lane width, there is no literature that documents the effect crash-rate changes from converting two-lane rural roads with of shoulder width and type on the safety of a roadway seg- full shoulders to four-lane undivided rural roads without ment. Moreover, the new AMFs developed for the HSM are shoulders. The results indicated that for roads with volumes in based mainly on an expert-panel approach and on the the 1,0003,000 vehicles/day range, crashes increased after the Harkey et al. work that is itself derived from Zegeer's work conversion. It should be noted here that the study used Texas (12, 13). roadways where, the report indicates, driving on the shoulder on two-lane rural roads is considered acceptable. This fact could Medians impact the findings of their study and therefore not provide any additional understanding of this shoulder-crash relationship. The most important objective for the presence of medians Harwood et al. (32) developed AMFs for the conversion is traffic separation. Additional benefits from medians include of shoulder types on rural two-lane roads. An expert panel the provision of recovery areas for errant maneuvers, accom- reviewed these factors and determined that they are appropriate modation of left-turn movements, and the provision for for use in both divided and undivided multilane roadways. emergency stopping. Median design issues typically address These estimates, shown in Table 4, were for converting turf the presence of median, along with its type and width. There or gravel shoulders to paved shoulders and turf shoulders to has been some research completed on these issues and their composite (partially paved) shoulders. implications on safety. Harkey et al. (27) also developed AMFs for rural multilane Hauer (33) conducted a review of studies that investi- roadways as part of a study that evaluated traffic engineering gated the effect of medians on rural multilane highway safety and ITS improvements (see Table 5). The study considered levels. This review, which was based on a few studies, did undivided roads with more than 2,000 vehicles per day, and not provide conclusive results on the effectiveness of the the AMFs developed were for roadways where the shoulder presence of medians on safety but did identify the potential related crashes were 35% of the total. Additional procedures for the median to impact safety. One of these studies (34) are available for roadways with lower volumes or different examined divided and undivided four-lane rural roadways percentages. in the context of the safety differences between two-lane and For divided highways, the draft HSM uses recommended four-lane roadways. The study concluded that the presence values from NCHRP Project 17-29 (22), which developed of a median had an effect on crashes that was related to the Table 4. AMFs for shoulder conversion for multilane roadways based on two-lane roads (21). Shoulder width (ft; one side) Treatment 3 4 5 6 7 8 Convert turf to paved 0.99 0.98 0.97 0.97 0.97 0.96 Convert gravel to paved 1.00 1.00 1.00 0.99 0.99 0.99 Convert turf to composite 1.00 0.99 0.98 0.97 0.98 0.98

OCR for page 12
13 Table 6. AMFs for paved shoulder width (22). (28) and Harkey et al. (27), and they account for the total number of crashes while considering median-related crashes. Paved shoulder width (ft) The recommended values are summarized in Table 8. Roadway 0 2 4 6 8 Median type has also been examined as it relates to roadway Undivided 1.18 1.11 1.05 1.00 0.95 safety. A meta-analysis of several studies conducted by Elvik Divided 1.18 1.13 1.09 1.04 1.00 and Vaa (36) suggests there is an effect due to the type of median used. Their analysis examined the relative effects of concrete, steel, and cable guardrail installations on multi- roadway volume (crashes for roads with medians as com- lane divided highways. The results indicate that the AMF for pared with roads without medians exhibited the relation- injury crashes for concrete barriers is 1.15, for steel barriers is ship 0.76 ADT-0.05)1. 0.65, and for cable is 0.71. The resulting AMF for all crashes Another study examined the effect of the median presence in for median guardrails is 1.24, indicating that the presence of Oregon and also reported crash reductions from the presence a median guardrail--and especially a concrete guardrail-- of medians (35). The study found that the AMF for median has the potential to increase crashes. Thus, designers must presence is 0.431, showing an agreement with the results of carefully consider whether the placement of a median barrier Council and Stewart (34), but a larger magnitude for its effect. will have an overall positive or negative influence on the safety Elvik and Vaa (36) also showed a similar finding with separate of a particular roadway segment. A barrier will result in a models for injury and property damage crashes in a meta- reduction of cross-median type crashes, but it also has the analysis of several studies where a median was added. Their potential to increase median-related crashes since its absence AMFs were 0.881 for injury and 0.821 for property damage could allow drivers opportunities to stop their vehicles in crashes. The interim report for NCHRP Project 17-27 recom- the median (37). As Hauer states: "The net effect of placing mended an AMF for the presence of median in the range of a barrier in the median is an increase in total accidents; an 0.85 to 0.50 (21). increase in injury accidents and its effect on the total number The contribution of width to the median effect has also of fatal accidents is at present unclear" (33). been examined. Hauer (33) found that it was not possible to Fitzpatrick et al. (38) developed AMFs for median barriers identify AMFs for median width but rather noted three safety on freeways and four- and six-lane rural highways in Texas. trends: (1) cross-median crashes (i.e., opposing vehicles) For rural highways the influence of the median barrier was are reduced with wider medians; (2) median-related crashes examined as a function of the available left shoulder width. increase as the median width increases, with a peak at about The study concluded that for roads with a barrier, increasing 30 ft, and then decrease as the median becomes wider than the left shoulder width by 1 ft will result in a 1.6% reduction 30 ft; and (3) the effect of median width on total crashes is of crashes for both four- and six-lane highways. questionable. Hadi et al. (25) used negative binomial models Other studies have demonstrated that the addition of to show that the median width has an influence on multilane a barrier could contribute to crash occurrence. Elvik (39) roadways; these authors produced two models based on the analyzed the results of 32 studies that examined the effect traffic volume range and number of lanes. This is the only of median barrier presence. His major conclusion was that study that examined the effect of median width on safety for " . . . the best current estimates of the effects of median bar- rural, multilane roads because the several studies reviewed by riers are a 30% increase in accident rate, a 20% reduction Hauer (33) and the NCHRP Project 17-27 interim report (21) in the chance of sustaining a fatal injury, given an accident, deal with freeway median width. and a 10% reduction in the chance of sustaining a personal Table 7, which is taken from the interim report for NCHRP injury, given an accident." These findings indicate that, in Project 17-27, presents a set of AMFs for the effect of median general, crashes can increase, but their severity may decrease. width on crashes for four-lane rural roadways; these values Miaou et al. also noted that crash rates are higher on roadways are based on one study. with median barriers when compared with roads without The HSM section on multilane rural roads developed as part them and that median barriers present a higher likelihood of NCHRP Project 17-29 (22) also proposes AMF values for of vehicle impact (28). rural multilane highways (see Table 5 in HSM). Two sets of A median-type treatment that may be used on multilane values were developed based on whether a median barrier was rural roads is a two-way left-turn lane (TWLTL). This median present. These values are based on the studies of Miaou et al. type is typically found on rural roads where some develop- ment may be present or anticipated. Such a median treat- ment is often associated with specific types of crashes that 1The values presented here are those stated in the NCHRP Project 17-27 are access related, that is, left turns in and out of an access interim report (21), and they have been adjusted from the original studies. point. An issue of concern in estimating safety impacts from