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52 Nearby Features RELATIVE SAFETY OF MEDIAN OPENING DESIGNS BASED ON TRAFFIC CONFLICT The proximity of a median opening to the following nearby POINTS features influences the operational and safety performance of the median opening: The classification of median opening designs provides a framework for comparing the relative safety of each design. Driveways, The results of the accident data analysis, presented in Chap- Adjacent median openings, ter 6 of this report, provide information on the relative safety Signalized intersections, and performance of those median opening designs for which suf- Unsignalized intersections. ficient data existed. However, data for several median open- ing designs were not sufficient to quantify the relative safety NCHRP Report 420 (7) suggests that access points intro- with certainty. In these cases, the relative safety of each duce conflicts and friction into the traffic stream. Vehicles median opening design must be estimated or hypothesized. entering and leaving the major road often slow the through The relative safety of median opening designs can be repre- traffic, and the difference in speeds between through and sented by the number of conflict points for each design. This turning traffic increases accident potential. A discussion in approach is useful in the absence of actual data, but is ulti- the Green Book (3) about driveway spacing could also be mately limited because it does not consider the volumes of applied to median openings and unsignalized intersections. vehicles that conflict at each point. The use of conflict points The Green Book (3) recommends that, ideally, driveways to assess safety is described below. should not be located within the functional area of an inter- Intersection conflict analysis is a well-understood means of section or in the influence area of an adjacent driveway. The addressing the complexity and relative safety of alternative functional area extends both upstream and downstream from intersection designs. For example, it has long been known that the physical intersection area and includes the longitudinal three-leg intersections operate more safely than four-leg inter- limits of auxiliary lanes. The influence area associated with sections because three-leg intersections have fewer conflict a driveway includes (1) the impact length (the distance from points at which conflicting traffic streams cross, merge, or a driveway at which vehicle operations begin to be affected), diverge. Figure 36 illustrates the four basic types of vehicular (2) the driver perception-reaction distance, and (3) the vehi- conflicts: diverging, merging, weaving, and crossing. cle length. Intersection conflicts reflect the crossing or conflicting paths A median opening placed too close to one or more drive- of vehicles moving from one leg to another. Depending on ways, intersections, or other median openings creates unde- the type of movement (i.e., U-turn, right or left turn, or cross- sirably short weaving areas. For example, minor-road drivers ing), any given vehicle movement can conflict with one or seeking to negotiate their way to a nearby median opening on more other vehicle movements. the major road should be able to enter the major-road traf- Figure 37 illustrates the vehicular conflicts at a conven- fic stream safely, select an acceptable gap in order to merge tional median opening at a four-leg intersection. The figure into the inside lane, and then move laterally into a median includes left-turn, right-turn, and through movements, but left-turn lane. does not explicitly consider U-turn movements. However, any The following are safety and operational problems that may given U-turn movement includes the same diverge maneuver occur when a median opening is located too close to a sig- as one left-turn movement and the same merge maneuver as nalized intersection: another left-turn movement. Table 15 summarizes the total number of conflict points A median opening within the physical length of a left- for each of the typical median opening designs discussed ear- turn bay should be avoided. It violates driver expectancy lier in Chapter 4 of the report. Table 15 suggests that median and restricts the sight distance of vehicles in the median opening. A median opening within the boundaries of regularly forming queues from neighboring intersections should be avoided. The problem with this situation is that when these queues build, "good Samaritans" may allow the left-turning vehicle through the queue, and the left- turning vehicle then crashes with a vehicle moving freely in an adjacent lane. When the queue in the through traffic lane extends past the median left-turn lane, vehicles seeking access to the median opening are trapped in the queue of the signal- ized intersection and cannot move into the left-turn lane until the queue advances. Figure 36. Types of vehicular conflicts (33).