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44 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections The distance that the treatment extends from the intersection proper should be related to stopping sight distance and/or distance to achieve the desired deceleration at a comfortable deceleration rate. The distance should also be selected to work in concert with other treatments or features such as in-pavement markings at pedestrian crosswalks, lane drops or adds, or warn- ing signs. The markings have potential to draw additional attention to those warning signs and to encourage drivers to reduce their speeds as they approach the intersection. The standard width for longitudinal pavement markings is four inches. Wider pavement markings generally range from 5 to 10 inches wide. 4.6.4 Speed Effects No specific information was found to describe the impacts of wider longitudinal pavement markings on speed at intersection approaches or roadway segments. The increased visibility and comfort associated with wider pavement markings could lead to increased speed in some appli- cations. Although this treatment may not directly affect reduced speeds, it may increase driver awareness of an impending intersection, thereby indirectly reducing speeds if drivers perceive a greater risk. FHWA is conducting demonstration projects in Alaska and Tennessee to evaluate the impacts and effectiveness of increasing the width of pavement marking edge lines from four inches to six inches; a report is due in June of 2009. 4.6.5 Safety Effects There is no conclusive data associated with the crash-reduction effects of wider longitudinal pavement markings. Cottrell (1988) conducted a before-and-after evaluation of wider pavement edge lines on rural two-lane highways in Virginia and found no evidence to indicate any safety benefit. Conclusive evidence suggests wider longitudinal pavement markings are easier for drivers to see, which can contribute to roadway safety. (Gates et al., 2002) The greatest improvement in visibility is achieved at night. 4.7 Roundabouts 4.7.1 Overview The high-speed intersection treatments discussed in this section are widely used throughout the United States, United Kingdom, France, and Australia. Roundabouts use intersection geometry to reduce speeds and conflict points. These treatments can be applied to a variety of applications. Design variations include roundabout type, size, number of lanes, and geometry. Secondary effects and considerations should include operations, right-of-way, access, horizontal and vertical geom- etry, and driver expectancy. 4.7.2 Applicability and Considerations A roundabout is a type of circular intersection with specific design and traffic control features to ensure that travel speeds on the circulatory roadway are typically less than 30 mph (50 km/h). These features include channelized approaches and geometric curvature. (FHWA, 2000b) The following qualities distinguish a roundabout from other circular intersections:

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Treatment Descriptions 45 Yield control is provided on all entries, Right-of-way is given to circulating vehicles, Pedestrians can cross only the roundabout approaches (behind the yield line), Parking is not allowed within the circulatory roadway or at the entries, and Vehicles circulate in a counter-clockwise direction. Roundabouts have been designed to improve some of the safety and operational deficiencies that occur in other types of circular intersections, such as traffic circles and rotaries. The specific design features of a roundabout can reduce speeds and the number and severity of collisions. Roundabouts are appropriate for locations with a high crash frequency or severity, intersec- tions where queues need to be minimized, intersections with irregular geometry, intersections that need to accommodate U-turns, and areas with a large amount of right-of-way available. Pedestrians are accommodated at a roundabout by crossings through splitter islands located around the perimeter of the roundabout. Roundabouts potentially are difficult for visually impaired pedestrians to navigate. NCHRP Project 3-78, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities, provides additional information on the impacts roundabouts have on visually impaired pedestrians. At single-lane roundabouts (see Exhibit 4-12), bicyclists have the option to mix with traffic or use the pedestrian crossings. At multilane roundabouts, bicycle paths should be separate and designated from the circulatory roadway, for example, by providing a shared bicycle-pedestrian path. Landscaping placed on the center island of a roundabout may need to be maintained to keep the intersection aesthetically pleasing and to manage intersection sight distance. A maintenance program should be developed for the landscape design of a roundabout. (FHWA, 2000b) 4.7.3 Treatment Layout/Design Roundabouts can be adapted to a variety of user and vehicle types based on the environment, number of lanes, and space used by the intersection. Identifying the proper dimensions of the (Credit: Oregon DOT) Exhibit 4-12. Single-lane roundabout.

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46 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections key design features--including an appropriate design vehicle--is critical to ensure proper oper- ations and safety for all users at roundabouts (FHWA, 2000b). To distinguish roundabouts from other types of circular intersections, key design features such as a central island, splitter islands, circulatory roadway, yield lines, pedestrian crossings and, in some cases, an apron, have been defined and are shown in Exhibit 4-13. A mountable apron may be designed around the perimeter of the central island to provide additional width required for tracking through a single-lane roundabout. Large vehicles may use the entire circulatory roadway at double-lane roundabouts to track and maneuver. (FHWA, 2000b) Single-lane, four-leg roundabouts have a typical daily service volume of approximately 20,000 entering vehicles per day. Two-lane, four-leg roundabouts have a typical daily service volume of approximately 40,000 entering vehicles per day. (FHWA, 2000b) 4.7.4 Speed Effects Roundabouts have the potential to lower speeds to allow drivers more time to react to poten- tial conflicts. Traffic measured at 43 locations revealed that the geometry yields 85th-percentile entry speeds between 13 and 17 mph. (Rodegerdts et al., 2007) 4.7.5 Safety Effects One of the most significant benefits of roundabout installation is the overall improvement to intersection safety. Crash reduction research for conversions of all types of intersections to roundabouts (55 sites studied) found a 35% reduction in all crashes and 76% reduction in injury crashes. Specifically for rural two-way stop-controlled intersections that have been converted to roundabouts (10 sites studied), research found a 72% reduction in all crashes and an 87% (Credit: FHWA, 2000b) Exhibit 4-13 . Key roundabout design features.