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Diagonal Dlverters Cul-de-Sac Closures intersection to convert the intersection into two unconnected streets, each making a sharp turn. Its primary purpose is to make travel through a neighborhood circuitous, while not preventing such travel. Used alone, the diverters will affect only the two specific streets involved. This application is most effective in reducing traffic volumes if used as part of a planned system for the neighborhood that will discourage through traffic. Smith et al. (1980, 31) note that "In a system of devices, traffic on streets with diverters can be reduced from between 20 to 70 percent depending on the system of devices in the area." Diverters are effective in reducing traffic volumes, whereas speed is reduced only in the immediate vicinity of the divdrter, within about 200-300 feet. Studies done in Seattle, Washington, and Richmond and Berkeley, California, have shown a significant r&iuction in the number of accidents in the neighborhoods. Usually, however, the actual number in each case was small originally (Smith et al. 1980, 31). In order to have diverters function safely and effectively, they should incorporate the following features: · Visibility. Devices should have painted curbs, rails, reflectors, directional signs, street lights, and elevated landscaping. · Delineation. Centerline pavement striping and, where possible, pavement buttons are helpful in identifying the driving path. · Emergency vehicle access. The design of the diverter should allow for passage of emergency vehicles while restricting automobile passage. · Pedestrian, bicycle, and disabled access. Sidewalks across the diverter should allow such access. ad-end Streets or Cul-de-sacs In some communities, traffic volumes in older residential areas have become so problematic that streets have been converted to dead- ends or cul-de-sacs to prevent cut- through traffic. A cul-de-sac is a complete barrier of a street at an intersection or mid-block that leaves the block open to local traffic at one end while physically restricting through traffic. Studies have shown cul-de-sacs or dead- end streets to be very effective in reducing traffic volumes. Due to the need for adequate turning radius, retrofitting an existing street can be very e~, pensive. On existing streets, it is often the case that only an 18- to 20-foot turning radius can be provided, whereas in new subdivisions 35 feet (10.5 m) is standard. The appropriate length of a street that can be dead-ended should be determined by traffic volume and the number of houses on the street. In general, however; cul-de-sacs should probably not be installed on streets longer than 500 feet when lots are 50 feet wide, meaning there would be approximately 20 houses on a street generating eight t~ 10 vehicle trips per day (NAHB 1990, 55). Streets longer than 500 feet tend to lose the advantages of inst~lllng a cul-de-sac because there are likely to be increases in traffic speeds and mid-block turnar~unds, a potential safety hazard. The number of properties on a longer street also means an increase in the volume of . trips on that street, again reducing the safety factor that the cul-de-sac should bring. A cul-de-sac should be clearly identified by signs indicating that the street is not a through street. In some cases, provision for passage of emergen.c~, vekicles through the cul-de-sac iixa-y be desirable. Existing movement of pedestrians, bicyclists, and people with disabilities will need to be evaluated and accommodated by provision of through sidewalks and/or ramps. The use of dead-end streets and cul-de-sacs to reduce traffic volumes is one of the most expensive and least desirable techniques employed for traffic calming due to issues of accessibility for emergency vehicles, buses, etc. Caution should be employed in making use of this technique. Semi-diverters, Neck-downs, Chicanes, Chokers, and Protected Parking A serai, diverter is a barrier to traffic at the intersection of two streets in which one direction of the street is blocked, but traffic from 11