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Most of the studies available in the literature indicate that the speed data on a highway follow the normal distribution. The present study indicates that the speed distribution may be a unimodal or a bimodal curve depending upon the variation in the speed of different categories of vehicles moving on the highway. The mathematical equations are suggested for unimodal and bimodal speed distribution curves, and their parameters are evaluated at 17 different sections of two-lane roads in India. It is observed that the proportion of slow moving vehicles is not a true indicator of bimodality in the speed data. The spread ratio defined as the ratio of the difference between 85th percentile and mean stream speed to the difference in mean stream speed and 15th percentile speed, is better correlated with the shape of the speed distribution curve. The speed data follow the unimodal curve only when the spread ratio is in the range of 0.69–1.35.
Dey et al. (Tue,) studied this question.