<Tr> <Td> Goods vehicles over 7.5 tonnes MLW </Td> <Td> n / a * </Td> <Td> 60 mph (97 km / h) </Td> <Td> 60 mph (97 km / h) </Td> </Tr> <Ul> <Li> a small, decreasing number of single - carriageway motorways do exist, both technical / "secret" and overtly signed, but they are almost universally either subject to a signed limit below 70 mph, or too short for any but the most powerful vehicles to exceed 60 mph for any significant distance . </Li> </Ul> <Li> a small, decreasing number of single - carriageway motorways do exist, both technical / "secret" and overtly signed, but they are almost universally either subject to a signed limit below 70 mph, or too short for any but the most powerful vehicles to exceed 60 mph for any significant distance . </Li> <P> Variable speed limits are used on some major traffic roads . These can be changed in response to weather, traffic levels, time of day or for other reasons with the currently applicable speed limit is displayed using an electronic road sign . Signs with the speed shown in a red circle are compulsory, signs where the speed is not within a red circle are advisory and exceeding these speeds while driving safely within the applicable national speed limit is not in itself an offence . Variable speed limits were introduced on some congested major routes as an element of controlled motorway techniques to improve traffic flows for given prevailing conditions . Part - time variable speed limits may also be used outside schools . </P>

When was the national speed limit sign introduced