Investigation of National Highway System Roadways in the Highway Safety Information System States

Motor vehicle crash rates and roadway characteristics for National Highway System (NHS) and non-NHS roads in seven states—California, Illinois, Maine, Michigan, Minnesota, North Carolina, and Washington— are presented. Accident and roadway data were extracted from the Federal Highway Administration’s Highway Safety Information System. Overall, crash rates on NHS roads were approximately 10 percent lower than those on non-NHS roads, perhaps the result of higher design standards. Crash rates on urban roadways were considerably higher than on the corresponding rural roadways in each state. For rural roadways, fixed-object crash rates were higher on NHS roads than on non-NHS roads. The distribution of crashes by severity was quite similar on NHS and non-NHS roads. For urban freeways, NHS Interstates usually had lower crash rates than NHS non-Interstates. The majority of NHS roads had lane widths of 3.4 m (11 ft) or more, and many had shoulder widths of 1.5 m (5 ft) or more. Compared to NHS non-Interstate roads, NHS Interstate roads were more likely to have lane widths of 3.7 m (12 ft) or over, shoulder widths of 2.4 m (8 ft) or over, paved shoulders, and improved median design. Highway designers and safety officials can use this type of information about accident rates and roadway characteristics to enhance safety by upgrading existing highways and improving the design of NHS highways to some specified roadway design standards and guidelines.