Reliability-based analysis of road geometric design has been reported in literatures as a trend in transportation safety study. In the reliability analysis of a horizontal curve, the performance function is usually formulated as a function of the failure mode of vehicle skidding only. This paper takes into account the failure modes of vehicle skidding and rollover in formulating the performance functions of cars and trucks, respectively. A comparative study of three different performance functions for calculating the probability of vehicle failure modes is conducted. The results qualitatively show that trucks are more nearly of skid and rollover with on wet pavement. A vehicle's is greatly influenced by vehicle suspension and roll motion. A sensitivity analysis investigating the influences of vehicle parameters and super elevation on vehicle skidding and rollover follows is done. The probability of failure for the radius from 700m to up by AASHTO at various super elevations and design speeds were more safe design.