Geometric Design Consistency of Multiple Horizontal Curves on Two-lane Rural Highways

Safety on rural highways is of great concern as nearly two third of road fatalities are found to take place on such roads. High speed of vehicles is the characteristic of rural highways. Geometry is one of the factors that control the speed of vehicles and consequently, the crash occurrence. Alignment is one of the components of geometry and an abrupt change in the alignment is a leading cause for crashes in rural highways. Presence of a curve after a long tangent or a sharp curve after a flat curve is an example for inconsistency in the alignment. Such an alignment brings about unnecessary and unhealthy speed changes that may lead to crashes. Thus, a highway design can be evaluated based on consistency in geometry. Operating speed is the measure adopted for consistency evaluation in this study. The objectives of this study are to develop operating speed models for different classes of vehicles at salient locations of multiple horizontal curves and to develop criteria for evaluating the geometric design consistency. Data of 30 sites were made use for this purpose. Multiple linear regression technique was adopted for modelling. Models were developed for the tangent section and middle of curved sections. Length of preceding tangent is the variable influencing operating speed at tangent. Radius and length of curve significantly influence operating speed at middle of first curve and radius of first and second curve influences operating speed at middle of second curve. Operating speed Deviation from Design speed (ODD) and Speed Reduction (SR) are the consistency measures and Equivalent Property Damage Only (EPDO) crashes is the safety measure used in the study for developing criteria. The criteria developed can be used to evaluate the curve as a single element, and as successive elements (combinations of tangent/curve or curve/curve). Alignment can then be classified as Good, Fair or Poor using these criteria, and it helps a designer to make suitable modifications in the design of a multiple horizontal curve from safety point of view.