Designing horizontal curves conforming to driver behavior is key to creating better-designed, safer highways. Doing so is assisted by a clear, quantitative understanding of driver behavior in a real highway environment. This study is concerned with collecting driver behavior data pertaining to steering behavior and using it to find desirable spiral lengths for horizontal curves. To realize this objective, a comparison between driver steering behavior and actual geometric alignment was performed. The profiles showed that drivers, in approaching horizontal curves, changed their behavior gradually to follow a natural spiral-curve-spiral path. Desirable spiral lengths were also related to geometric characteristics of the curve and were found to correlate well with the radius of curvature for two-lane highways and freeways with high coefficients of determination. In addition, the desirable spiral lengths were compared with the different controls of spiral length found in the North American design guides. The comparison revealed discrepancies in the design procedure of the spiral length. It was found that the minimum criteria found in these guides should be revised to better describe true driver behavior. In addition, the paper showed how new recommended values for the spiral parameter and rate of change of lateral acceleration would achieve spiral lengths that conform well to driver steering behavior.

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Series Transportation Research Record
Said, D. (Dalia), Halim, A.O, & Hassan, Y. (2009). Desirable spiral length based on driver steering behavior. Transportation Research Record. doi:10.3141/2092-04