Abstract: This paper evaluated the significance of the posted differential speed limits (DSL) on traffic safety and operation along interstate highways in South Carolina. This paper defines DSL as the difference between the posted maximum and minimum speed limits. The paper used vehicle speed and traffic crashes for analysis. The evaluation results showed over 99% of all vehicles complied with the posted 45 mph minimum speed limit and more than 50% of drivers drove above the posted maximum speed limit regardless of the posted maximum speed limits. The results also indicated that speed variations increased as the DSL increased. However, the safety evaluation showed conflicting results on the impacts of the DSL on safety. The results showed the DSL had negative implications on crashes per AADT per lane. This is attributed to increased interactions between slower and faster-moving vehicles in the traffic stream. On the contrary, it was observed that segments with higher DSL had fewer crashes per mile per lane than the segments with the lower DSL. These results suggest there is a need to conduct more research to quantify the efficacy of posting the minimum speed limit because there were no problems observed with motorists driving too slowly on interstate highways. The results provide important information on the efficacy of posting minimum speed limits to improve the uniformity of traffic flow and safety.