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7 · At least two times the established median opening N = 0.000783 Vd0.455 Vc0.633 (1) spacing interval when two major traffic generators are involved. where NCHRP Report 375 (7) found that very few state high- N = expected number of intersection accidents per year way agencies have formal policies on the minimum spac- Vd = ADT volume entering the intersection from the divided ing between median openings. Those agencies that do have highway (veh/day) criteria generally use a spacing between median openings in Vc = ADT volume entering the intersection from the cross- the range from 0.4 to 0.8 km (0.25 to 0.5 mi). road (veh/day) The Florida DOT Median Handbook (8) identifies the fol- lowing factors that should be considered in determining the This finding is evidence that concentrating cross-traffic at a spacing of median openings: few locations, by closing low-volume-crossroad intersections and providing frontage roads, may effectively reduce the · Deceleration length, number of intersection accidents. · Queue storage, The Ohio study, conducted by Priest (10) in 1964, included · Turn radius, and 3 years' worth of accident data for 316 at-grade intersections · Perception/reaction distance. on divided highways with partial or no control of access. Most of the intersections were unsignalized; however, the Based on consideration of all of these factors, Florida has author does not explicitly state the type of traffic control used identified a 330-m (1,070-ft) spacing between median open- at the intersections studied. Priest, like McDonald, also found ings as being a realistic minimum for urban arterials. that accident frequency is more sensitive to the crossroad traffic volume than to the divided highway traffic volume. Figure 2 illustrates the relationship between the average num- SAFETY OF MEDIAN OPENINGS ber of accidents per year and traffic volume at divided high- Much of the safety research of median openings has been way intersections developed by Priest. conducted at intersections on divided highways. These studies In a 1967 study of divided urban and rural highways in generally document the safety performance of the intersection North Carolina, Cribbins et al. (11) found that median open- as a whole, but usually do not provide specific information on ings do not necessarily experience high accident rates under the safety performance of the median opening area by itself. conditions of low volumes, wide medians, and light roadside Furthermore, safety research on midblock median openings development. However, as traffic volumes and roadside devel- (i.e., median openings not located at intersections or drive- opment increase, the frequency of median openings does ways) is limited and usually addresses a "system" or combi- affect accident potential significantly. Specifically, Cribbins nation of intersections and midblock median openings, such as et al. found the following relationships: for indirect left-turn treatments. However, even research on divided highway intersections that is broader in scope than · As traffic volumes increase, median openings experience just the median opening area can provide valuable informa- a sharp increase in accident frequency. When combined tion on the safety performance of median openings. with intensive roadside development, this increase in Two published research studies have specifically addressed accident frequency becomes even more pronounced. the safety performance of divided highway intersections, one · Signalization of median openings does not necessarily in California and one in Ohio. A 1953 California study by reduce accident experience under high-volume condi- McDonald (9) developed relationships between the number of tions, but it makes the traffic flow more efficiently by dis- accidents and traffic volume at divided highway intersections. tributing time for each movement. This study was based on the accident experience over periods · As roadside development increases, and crossovers of of 6 months to 5 years at 150 at-grade intersections on 290 km any type are permitted, accidents will increase. (180 mi) of rural divided expressways on the state highway system in California. Most of the intersections were two-way stop-controlled intersections, with stop control on the minor In another study of multilane highways in North Carolina, road and no control on the divided highway, although a few Cribbins et al. (12) used the same data to determine the signalized intersections were included. effects of selected roadway and operational characteristics on The analysis of these California data found that low- accidents on multilane highways. Cribbins et al. correlated crossroad-volume intersections have higher accident rates per eight highway characteristics with all injury accidents: median crossroad vehicle than do high-crossroad-volume intersec- width, speed limit, traffic volume, level of service, access point tions. The following relationship between the number of acci- index, intersection openings per mile, signalized openings per dents and the traffic volume at divided highway intersections mile, and median openings per mile. The following conclu- was developed: sions were drawn from the analysis: