Operational and Safety Effects of Transition Curves in Highway Design - a Driving Simulator Study

The critical point in rural road safety and road design is evaluation of perceptual properties of road curves. This research project deals with geometrical aspects of horizontal curves design that enhance road safety and it covers experimental studies of driver’s perception, using CRISS simulator. Experimental research goal is to test and to evaluate the effect of geometric road parameters (width and number of driving lanes, curvatures, transition curves, etc.) and road environment characteristics (visibility related) on drivers behavior expressed by the speed adopted indicators which are related to driving safety. The main objective of this work was to test the effect of rural road design parameters, especially horizontal curves geometry, on driver’s behaviour directly related to road safety. The detailed objectives related to driving behaviour on curves, were: 1. To test the effect of curve radius on driving behaviour and risk perception, 2. To investigate the effectiveness of spiral transition curves (clothoids), 3. To evaluate the road visibility effect on driver’s behaviour (differences in risk perception on curves without and with restricted visibility), 4. To test the effects of road category (different design speed and cross- section parameters) on driver’s behaviour in adopting driving speed and trajectory. A driving simulator experiment was designed, where four independent variables (cross-section and horizontal curve geometric parameters) were manipulated. The laboratory experiment, pilot studies and the main experimental works were conducted at driving simulation laboratory of The Italian Interuniversity Research Centre of Road Safety, CRISS. Analysis presented in the paper are focusing on relations between experimental variables and driver’s behavior related to driving speed and other advanced behavioral indicators connected to driving trajectory, speed choice, risk perception and curve geometry. The results obtained demonstrate the effectiveness of clothoids as transition curves in road paths in terms of road safety. The variable effects of road visibility on driver’s behavior under different geometrical conditions and road curve scenario are shown. The general results show that the advanced techniques of visualization and simulation of road space can disclose the relationships between design road parameters and behavioral aspects important to create safer road infrastructure.