In December 1973, when the Opec countries cut back supplies of oil to the west, the UK and the US introduced a maximum national speed limit of 50mph on all roads in order to reduce petrol consumption (Netherlands lowers speed limit to comply with pollution court ruling, 14 November). This remained in force until March to May 1974. Maximum speed restrictions, aimed at reducing energy consumption, were again introduced from December 1974 through to May 1977.
When the speed of a petrol-engined car is reduced from 70mph to its optimal speed of 55mph to 60mph, its petrol consumption falls by some 17%. Given that demand for road transport fuels is around 44m litres of petrol and a little over 81m litres of diesel per day, that brings petrol consumption down by 7.5m litres and diesel by 13.75m litres per day. The 1970s restrictions were introduced to save energy. But they also brought down pollution and carbon emissions: every litre of petrol consumed produces 2.3kg of carbon dioxide (diesel produces 2.68kg).