Many arterial highways and streets are divided by raised, depressed, or flush medians. Where narrow raised or depressed medians are used, larger vehicles, such as trucks, buses, and recreational vehicles, may encounter traffic operational and safety problems in making left-turn and crossing manoeuvres through the median at intersections or driveways with median openings. In addition, where left turns cannot be made because median openings are not provided, larger vehicles may be required to make U-turns at a downstream location and proceed to their destination, or may find it necessary to use an indirect route to reach their destination. Traffic operational and safety problems can also result from such U-turn manoeuvres and indirect routings. Median widths used by highway agencies on divided arterials in both rural and urban/suburban areas vary widely. The geometric design policies of the American Association of State Highway and Transportation Officials (AASHTO) specify a minimum median width of 1.2 m (4 ft) for divided highways have wider medians and such wider medians are desirable to accommodate larger vehicles. The size of larger vehicles in the traffic stream has been growing steadily. The AASHTO design vehicles now used in the planning and design of roadways and intersections range up to 2.6 m (8.5 ft) in width and 35.9 m (118 ft) in length. Although the largest trucks are usually permitted only on freeways and toll roads, trucks of up to 23 m (75 ft) in length are now common on rural highways and urban arterials. Highway agencies face the challenge of providing access for these large trucks to reach their destinations. Narrow medians are one of the constraints that make it difficult for trucks to manoeuvre safely at intersections and avoid interfering with other traffic. Divided highway median widths at rural unsignalised intersections should generally be as wide as possible. It is desirable to provide a median width sufficient to store any larger vehicles likely to use the intersection frequently. At urban/suburban unsignalised intersections on divided highways, accidents and undesirable driving behaviour increase with increasing median width. Therefore, medians should generally be only as wide as necessary to accommodate current or planned left-turn treatments. However, at intersections used frequently by larger vehicles, it may be desirable to choose a median width sufficient to store a selected design vehicle with adequate clearance to the through lanes of the divided highway at both ends of the vehicle. This presents a difficult trade-off for highway agencies in designing urban/suburban unsignalised intersections on divided highways. Wider medians provide more storage space in median openings and therefore, reduce the likelihood that vehicles stopped in the median will encroach on the through traffic lanes. However, research has found wider medians at intersections and driveways in urban and suburban areas are also associated with an increased frequency of undesirable driving behaviour and accidents. By contrast, at signalised intersections on divided highways, the length of a specific design vehicle is non usually a consideration in selecting the median width because any larger vehicles that stop in the median should be able to clear the intersection at the end of any given signal phase. This synthesis identifies a range of traffic operational and safety problems encountered by larger vehicles at divided highway intersections. These include problems related to: insufficient storage space in the median opening area, larger vehicles turning onto and off the divided highway, larger vehicles crossing the divided highway, and U-turn manoeuvres by larger vehicles seeking to reach destinations at which a median opening is not provided or at which a median opening is not or at which a median opening is provided but the median is too narrow for effective use. Only limited research was found that deals directly with problems encountered by larger vehicles at intersections with narrow medians. However, this synthesis includes findings of broad range of published research on the topics of median type and width. Were passenger vehicles have encountered problems related to median width, such problems are likely to be even more critical when larger trucks use the same median. The synthesis identifies and discusses mitigation techniques that have been used by highway agencies to address such problems at existing intersections with narrow medians on divided highways. The synthesis also identifies design techniques that can be used by highway agencies in new construction and reconstruction projects to avoid introducing such problems in the future. (A)
Samenvatting