Research in the UK and USA over many decades established that speed limits set at the 85th
percentile produce the smoothest traffic flow, minimum speed variance and lowest accident
risk. The safest drivers are those who travel at 5–10mph above the mean speed. By changing
the basis of speed limit setting to the mean speed, half of all drivers, including the safest, are
criminalised if they continue to travel at a safe speed above the limit.
The safe system approach would lower speed limits still further, to the point where drivers
could crash without suffering serious injury or death. Speed limits set this low, however,
would lose all credibility with drivers, leading to high levels of non-compliance, increased
speed variance and consequently higher accident risk. They would also boost the highly
lucrative speed enforcement industry still further and increase the cost of road transport.
Since the road network carries the vast majority of passenger and freight movements in the
UK, the economic consequences could not be ignored.