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8 Figure 2. Average number of accidents per year related to traffic volume at divided highway intersections (10). The number of median openings, excluding intersections, sively by Harwood et al. in NCHRP Report 375: Median Inter- affected the accident rate significantly. section Design (7). However, this evaluation addressed median The two roadway characteristics having the least effect width for median openings at intersections--not median open- on the accident rate were median width and speed limit. ings at driveways or median openings used solely for U-turns. Whenever storage lanes are installed at openings, the NCHRP Report 375 used two separate approaches to address median-opening accident rate is no longer significantly the relationship between median width and accidents at divided affected by (1) the number of openings excluding inter- highway intersections: an accident study and a field observa- sections, (2) median width, (3) speed limit, or (4) ADT. tional study. The traffic accident analysis of divided highway intersec- Research by Harwood et al. in NCHRP Report 375 (7) tions was conducted for NCHRP Report 375 with a statewide found that the median width and median opening length have database of accident, geometric, traffic control, and traffic a strong influence on the safety performance of median open- volume data for state highways in California. ings. These issues are addressed in the next two sections of The findings of the analysis concerning median width are this report. as follows: At rural, four-leg, unsignalized intersections, accident MEDIAN WIDTH frequency decreases as median width increases. At rural, three-leg, unsignalized intersections, no statis- The safety and operational effects of median width at sig- tically significant relationship exists between accident nalized and unsignalized intersections were evaluated exten- frequency and median width.

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9 At urban/suburban, four-leg, unsignalized intersections, ing and crossing volumes of large vehicles (such as school accident frequency increases with increasing median buses or trucks) are present, highway agencies may find it width over the range of median widths from 4 to 24 m appropriate to select an appropriate median width to store a (14 to 80 ft). design vehicle of that type safely in the median. At urban/suburban, three-leg, unsignalized intersections, One issue of interest to the research for NCHRP Report the intersection accident frequency increases with increas- 375 was how drivers making opposing left turns are influ- ing median width. enced by the median width. Specifically, it was hypothesized that, at intersections with narrow medians, drivers making The field observational study in NCHRP Report 375 inves- opposing left turns tend to turn in front of one another and, tigated the effect of median width on three types of undesir- at intersections with wide medians, drivers making opposing able driving behavior as commonly observed in the median left turns turn behind one another. Although no quantitative opening area at intersections on divided highways: information exists on the median width at which drivers cease to turn in front of one another and begin to turn behind one Encroachment on through lanes by vehicles in the median another, an analysis of rural, unsignalized intersections found opening area, that, at intersections with median widths of less than 15 m Side-by-side queuing of vehicles in the median opening (50 ft), vehicles making opposing left turns tend to turn in front area, and of one another. In contrast, at intersections with median widths Angle stopping by vehicles in the median opening area. of more than 15 m (50 ft), vehicles making opposing left turns tend to turn behind one another. A similar pattern was found Figure 3 illustrates side-by-side queuing of vehicles in an for suburban, unsignalized intersections with median widths of unsignalized median opening. less than 15 m (50 ft). There were no comparable suburban, NCHRP Report 375 reached the following conclusions unsignalized intersections to verify whether the same turn- concerning the effect of median width on accidents and unde- behind behavior observed at the rural, unsignalized intersec- sirable driving behavior at unsignalized intersections: tions with median widths of more than 15 m (50 ft) occurred at similar suburban, unsignalized intersections. At rural unsignalized intersections, the frequency of both A 1964 Ohio study by Priest found that, except at very accidents and undesirable driving behavior decreases as low volume levels, intersection accident rates decrease with the median width increases. increasing median width (10). However, the difference in acci- At suburban unsignalized intersections, the frequency of dent rates between medians less than 6 m (20 ft) wide and both accidents and undesirable driving behavior increases medians 6 to 12 m (20 to 39 ft) wide is greater than the differ- as the median width increases. ence in accident rates between medians with widths of 6 to 12 m (20 to 39 ft) and medians with widths of 12 m (40 ft) Based on these findings, NCHRP Report 375 recommended or more. These results are illustrated in Figure 4. that rural unsignalized intersections should have medians that are as wide as practical, as long as the median is not so wide that approaching vehicles on the crossroad cannot see both roadways of the divided highway. At suburban unsignalized intersections, by contrast, medians should generally not be wider than necessary to provide whatever left-turn treatment is selected. At specific intersections where substantial turn- Figure 4. Variation of annual accident frequency at divided highway intersections as a function of median Figure 3. Side-by-side queuing at unsignalized median width and exposure index (the product of the ADTs of the opening. intersecting roadways) (10).