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Operating Speeds on Curves With and Without Spiral Transitions

Although the design policies of many countries encourage the use of spiral transition curves, questions exist about whether they have any benefit in roadway design. In addition, United States speed prediction models do not account for the presence of spiral transitions. To determine if spiral transitions affect the speed at which vehicles traverse horizontal curves on rural two-lane roadways, speeds of free-flow passenger cars on spiral transition curves were compared with speeds of free-flow passenger cars on circular curves that had similar geometric characteristics. Operating speeds and geometric data were collected at 12 spiral transition curves and 39 circular curves in six states. Using regression techniques, it was concluded that spiral transitions did not significantly affect the 85th percentile speed of drivers on horizontal curves. Analysis of the cumulative speed distributions for different curve radii showed that speeds were higher for spiral transition curves than for circular curves for a 145-m radius (smallest radii studied); however, no data were available to test the hypothesis further. It is believed that spiral transition curves may affect vehicle speeds as the curve radius decreases. It was concluded that spiral transitions did not produce significant differences in passenger-car operating speeds.