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39 6 s per through vehicle, depending on pedestrian volume. Left- and right-turn lane treatments at intersections on Results from the project's economic analysis procedure all roadways with operating speeds of less than 40 mph. developed a method to identify where installation of right- Right-turn treatments on roadways with operating speeds turn lanes at unsignalized intersections and major drive- equal to or greater than 40 mph." ways would be cost-effective, indicating combinations of through-traffic volumes and right-turn volumes for which Where raised channelization is implemented at intersections, provision of a right-turn lane would be recommended. The they also recommended that median and island curb sides and research team stated that their economic analysis proce- curb horizontal surfaces be treated with retroreflectorized dure can be applied by highway agencies using site-specific markings and be maintained at a minimum luminance contrast values for ADTs, turning volumes, accident frequency, and level of 2.0 with overhead lighting or 3.0 without overhead construction cost for any specific location (or group of similar lighting. locations) of interest. Intersection Sight Distance Kikuchi et al. (2007) examined the lengths of turn lanes when a single lane approached a signalized intersection and Where determinations of intersection sight distance (ISD) was divided into three lanes: left-turn, through, and right- requirements for any intersection maneuver that is performed turn. Their objective was to determine the appropriate length by a driver on either a major or a minor road incorporate of each turn lane. From analysis of the vehicle queue pat- a PRT component, the FHWA Highway Design Handbook tern at the entrance to the turn lanes, they developed a set for Older Drivers and Pedestrians (Staplin et al. 2002) rec- of formulas to compute the probabilities of the occurrence ommends that a PRT value of no less than 2.5 s be used to of turn-lane overflow and turn-lane blockage. The recom- accommodate the slower decision times of older drivers. It mended lane lengths were calculated so that the probabili- also recommends that "where determinations of intersection ties that a lane did not overflow and that the entrance of the sight-distance requirements for a left-turn maneuver from a lane was not blocked were greater than a threshold value of major roadway by a stopped passenger car are based on a gap 0.95. Recommended turn-lane lengths, presented in a series model, a gap of no less than 8.0 s, plus 0.5 s for each addi- of tables, were found to be shorter than those recommended tional lane crossed by the turning driver, be used to accom- by AASHTO. modate the slower decision times of older drivers." In a subsequent study, Kikuchi and Kronprasert (2008) Yan and Radwan (2005) conducted research to develop developed analytical and computational processes for deter- sight distance geometric models for unprotected left-turning mining the length of the right-turn lane at a signalized inter- vehicles from the major road to the minor road at signalized section. They examined the factors that influenced length, intersections; they also sought to evaluate sight improvement reviewed available literature and practices, derived recom- effects of two offset methods and analyze the relationship mended lengths analytically, and developed a set of tables between available sight distance and selected geometric param- of recommended lane lengths as a function of approach eters. According to their conclusions, sight distance problems volumes (right-turn, through-traffic, and cross-traffic volumes) could occur for passenger cars on traditional left-turn lane and signal timing. Their analysis compared conditions designs with 14- to 18-ft medians at high design speeds. when right-turn-on-red (RTOR) was not permitted and when Using sensitivity analyses, they also developed equations it was permitted. Based on achieving desired probabilities showing a relationship between sight distance and offset of turn-lane overflow and turn-lane blockage, they calcu- value for parallel left-turn lanes and between sight distance lated recommended lane lengths based on the number of and taper angle for taper lanes. Left-turn lane length was also vehicle spaces and described a procedure to convert that cited as an important variable that affects sight distance. number to actual distance. They compared their guidelines that account for arrival rates of both right-turn and through Easa and Ali (2006) developed an extension of a previous vehicles with guidelines that only considered right-turn ISD model to consider sight distance for stop-control inter- vehicles; as a result, they concluded their proposed lane sections on three-dimensional alignments. Although the pre- lengths were different than those in existing guidelines. vious model accounted for obstructions inside the horizontal Their recommended lengths for RTOR conditions were curve and for intersections and major-road vehicles (objects) somewhat shorter than non-RTOR conditions when the right- on the curve, their model was expanded by (1) allowing the turn arrival rate was greater than the arrival rate for through object to be anywhere on the horizontal curve or tangent, vehicles. (2) allowing the horizontal and vertical curves to overlap par- tially, and (3) considering the case in which the obstruction lies FHWA's Highway Design Handbook for Older Drivers outside the horizontal curve. The obstruction location was for- and Pedestrians (Staplin et al. 2002) recommends "raised mulated through use of a simple variable that takes the value of channelization with sloping curbed medians rather than +1 or -1 for an obstruction, respectively, inside or outside the channelization accomplished through the use of pavement horizontal curve. They presented design aids for the required markings, for the following operating conditions: minimum lateral clearances (from the minor and major roads)