Influence of Curve Geometry Factors on Driver’s Speed Decision Making When Passing through a Horizontal Curve

The curve geometry influences the driver’s speed choice behavior when passing through a horizontal curve. Hence, speed prediction models are essential to ensure highway safety, particularly under heterogeneous traffc conditions prevailing in India, where various classes of vehicles occupy the roadway space. Toward this, the present study aims to develop an operating speed increment and decrement model to identify the influence of the highway geometry variables on the drivers’ speed choice behavior. The National Highway NH-953 is located on hilly terrain, from which 15 curves with various geometric characteristics were selected. The speed data were collected using the radar gun for three dominant vehicle types: motorized two-wheelers (2Ws), cars, and heavy commercial vehicles (HCVs) at three curve points. After that, the two types of speed differences are calculated: the differential of 85th percentile speed ($\mathrm{\Delta }{V}_{85}$) and 85th percentile speed differential (${\mathrm{\Delta }}_{85}V$). The backward stepwise regression (BSR) technique was used on the collected speed data to develop the operating speed increment and decrement models. The results revealed that the curve geometric variables like curve radius, deflection angle, curve length, and tangent length significantly influence the drivers’ speed choice behavior passing through the horizontal curves. Besides, for all three vehicle types, speed increments and decrement models $\mathrm{\Delta }{V}_{85}$ underestimated ${\mathrm{\Delta }}_{85}V$ by 3 to $13\mathrm{km}/\mathrm{h}$.