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26 guidelines and describes current state, county, and local spac- Stopping sight distance, ing requirements. Among the various agencies, there is little Intersection sight distance, consensus on unsignalized intersection spacing; however sight Maneuver distance, distance requirements and driver response times are key param- Right-turn conflict overlap, eters. The authors recognize the need for additional research Maximizing of egress capacity, and on unsignalized spacing and corner clearance criteria and their Corner clearance. applicability in various urban, suburban, and rural settings. Finally, the circular presents some established traffic engi- In a paper addressing the effect of street spacing on scale neering and roadway design and planning principles related (65), Levinson compares the spacing and design of city to unsignalized access spacing: streets and suburban highways, identifies the strengths and weaknesses of each, and suggests spacing guidelines for var- Limit the number of conflicts. ious urban and suburban environments. The paper demon- Separate basic conflict areas. strates how the provision of arterial streets at closer intervals Reduce interference with through traffic resulting from improves access opportunities, reduces traffic concentrations turns into or out of a site. where these streets meet, and allows reduced cross-section. Provide sufficient spacing between at-grade intersections. In another paper by Levinson (66), a method is presented Maintain progressive speeds along arterials. for predicting the safety of arterial roads based on arterial Provide adequate on-site storage areas. traffic volumes, access road volumes, and access density. The procedure applies the long-established relationship between NCHRP Report 348 (5) suggests that access spacing guide- intersection accidents and the product of conflicting traffic lines be keyed to allowable access levels, roadway speeds, volumes. Safety indices are provided that relate only to the and operating environments. The guidelines should apply to change in access density; these indices are generally consis- new developments and to significant changes in the size and tent with those reported in NCHRP Report 420 (4). The indices nature of existing developments. It goes on to add that the also show that the increase in accidents is equal to the square guidelines do not have to be consistent with existing prac- root of the increase in access density. tices; because of historical conditions, access to land parcels that do not conform to the spacing criteria may be necessary when no alternative reasonable access is available. In addi- EFFECTS OF ADJACENT TRAFFIC SIGNALS tion, NCHRP Report 348 observes that research and prac- tices have not identified any clear method of establishing The effects of adjacent traffic signals on the operation of spacing standards for unsignalized intersections and that, median openings and U-turn maneuvers include queue spill- moreover, many proposed guidelines have never been imple- backs that block a median opening and the influence of the mented. Standards may be based on speed, stopping sight adjacent signal on available gaps in traffic. distance, roadway function, type of traffic generator, or other The Florida Median Handbook (8) addressed queue spill- considerations. The report presents guidelines for unsignal- backs and states that median openings should not be placed ized driveway spacing that are based on speed, access level, across regularly forming queues from neighboring intersec- size of activity center, and environment (e.g., urban). In gen- tions. Median openings placed too closely to an intersection eral, spacing increases as the size of the activity center and cause both safety and operational problems. The safety prob- operating speed increase. For example, for a minimum use lem is that when these queues build, "good Samaritans" might activity in an urban area on a low-speed roadway with a high allow vehicles in the median opening through the queue with- degree of access allowed, the spacing could be about 15 m out an adequate gap in the adjacent lane, creating a potential (50 ft). In a rural area for a major activity on a high-speed collision with a vehicle moving freely in the adjacent lane. A roadway with allowable access more limited, the spacing traffic operational problem is that when the queue in the could be about 150 m (500 ft). through lane extends past the median left-turn lane, vehicles In a manual for the NHI short course on Access Manage- wanting access to the median opening are trapped in the queue ment (33), various conditions that should be considered in the and are unable to move into the turn bay until the queue determination of unsignalized access spacing are presented: advances.