Parameters for Evaluation of Speed Differential

Abstract

This paper describes the results of a study performed with the interactive fixed-base driving simulator of the Inter-University Research Center for Road Safety and seeks (a) to select, from the parameters proposed in the literature for the evaluation of the speed differentials between tangents and curves, those that best reflect the real speed reductions experienced by drivers and (b) to develop new models for estimating speed differential values. Thirty people drove the driving simulator on a two-lane rural road with 16 different tangent-curve configurations. On the basis of measured speeds, the parameters were calculated for the 85th-percentile of the distribution of maximum speed reduction experienced by each driver (85MSR) as well as the differential speed not exceeded by 85% of the drivers traveling under free-flow conditions (Δ85V). The statistical analysis performed reveals that the differences between the two parameters were statistically significant and, in so doing, indicates that using Δ85V to measure speed differentials leads to an underestimation of the speed differences adopted by drivers. Three speed differential predicting models were also developed. The models being proposed are similar to those in the literature. These models show that speed differentials are influenced by the same variables that were uncovered in previous field research. The results of the present study corroborate the outstanding potential of the driving simulator as a useful instrument for the analysis of speeds on two-lane rural roads as well as for the effects of road alignment configurations on drivers.