Safety

Crash forces are proportional to the square of the speed. This means that crash forces rise much faster than speed. For example, the percentage increase in crash forces between 70 MPH and 60 MPH is twice the percentage speed increase.

While it is true that higher speed usually increases the severity of crashes, there is not an automatic correlation between speed or a speed limit and aggregate safety. Sometimes a speed limit can be too high, i.e. that a decrease of the speed limit will increase overall safety. Sometimes the speed limit can also be too low, so that an increase of the speed limit increases overall safety.

Many factors define the safety value of speed limits, and some have nothing to do with an individual motorist's vehicle speed. Examples are driver alertness, distractions, road conditions, weather, other drivers, and wildlife. However, unlike speed, some of these factors are not under the full control of the driver. This suggests that limiting vehicle speeds can play a role in reducing the frequency and severity of crashes if only to give the driver more time to respond appropriately in the face of unexpected dangers. However, the efficacy of speed limits in restraining driver speed is subject to debate as several university and government studies in the US have found a weak link between speed limits and actual speeds.[22]

Crash causation should not be regarded as being due to speed or other factors in isolation. These other factors, such as the weather and the condition of the road, interact with speed. This combination of factors may lead to a crash. In many cases, the crash probability could have been reduced if the speed had been reduced. For example, a slower vehicle is less likely to skid on a wet road. Speed plays a part of the causal chain which leads to crashes.

Another view is that speed magnifies the consequences of other unsafe acts, and in of itself, speed is often not a causitive factor. This viewpoint is reinforced by the fact that if speed is a crash factor, it is usually accompanied by other crash factors. Removing one or more of these other factors often can prevent the crash. In the right cases, reducing the speed could reduce the probability of a crash, but it cannot guarantee no crash.

This view can be illustrated with a hypothetical situation: suppose a motorist ignores a traffic control device, like a yield sign, and causes a crash with another vehicle that had right of way but was traveling 10 MPH over the speed limit. Even though under US standards speed could be listed as a crash factor, slowing down the vehicle having right of way would have an unclear effect on preventing that crash. Furthermore, the crash would not have occurred at all had the offending motorist yielded right of way.

Several studies performed in the U.S., including ones performed after speed limit increases in the late 1980s and mid- to late 1990s, produced mixed empirical evidence on the relationship between speed and safety[23]. Sometimes no overall effects was noticed, and sometimes overall fatalities and crashes declined. Some studies found more fatalities, but they are sometimes criticized for inadequately explaining why adjacent roads often had fewer crashes or why systemwide safety effects were minimal or sometimes positive. A theory is that higher speed limits on Interstate roads invite travel on safer roads. Another theory is that higher speed limits encourage slower drivers to match the faster flow of traffic, thus reducing speed differentials, reducing lane changes, and reducing driver frustration.