PURPOSES: The objective of this study is to establish the traffic volume-based warrants of right-turn lanes at unsignalized intersections and to introduce a risk probability methodology based on the warrants.METHODS : In this study, a risk probability of a potential rear-end collision is applied between a right-turn vehicle and the immediately following through vehicle. Using the shifted negative exponential model and the compound probability theorem, the risk probability can be expressed as the function of directional volumes and the percentage of right-turns for a two-lane and four-lane highway, respectively.RESULTS : Based on the risk probablity, guidelines for installing right-turn lanes on two-lane and four-lane highways were developed. The risk probability also showed rationality by comparing with right-turn same-direction conflicts observed in-situ.CONCLUSIONS : The results of our study define the total approaching volumes to encourage a right-turn lane as a function of operating speed, percentage of right-turn, and number of lanes.