This study presents an empirical investigation into the use of a new measure, percent impeded, for estimating performance on two-lane highways. Percent impeded is a point estimate of the percentage of vehicles that are impeded by slower vehicles in the traffic stream due to platooning, a common phenomenon on two-lane, two-way highways. This point measure, by definition, is closest in concept to percent-time-spent-following, a spatial measure of performance on two-lane highways that is used in the current practice yet is impractical to measure in the field. Field data were collected from two two-lane highway study sites with passing lanes in the state of Montana. Performance was examined at a single location upstream and multiple locations downstream of the passing lane at each study site. As a reference, the analyses utilized some of the currently used or suggested performance measures on two-lane highways. Overall, the percent impeded patterns and trends were consistent with the mechanism of platoon break-up and formation in the vicinity of passing lanes, a concept that is well-documented in the literature. Further, study results suggest that percent impeded has more logical and consistent sensitivity to platooning variables compared with other performance measures investigated by this study.