Effect of geometric road characteristics on deceleration rate in transition from tangent to horizontal curve for two-lane rural roads

Evaluation of geometric design consistency presents an important safety aspect in road design. The most commonly used consistency criteria are based on defining operating speed variations along the road segment, usually by depicting operating speed profiles. Many models have been developed worldwide to predict operating speed on curves and tangents. However, it is not possible to create a continuous operating speed profile if transition acceleration and deceleration rates are not defined. Under this scope, this paper presents a study of the deceleration phenomenon based on speeds continuously measured on a segment of a two-lane state road by using a GPS device. Dependence of driver behaviour in tangent to curve transition on geometric characteristics of the road was analysed. The regression analyses show that the deceleration rate is only affected by the curve radius. More specifically, higher deceleration rates generally correspond to transitions between independent tangents to sharper curves that represent inconsistent road alignment.