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24 TABLE 9 Accident rate differences--U-turns as alternate to direct left turns (4 ) Difference in Location Treatment accident rate US-1, Florida Driveway left turns replaced by right- 22% turn/U-turn Michigan Bi-directional crossover replaced by +14% directional crossover Michigan TWLTL replaced by directional 50% crossover · Minor-road intersections that are unsignalized become erable amount of literature focuses on access management two T-intersections, so there are no direct unsignalized issues, and several studies have evaluated the relationship crossings of the median. between safety and access management. Although none of these studies specifically address the safety at unsignalized The safety and operational benefits included lower accident median openings, the research that has established relation- rates, increased capacity, and reduced travel times. ships between access density and safety can be useful in this Recently, the Florida DOT sponsored a study to evaluate study as well. the safety effects of replacing full median openings with The research performed for NCHRP Report 348 (5) inves- directional median openings, resulting in the indirect left- tigated and documented the state of the art in access control turn treatment that forces drivers to make a right turn fol- and the broader concept of access management. The report lowed by a U-turn at a midblock U-turn lane (61). Over 250 defines access management as ". . . providing (or managing) sites were evaluated in this study, including 125 sites involv- access to land development while simultaneously preserv- ing right turns followed by U-turns and 133 sites involving ing the flow of traffic on the surrounding road system in direct left turns. A cross-sectional comparison was used to terms of safety, capacity, and speed." The report also defines measure the safety effects. The cross-sectional comparison the overall concept of access management, reviews current method compares the crash rates of sample sites with two dif- practices, and sets forth basic policy, planning, and design ferent egress designs: (1) direct left turns and (2) right-turns/ guidelines. The guidelines include possible legislative changes U-turns. If the average crash rate of right-turn/U-turns is less and enforcement procedures, as well as strategic design and than that of direct left turns at a certain statistical level of sig- operating guides. nificance, it is presented that right-turn/U-turn movements NCHRP Report 420 (4) presents methods to predict and could improve safety conditions. An assumption behind this analyze the safety and traffic operational effects of selected comparison is that all the traffic patterns and geometric con- access management techniques for different roadway variables ditions remain consistent during the study period. The com- and traffic volumes. Over 200 roadway segments, involving parison concluded that on six-lane divided arterials with large more than 37,500 accidents, were analyzed in detail. Acci- traffic volumes, high speeds, and high driveway/side-street dent rates were derived for various spacings and median types. access volumes, the implementation of the right-turn/U-turn Key findings related to accident density and safety at unsignal- treatment leads to a statistically significant reduction in total ized intersections were as follows: crash rate (26.4-percent reduction) as compared with direct left turns. The injury/fatality crash rate for right-turn/U-turns is significantly less than that of direct left turns (32.0-percent · Accident rates rise as the density of unsignalized access reduction). For eight-lane arterials, replacing direct left-turn connections per mile increases. openings with right-turn/U-turn openings leads to a reduction · The number of affected through vehicles traveling in the in the crash rate while the four-lane group results in an curb lanes increases as high-volume driveways are spaced increased crash rate; however, the results for the four-lane and closer together. The likelihood of spillbacks across a eight-lane groups were not statistically significant because of driveway rises with either an increase in the traffic vol- small sample sizes. umes entering driveways and/or the driveway density. · Access spacing or setback distances on arterial roadways near freeway interchanges are generally inadequate for ACCESS MANAGEMENT the weaving and left-turn storage movements that must be accommodated. The safe and efficient operation of the highway system depends heavily on the effective management of access to adjacent developments. Access management is generally A planning and access management guide (62) for Florida understood to preserve the flow of traffic on the surrounding cities and counties presents two recommendations related to roadways, maintain mobility, and improve safety. A consid- the location of driveways: