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37 FIGURE 9 Example of approach curvature (Robinson et al. 2000). types of vehicles with unique sight-line obstructions. They · Adding left-turn lanes is effective in improving safety derived equations and nomographs for calculating available at signalized and unsignalized intersections. Installing sight distance that included the influence of factors such a single left-turn lane on a major-road approach would as geometry (intersection angle, lane width, shoulder width, be expected to reduce total intersection accidents at position of stop line), vehicle dimensions, and the driver's rural unsignalized intersections by 28% for four-leg field of view. They concluded that findings from their intersections and by 44% for three-leg intersections, research provided evidence that a skew angle greater than with corresponding reductions of 27% and 33% at urban 20 degrees should not be used in design when the design unsignalized intersections. At four-leg urban signal- vehicle is a large vehicle or semitrailer, because available ized intersections, installation of a left-turn lane on one sight distances were less than the required stopping sight dis- approach would be expected to reduce accidents by 10%, tance, even with a low value of design speed for intersection and installation on both major-road approaches would be angles less than 70 degrees. expected to increase, but not quite double, the resulting effectiveness measures for total intersection accidents. The Highway Design Handbook for Older Drivers and · Positive results can also be expected for right-turn lanes, Pedestrians (Staplin et al. 2002) recommends establishing with reductions in total intersection accidents of 14% at 15 degrees as a minimum skew angle as a practice to accom- rural unsignalized locations and 4% at urban signalized modate age-related performance deficits at intersections where locations for installations on single approaches. "Instal- right-of-way is restricted; "at skewed intersections where lation of right-turn lanes on both major-road approaches the approach leg to the left intersects the driver's approach to four-leg intersections would be expected to increase, leg at an angle of less than 75 degrees, the prohibition of but not quite double, the resulting effectiveness mea- right turn on red (RTOR) is recommended." The Handbook sures for total intersection accidents." cites multiple studies documenting restricted neck move- · "In general, turn-lane improvements at rural intersec- ment in older drivers, making detection of and judgments tions resulted in larger percentage reductions in acci- about potential conflicting vehicles on crossing roadways dent frequency than comparable improvements at urban much more difficult. intersections." · "Overall, there [was] no indication that any type of turn-lane improvement is either more or less effective Auxiliary Lane Design for different accident severity levels." Harwood et al. (2002) conducted a study to investigate the FHWA's Highway Design Handbook for Older Drivers safety effectiveness of left- and right-turn lane treatments. and Pedestrians (Staplin et al. 2002) states that "two fac- The research team collected geometric design, traffic con- tors can compromise the ability of older drivers to remain trol, traffic volume, and traffic crash data at 280 improved within the boundaries of their assigned lanes during a left sites in eight states and at 300 similar intersections that were turn. One factor is the diminishing ability to share atten- not improved during the study period. The types of improve- tion (i.e., to assimilate and concurrently process multiple ment projects evaluated included installation of added left- sources of information from the driving environment). The turn lanes, installation of added right-turn lanes, installation other factor involves the ability to turn the steering wheel of added left- and right-turn lanes as part of the same proj- sharply enough, given the speed at which they are traveling, ect, and extension of the length of existing left- or right-turn to remain within the boundaries of their lanes." Data sources lanes. Based on the results of the analyses, they concluded cited by the Handbook's authors indicated that a 12-ft lane the following: width provides the most reasonable tradeoff between the
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38 need to accommodate older drivers, as well as larger turning They compared field-measured delays with delays estimated vehicles, without penalizing the older pedestrian in terms of by the HCM with the use of regression models for lane uti- exaggerated crossing distance. The Handbook's correspond- lization. They stated that even with the new models for lane ing recommendation was for a minimum receiving lane utilization, the HCM consistently overestimated delay for all width of 12 ft accompanied, wherever practical, by a shoul- types of lane-drop intersections with low lane utilization and der of 4 ft minimum width. suggested that a reassessment of the effect of lane utilization on capacity may be in order. The study also found that the The Handbook further recommended that, for new or downstream lane length and traffic intensity positively corre- reconstructed facilities, unrestricted sight distance, achieved lated with the lane utilization factor, existence of a TWLTL through positive offset of opposing left-turn lanes, be pro- or midblock left-turn bay increased the lane utilization fac- vided whenever possible. This recommendation was made in tor, lane drops resulting from lane usage change had more anticipation of providing "a margin of safety for older driv- equal lane volume distribution than the midblock taper lane ers who, as a group, do not position themselves within the drop, and that some geometric variables at the approach may intersection before initiating a left turn." Where the provi- also influence lane utilization. sion of unrestricted sight distance is not feasible, positive left-turn lane offsets were recommended to achieve the mini- Fitzpatrick et al. (2006) conducted a study to determine mum required sight distances appropriate for major roadway variables that affected the speeds of free-flow turning vehicles design speed and type of opposing vehicle. in an exclusive right-turn lane and explore the safety expe- rience of different right-turn lane designs. Their evaluations Long (2002) reviewed the characteristics of intervehicle found that the variables affecting the turning speed at an exclu- spacing for the purpose of auxiliary lane design. He concluded sive right-turn lane included the type of channelization pres- that the value of 25 ft per vehicle used by the CORSIM mod- ent (either lane line or raised island), lane length, and corner eling software was a severe underestimation for determining radius. Variables that affected the turning speed at an exclu- queue lengths, as was the 3-ft distance between vehicles. He sive right-turn lane with island design included (1) radius, developed new models for estimating average queue lengths lane length, and island size at the beginning of the turn, and and maximum lengths at a given probability; models based (2) corner radius, lane length, and turning-roadway width near on an intervehicle spacing of 12 ft, a passenger car length of the middle of the turn. The authors compared this with 15 ft, a 65-ft length for combination trucks, and 30 ft for other previous research treatments that had the highest number of vehicles. crashes were right-turn lanes with raised islands. In their anal- ysis, they found this type of intersection had the second high- Kikuchi et al. (2005) developed a method for estimating est number of crashes of the treatments evaluated in this study. the needed length of dual left-turn lanes (DLTL). Their pro- In both studies, the "shared through with right lane combina- cedure first surveyed how drivers choose a lane of the DLTL tion" had the lowest number of crashes. They recommended in the real world and analyzed the relationship between lane that these findings be verified through use of a larger, more use and the volume of left-turn vehicles. Second, the pro- comprehensive study that includes right-turning volume. cedure calculated the probability that all arriving left-turn vehicles during the red phase could enter the left-turn lanes NCHRP Project 3-72 was tasked with developing design (i.e., no queue spillback of vehicles from the DLTL and no guidance related to right-turn lanes on urban and suburban blockage of the DLTL by the queue of through vehicles). arterials. The research team from Midwest Research Insti- This probability was presented as a function of the length tute discussed results from their research with respect to of the DLTL and the arrival rates of left-turn and through right-turn deceleration lanes (Potts et al. 2007a). They con- vehicles. The adequate lane length was derived such that the ducted a computer simulation study of motor vehicles and probability of the vehicles entering the DLTL is greater than pedestrians at right-turn lanes to determine their operational a threshold value. Third, the recommended adequate length effects. They also performed a benefit-cost analysis of was expressed in number of vehicles, then converted to the right-turn lanes that considered both operational and safety actual distance required based on the vehicle mix and prefer- effects. The researchers determined that right-turn maneuvers ence between the two lanes. Resulting recommended lengths from a two-lane arterial at an unsignalized intersection or were presented as a function of left-turn and through volumes driveway can delay through traffic by 0 to 6 s per through for practical application. vehicle where no right-turn lane was present. Delays to through traffic owing to right turns in the same situation Lee et al. (2005) developed models to predict lane utiliza- on a four-lane arterial were substantially lower, in the tion factors for six types of intersections with downstream range from 0 to 1 s per through vehicle. They concluded lane drops and to assess how low lane utilization affects the that pedestrians at unsignalized intersections or driveways observed intersection capacity and level of service. They col- can have a substantial impact on delay to through vehicles lected traffic and signal data at 47 sites in North Carolina. On owing to slowing of right-turning vehicles yielding to pedes- the basis of 15 candidate factors, multiple regression mod- trians, but provision of a right-turn lane could reduce els were developed for predicting the lane utilization factor. pedestrian-related delays to through traffic by as much as