Evaluation of Criteria for Setting Speed Limits on Gravel Roads

Even though speed limits of gravel roads are typically set based on statutory guidelines, there are some instances where exceptions are allowed and in place. In the absence of detailed literature on the subject, this study made an effort to evaluate and compare two criteria used in setting speed limits on low-volume gravel roads, with the intention of providing guidelines for future use. Field speed data were collected at a number of straight, level sections of gravel roads in Kansas where there is sufficient sight distance. Several sites located in two adjoining counties that use two different criteria for setting speed limits were also specifically selected for comparison purposes. One of the two counties adopts 35-mph speed limits on gravel roads and posts speed limit signs similar to those on paved roads. Characteristics of gravel roads in the adjoining county are very similar except that the speed limit is governed by the statutory limit (55 mph) and not posted. Collected actual speed data indicated no significant differences between the two speed data sets. In addition to operational speed comparisons, crash experience was also taken into consideration, which showed no major differences in terms of safety. When looking at the findings of this study, it is clear that lowered, posted speed limit values on gravel roads have not necessarily helped in improving either operational conditions or safety. Rather, reduced posted speed limits on gravel roads have increased the percentage of speed limit violators. Accordingly, care should be taken in making such decisions in the future for new situations.