Abstract
The 70 mile-h speed limit was introduced on all roads in Great Britain (except those already subject to a lower limit) in December 1965. During a speed limit trial there was a marked reduction in the speeds of cars on motorways and an accompanying reduction of about 20% in motorway causalities compared with expected. On all-purpose main-roads subject to the 70 m.p.h. speed limit there was only a slight reduction in speeds and a small reduction in accident rates during the trial year. On the remaining all-purpose roads there was virtually no difference between the actual and expected numbers of accidents.