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
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.
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. 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.