Local street roundabouts - design for speed regulation

The road geometry provided by the construction of a local-street roundabout, primarily introduces a Designed sub-standard curve in an otherwise straight section of road. Motorists are forced to reduce their vehicle speeds to traverse these devices in comfort. Research using spot-speed readings of free flowing traffic through eight roundabouts has established an empirical formula based on radius of travel path and the 95th percentile vehicle speed through the roundabout. Currently the design of roundabouts in accordance with the NAASRA design guide for roundabouts produces a device which is ineffective in the reduction of traffic speeds. The use of the empirical formula and the design process developed has produced local street roundabouts that are efficient and effective in moderating vehicle speeds. Spot speed surveys from recently constructed roundabouts designed by this process confirms the speed regulating effect of these roundabouts (A).