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