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16 There is sufficient space to separate the signalized inter- Parker et al. (34) determined that the addition of left-turn section and the opening. lanes at rural intersections along two-lane highways can reduce the potential for passing-related accidents. On urban The Handbook also provides three design options to four-lane roadways, McCoy and Malone (35) found that accommodate U-turns: (1) wide medians, (2) median "bulb- installation of left-turn lanes reduced rear-end, sideswipe, out" or loon, and (3) flare-out (jughandles). and left-turn accidents. Foody and Richardson (36) found that accident rates decreased by 38 percent with the addition of a left-turn lane at signalized intersections and by 76 per- cent at unsignalized intersections. Hauer (37) reported that LEFT-TURN LANES left-turn channelization reduced accidents to varying degrees, depending on the intersection configuration; and Vehicles turning left from a multilane highway may pose Gluck et al. (4) reported accident rate reductions ranging safety and operational problems at median openings. They from 18 to 77 percent as a result of the installation of left-turn not only increase conflicts with and delays to other vehicles, but also pose a major safety problem with the large speed dif- lanes. A report by ITE indicates that median deceleration and ferential between left-turning and through vehicles. storage lanes installed at intersections generally provide sig- The FHWA National Highway Institute (NHI) has devel- nificant safety and operational benefits (38). Agent (39) per- oped a short course on Access Management (33). The man- formed an accident analysis of unsignalized intersections in ual for the course recommends installing left-turn lanes at Lexington, Kentucky, and found that the left-turn accident existing median openings to rate was 77 percent lower for intersections with left-turn lanes than intersections without left-turn lanes. Cribbins et al. (11) also reported that the number of rear-end collisions Allow turning vehicles to clear the through traffic lane is less where storage lanes are provided. with an acceptable speed differential, When implemented with additional safety measures, left- Provide queue storage without interference with through turn lanes have been found to be very effective in increasing traffic, Reduce rear-end collisions and sideswipes, and safety. Hauer (37) reported that the provision of left-turn Increase capacity and decrease delay. lanes at unsignalized intersections, when combined with instal- lation of curbs or raised medians, reduced accidents by 70, 65, and 60 percent in urban, suburban, and rural areas, respec- The course also recommends increasing the length of an tively. When the channelization was painted, rather than existing left-turn bay at all existing median openings where raised, accidents decreased only by 15, 30, and 50 percent in urban, suburban, and rural areas, respectively. At signalized Deceleration in the through traffic lane results in an unde- intersections, installation of left-turn channelization accom- sirable speed differential between left-turning vehicles panied by a left-turn signal phase reduced accidents by 36 per- and through traffic. cent; however, without the left-turn phase, accidents decreased The left-turn queue exceeds the length of the full-width only by 15 percent. At unsignalized intersections, findings of left-turn lane. a California study indicate greater reductions in accidents with the use of a left-turn lane in a raised median than with painted Left-turn lanes are often installed at median openings to left-turn lanes (40). Similarly, Lacy (41) found that a left-turn accommodate high left-turning volumes. NCHRP Report 420 lane, when coupled with several other safety improvements, (4) summarizes the following safety benefits of left-turn lanes: reduced accident frequency by 35 percent and accident sever- ity by 80 percent. Dale (42) found that installation of a traf- They remove the turns from the through travel lanes, fic signal and left-turn channelization at intersections along thus reducing rear-end collisions. rural two-lane highways reduced the total number of acci- They improve the visibility of oncoming traffic for vehi- dents by 19.7 percent, while the installation of a traffic sig- cles turning left, thus reducing right-angle collisions. nal without any channelization reduced the total number of accidents by only 6 percent. Installation of left-turn lanes has been the focus of many Several predictive models and accident modification fac- research studies. Various safety-related factors have been tors have been developed that indicate left-turn lanes have a documented based on the type of intersection (e.g., signal- positive effect on safety. Maze et al. (43) developed a model ized, unsignalized, and four-leg) where the left-turn treat- that predicted a reduction in left-turn accident rate of 5.5 per- ment was implemented. Although many of these studies focus cent as a result of the installation of a left-turn lane with per- on left-turn treatments on two-lane highways, the safety rela- mitted signal phasing and a reduction of approximately tionships can be useful for a broader range of roadway types, 35 percent from installation of a left-turn lane with protected/ including divided arterials. permitted signal phasing. Vogt (44) developed a model for a

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17 four-leg rural intersection of a four-lane major road with increase in right-angle collisions with installation of a left-turn STOP-controlled two-lane minor roads, which yielded an acci- lane. However, at unsignalized intersections on rural two-lane dent reduction factor for total accidents of 38.4 percent as a highways, McCoy et al. (47) found no significant difference in result of the installation of a left-turn lane along the major road. rear-end and left-turn collision rates between intersections In another study, Harwood et al. (45) developed algorithms with and without left-turn lanes. Poch and Mannering (48) also to predict the expected safety performance of rural two-lane found some situations in which accidents of specific types highways. The prediction algorithms combined elements of increased with installation of left-turn lanes. historical accident data, predictions from statistical models, NCHRP Report 348 (5) indicates that, although turning results of before-and-after studies, and expert judgments made lanes may be required for some or all access locations to by experienced engineers. As part of the research, an expert major activity centers, they are not always required for smaller panel of engineers developed accident modification factors developments. The report cites reference materials, such as (AMFs) for specific geometric design and traffic control fea- the Highway Capacity Manual (49), that should be consulted tures. AMFs are used in the accident prediction algorithms to for information to help guide the decision of whether turn represent the effects of safety of the respective features. The lanes are needed. base value of each AMF is 1.0. Any feature associated with An emerging issue in the design of left-turn channelization a higher accident experience than the base condition has an is the restriction in sight distance that opposing left-turn vehi- AMF value greater than 1.0, and any feature associated with cles cause one another. As an indication of this safety problem, lower accident experience than the base condition has an AMF David and Norman (50) determined that for ADT volumes value less than 1.0. In developing AMFs for the installation of left-turn lanes on between 10,000 and 20,000, four-leg intersections with oppos- the major-road approaches to intersections on two-lane rural ing left-turn lanes had more accidents than those without. A highways, the expert panel reviewed various sources of potentially effective countermeasure for safety problems information related to the accident reduction effectiveness of where opposing left-turn lanes are present is to eliminate the left-turn lanes. However, the panel did not find any well- sight restrictions by offsetting the left-turn lanes, as shown in designed before-and-after studies. Therefore, the panel com- Figure 5. NCHRP Report 375 reviewed the safety perfor- bined results from several sources and developed AMFs for mance of a limited set of tapered and parallel offset left-turn left-turn lanes, which are presented in Table 6. The AMFs rep- lanes and found no safety problems (7). Both McCoy et al. resent a judgment by the panel. The panel estimated that instal- (51) and Joshua and Saka (52) developed procedures to com- lation of a left-turn lane along one major approach reduces pute the amount of offset required for clear sight lines. How- intersection-related accidents by 18 to 24 percent, depending ever, no evaluations of the accident reduction effectiveness on the type of traffic control and the number of legs, and instal- of offset left-turn lanes have been found. Although offset lation of left-turn lanes along both major approaches to a four- left-turn lanes have been used primarily at signalized leg intersection reduces intersection-related accidents by 33 median openings, they have been used by at least two agen- to 42 percent, depending on the type of traffic control. cies at unsignalized median openings. Offset left-turn lanes Not all studies, however, have shown that left-turn lanes are a potential concern because they may make U-turn maneu- reduce accidents. Bauer and Harwood (46) found that left- vers more difficult to complete because they move the starting turn lanes had a statistical association with higher frequen- point for the U-turn maneuver closer to the opposing roadway. cies of both total multiple-vehicle accidents and fatal and This potential problem is not addressed in the literature. injury multiple-vehicle accidents. However, this result was not In another study, Harwood et al. (53) conducted a before- advanced by the authors as a basis for policy because the direc- and-after evaluation of the safety effects of providing left- tions of specific effects in predictive models often represent and right-turn lanes for at-grade intersections. Geometric the surrogate effects of other variables, rather than the true design, traffic control, traffic volume, and traffic accident data effect of the variable of interest. At unsignalized intersections, were gathered for 280 improved intersections, as well as for McCoy and Malone (35) determined there was a significant 300 similar intersections not improved during the study TABLE 6 Accident modification factors for installation of left-turn lanes on the major- road approaches to intersections on two-lane rural highways (45) Number of major-road approaches on which Intersection traffic left-turn lanes are installed Intersection type control One approach Both approaches a Three-leg intersection STOP sign 0.78 Traffic signal 0.85 a Four-leg intersection STOP sign 0.76 0.58 Traffic signal 0.82 0.67 a STOP signs on minor-road approach(es).