The speed at which drivers operate their vehicles directly affects two performance measures – mobility and safety. Higher speeds provide for lower travel times, a measure of good mobility. Designers of highways use a designated design speed to establish design features; operators set speed limits deemed safe for the particular type of road; but drivers select their speed based on their individual perception of safety. According to the Federal Highway Administration, however, the relationship of speed to safety is not as clear cut – there exists no established scientific correlation between motor vehicle speed and crash rates:
The relationship between speed and safety is complicated and unclear. There are very few points of consensus on the effects of speed on crash probability. One of the many complicating factors is that high-speed highways (e.g., Interstates) have low crash rates. However, since Interstate highways also have distinguishing design features (e.g., limited access control and wide clear zones), it is difficult to separate the effects of speed from other characteristics.