An empirical bayes before-after study was conducted to examine the impacts of differential and uniform speed limits. Treatment sites included about 100 miles of interstate roads in Illinois where car speed limits were increased to 65 mph and truck speed limits remained at 55 mph in 1995/1996, and about 95 miles in North Carolina where both car and truck speed limits were increased to 65 mph. Reference groups in both States included urban sections where the speed limits remained at 55 mph and rural sections where it remained at 65 mph. The results regarding truck-involved crashes were inconclusive and depended on the reference group that was chosen.