<P> Speedometers were made compulsory for new cars in 1937 . </P> <P> A 20 mph (32 km / h) night - time speed limit for built - up areas was introduced in 1940 as an attempt to halt the increase in the number of road casualties occurring during the World War II blackouts . Following the introduction of blackouts fatalities rose on speed - limited roads from 289 in March 1939 to 325 in March 1940 . For October 1940 the total number of deaths during daylight (when the speed limit didn't apply) fell, in relation to those for October 1939, from 511 to 462, whereas the figures for the black - out hours (when the speed limit did apply) rose from 501 to 684 . The highest number of deaths in any one year in the UK occurred the following year (9,196 people in 1941). </P> <P> On 1 October 1956, the 30 mph (48 km / h) speed limit for built - up areas became permanent under the Road Traffic Act 1956 . The speed limit, introduced on a trial basis in 1935, had relied on being renewed by Parliament each year . The maximum speed limit for goods vehicles was raised from 20 mph (32 km / h) to 30 mph (48 km / h) in 1957 . </P> <P> In addition, around 1958 some 30 mph roads had the limit raised to 40 mph to improve transit times, an early example being on Croydon Road in Mitcham, Surrey, saving, it was estimated, 33 seconds in journey time across Mitcham common . </P>

When did speed limit change on dual carriageways