In many states, single-vehicle run-off-the-road (ROR) accidents constitute one of the most significant traffic accident problems. Described in this paper is a study of the effectiveness of one potentially useful countermeasure--the application of 8-in. wide edgelines. The critical rate technique was used to identify approximately 530 mi of rural two-lane highway with unusually high ROR accident rates. In 1984, 100 of these miles were treated with wide edgelines, and the following year, an additional 76 mi were marked. The remaining mileage was used for comparison purposes. The accident experience on the treatment and comparison sections was monitored after the application of this countermeasure. From the research it is concluded that wide edgelines do not have a significant effect on the incidence of ROR accidents. In addition, this treatment does not have a significant effect on the rate of ROR accidents at night or on curves, or on accidents involving the opposing flow of traffic. It is recommended that this treatment be discontinued on rural highways in New Mexico.