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5 CHAPTER 2 LITERATURE REVIEW The objectives of the literature review are as follows: undesirable acceleration and deceleration maneuvers. There- fore, if traffic safety on multilane highways is to be preserved, To document current knowledge of the safety effect of the location of median openings must be given careful con- installing nontraversable medians on multilane highways, sideration. Some factors that influence median opening loca- To identify key issues related to the increase of U-turns tions include the following: at unsignalized median openings, and To document the safety effect of installing or removing Spacing between median openings, median openings. Stopping sight distance, Intersection sight distance, The literature review includes the following issues related Operating speeds, to the safety and operation of U-turns at unsignalized median Length of turn lanes, openings: Right-turn conflict overlap, and Size and type of traffic generator. Location of median openings, Spacing of median openings, A committee of the Institute of Transportation Engineers Safety of median openings, developed a list of factors to consider in locating median Median width, openings (1). These included the potential number of left Median opening length, turns into driveways, length of frontage along the street right- Safety effects of median treatments, of-way line of the property proposed to be served, distance Safety effects of increasing U-turn maneuvers through of proposed opening from adjacent intersections or other use of nontraversable medians, openings, length and width of the left-turn storage lane as Left-turn lanes, functions of the estimated maximum number of vehicles to Median acceleration lanes, be in the lane during peak hours, and traffic control. The Loons (i.e., paved aprons opposite median openings to committee noted the need to consider circuitous routing and assist larger vehicles in completing U-turn maneuvers), added intersection turns that may be caused by closing a Sight distance at median openings, median opening. Indirect left-turn maneuvers, Research for the Florida Department of Transportation (2) Access management, found that the overall reductions in the number of median openings along the roads studied in their analysis resulted in Spacing between access points, and accident rate reductions, despite the increased through traffic Effects of adjacent traffic signals. flow and higher density of traffic flow per median opening. The Florida research, performed for four-lane and six-lane roadway Literature related to each of these topics is summarized below. sections, also found that the reduction in conflict points can improve traffic flow characteristics without increased risk of LOCATION OF MEDIAN OPENINGS accidents at the remaining median openings. Policies recommended at the national level for geometric The growing number of multilane highways with raised or design of median openings are presented in the AASHTO A depressed medians and without access control has created the Policy on Geometric Design of Highways and Streets, com- need to provide median openings, or crossovers, at various monly known as the Green Book (3). The Green Book states locations along such facilities to permit vehicles to reach abut- that "median openings on divided highways with depressed ting property or reverse their direction of travel. Median open- or raised curbed medians should be carefully considered. ings, however, may also become points of increased conges- Such openings should only be provided for street intersec- tion and accident exposure. Turbulence in traffic flow created tions or for major developments." Regarding the location of by vehicles turning on or off high-speed roadways causes median openings, the Green Book recommends that median