Abstract
Although the design policies of many countries encourage the use of spiral transition curves, questions still exist as to whether spiral transition curves have any benefit in roadway design. The findings of previous research based on simulation models and driver behavior on circular curves have produced varying conclusions. In addition, speed prediction models that have been developed in the United States do not account for the presence of spiral transitions. The objective of this research was to determine if the presence of spiral transition curves affected the speed in which vehicles traversed a horizontal curve. If spiral transitions were found to have a significant effect on vehicle speed, a parameter quantifying the effect would be created and included in speed-profile models. To accomplish the objective of this research vehicle speeds on curves with spiral transitions were compared to circular curves with similar geometric conditions. Using regression techniques and the full data set, the presence of spiral transitions was not found to significantly affect the 85th percentile speed of drivers on horizontal curves. A model was created to predict 85th percentile speed for both spiral transition curves and circular curves which had the same form as previous models using only circular curves. Analysis of the individual speed data categorized by curve radius showed that for a curve radius of 145 meters, speeds were slightly higher for curves with spiral transitions. Although no data were available to further test the hypothesis, it is believed that the presence of spiral transitions may have a greater affect on vehicle speeds as the curve radius decreases. Based on the results of this research, it was concluded that for the range of data analyzed, the presence of spiral transitions was not shown to produce any significant differences in operating speeds for passenger cars. Therefore, it is recommended that speed prediction models do not have to account for the presence of spiral transitions when used to estimate 85th percentile speeds of passenger cars on horizontal curves.
Passetti, Karl Andrew (1998). Comparison of operating speeds at spiral transition and circular curves. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1998 -THESIS -P377.