A time series analysis was performed on fatal accidents, injury accidents, vehicle miles travelled, and vehicle speeds over the 5 years preceding and 1 year following the increase in the national maximum speed limit (NMSL) allowed during the spring of 1987 on rural interstate highways. In the states that raised their limits to 65 mph, speeding on rural interstates increased by 48% and fatal accidents increased by 27% over projections based on previous trends. A 9% increase in speeding and a 1% increase in fatalities were observed on highways still posted at 55 mph. In the states that retained the 55-mph limit, fatal accidents increased by slightly more than 10% both on rural interstates and other posted highways coincident with the change in the NMSL. Speeding on the two classes of highways increased by 18% and 37%, respectively. The total increase in fatal accidents attributed to the raised speed limit, both in 65-mph and 55-mph states, was estimated at approximately 300/year. A shift of high-speed traffic to rural interstates from other highways may have contributed to the changes occurring in the 65-mph states. The increase in fatal accidents on 55-mph non-interstate highways in states that did not raise their limits may have been caused, in part, by the absence of such a shift. This paper appears in Transportation Research Record No. 1281, Human Factors and Safety Research Related to Highway Design and Operation 1990.
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