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 79
79 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS This chapter presents the conclusions of the research and left turns represent 42 percent of the median opening recommendations for future work. movements. Based on these limited accident frequen- cies, there is no indication that U-turns at unsignalized median openings constitute a major safety concern. CONCLUSIONS · Average accident rates per median opening movement (U-turn plus left-turn maneuvers) have been estimated The conclusions of the research are as follows: for specific median opening types in both urban and · As medians are used more extensively on arterial high- rural arterial corridors. No satisfactory regression rela- ways, with direct left-turn access limited to selected tionships relating median opening accident frequency to locations, many arterial highways experience fewer mid- the volume of U-turn and left-turn maneuvers through block left-turn maneuvers and more U-turn maneuvers at the median opening could be developed. Given the low unsignalized median openings. accident frequencies, this is not surprising. · Field studies at various median opening types in urban · For urban arterial corridors, median opening accident arterial corridors have found estimated U-turn volumes rates are substantially lower for midblock median open- ranging from 2 to 977 veh/day, representing from 0.01 ings than for median openings at three- and four-leg to 3.2 percent of the major-road traffic volumes at those intersections. For example, the accident rate per mil- locations. At median openings in rural areas, U-turn vol- lion median opening movements (U-turn plus left-turn umes were found to range up to 374 veh/day, represent- maneuvers) at a directional midblock median opening is ing at most 1.4 percent of the major-road traffic volumes typically only about 14 percent of the median opening at those locations. accident rate for a directional median opening at a three- · Review of accident data for median openings at which leg intersection. U-turn maneuvers are made have found that accident · For urban arterial corridors, average median opening acci- report data do not distinguish clearly between accidents dent rates are slightly lower for conventional three-leg involving U-turn maneuvers and those involving left- median openings than for conventional four-leg median turn maneuvers. In particular, at some median openings openings where U-turn maneuvers can be made but no left-turn · For urban arterial corridors, average median opening maneuvers are feasible, investigating officers classified accident rates for directional three-leg median openings a substantial proportion of the accidents involving turn- are about 48 percent lower than the accident rates for ing movements through the median as related to left- conventional three-leg median openings. turn maneuvers. For this reason, reliable evaluation of · For urban arterial corridors, average median opening median opening accident frequency must necessarily accident rates for directional four-leg median openings consider the frequency of U-turn-related and left-turn- are about 15 percent lower than for conventional four- related accidents combined. leg intersections. · Accidents related to U-turn and left-turn maneuvers at · For rural arterial corridors, the average median open- unsignalized median openings occur very infrequently. ing accident rate is lower for median openings at three- The 103 median openings in urban arterial corridors eval- leg intersections than for median openings at four-leg uated in detail in this research experienced an average intersections. However, the sample size of median open- of 0.41 U-turn plus left-turn accidents per median open- ings and median-opening-related accidents for rural ing per year. The 12 median openings in rural arterial arterial corridors is so small that no firm conclusions can corridors evaluated in detail in this research experienced be drawn. an average of 0.20 accidents per median opening per · Where directional median openings are considered as year. Overall, at these median openings, U-turns repre- alternatives to conventional median openings, two or sent 58 percent of the median opening movements and more directional median openings are usually required