Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 81

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 80
80 to serve the same traffic movements as one conventional Unsignalized median openings may be used for a broad median opening. Therefore, design decisions should con- range of major- and minor-road traffic volumes. However, sider the relative safety and operational efficiency of all if the major- and minor-road volumes exceed the traffic directional median openings in comparison with the sin- volumes given in the MUTCD signalization warrants, sig- gle conventional median opening. nalization of the median opening should be considered. A comparison of the total median opening accident rates When evaluating the safety performance of unsignal- for the range of median opening types considered in the ized median openings, highway agencies should take into research is presented in Table 35. Comparable results for account both U-turn and left-turn maneuvers because fatal-and-injury and property-damage-only accident rates accident report data do not distinguish clearly between at median openings are presented in Tables 36 and 37. accidents involving U-turn maneuvers and those involv- Analysis of field data found that, for most types of median ing left-turn maneuvers. openings, most observed traffic conflicts involved major- Median widths at suburban unsignalized intersections road through vehicles having to brake for vehicles turn- generally should be as narrow as possible while provid- ing from the median opening onto the major road. ing sufficient space in the median for the appropriate For median openings at four-leg intersections without left-turn treatment and to accommodate U-turn maneu- left-turn lanes on the major road, most of the observed vers by a selected design vehicle. traffic conflicts involved major-road through vehicles Rural unsignalized intersections should have medians having to brake for vehicles turning left into the median that are as wide as practical, as long as the median is not opening. so wide that approaching vehicles on the crossroad can- The minimum spacing between median openings cur- not see both roadways of the divided highway. rently used by highway agencies in rural areas ranges Median opening lengths at urban and suburban divided from 152 to 805 m (500 to 2,640 ft). In urban areas, the highway intersections may be as long as necessary. In minimum spacing between median openings ranges from contrast, median opening lengths at rural divided high- 91 to 805 m (300 to 2,640 ft) in highway agency policies. way intersections generally should be kept to the mini- In most cases, highway agencies use spacings between mum possible. median openings in the upper end of these ranges, but Intersection sight distance based on the criteria in the there is no indication that safety problems result from AASHTO Green Book (3) for Cases B1, B2, and F occasional use of median opening spacings as short as should be available to accommodate U-turns and left 91 to 152 m (300 to 500 ft). turns at unsignalized median openings. Where a large truck is used as the design vehicle for a median opening and a median width of 21 to 31 m RECOMMENDATIONS (70 to 100 ft) cannot be provided, consideration should be given to providing a loon. The following recommendations have been developed based Left-turn lanes on the major road are desirable to mini- on these conclusions: mize conflicts between through and turning vehicles at unsignalized median openings. Guidelines for the use, location, and design of unsignal- Midblock median openings should be considered, where ized median openings are included in Appendix C of appropriate, as a supplement or an alternative to median this report. openings at three-leg or four-leg intersections. The guidelines include tables presenting the advantages Directional median openings at three-leg intersections, and disadvantages of typical median opening designs; combined with a directional midblock median opening, these tables should be used by designers as a resource should be considered as a supplement or an alternative to when considering alternative median opening designs. conventional median openings at three-leg intersections. A methodology for comparing the expected safety per- Directional median openings at four-leg intersections, formance of median opening design alternatives, as pre- combined with two directional midblock median open- sented in Appendix C, should be used to assist in the ings, should be considered as a supplement or an alter- selection of median opening types and the comparison native to conventional median openings at four-leg of alternative median opening arrangements. intersections.