- Typical speed limit: Between 55 and 75 mph
- Fatalities per 100 million vehicle-miles traveled, 2021: 0.62 (5,027 fatalities)
- Fatalities involving speeding: 1,430 fatalities, 28.4% of total
Interstate roadways that cross state lines are funded and maintained by the U.S. Department of Transportation—which is to say by the federal government, as opposed to DOTs in the respective states they pass through. "Principal arterial" roads generally refer to heavily used highways that see higher speeds because they stretch for longer distances.
The Interstate Highway System grew out of the Federal-Aid Highway Act of 1956. One of President Dwight D. Eisenhower's crowning achievements, the IHS has in subsequent decades continued to expand. According to nonprofit organization TRIP, the IHS, while comprising only 3% of the nation's roadways, handles around 26% of all national vehicular traffic. What's more, between 1956 and 2019, just prior to the onset of the pandemic, vehicle-miles traveled on the interstate system have grown by 427% to reach 3.3 trillion miles driven.
In more recent years, the IHS has undergone a sociopolitical reevaluation for the ways in which the initiative disproportionately impacted Black communities through the use of eminent domain. In terms of driving safety, interstates have over time evolved to implement measures that, as of 2019, save an estimated 6,555 lives annually, according to TRIP. These measures include a minimum of four lanes, distance from other roads and train tracks, more gradual curves, median barriers, and rumble strips, which alert drivers when they're leaving the roadway and need to course-correct.