In the United States, where one can pass in any lane, virtually all traffic rigorously obeys the speed limits, and will travel precisely at these speeds, maintaining them for hours on end. Because of this mature attitude to driving, one is seldom overtaken - or undertaken - as vehicles merge between lanes only as they need to make a lane change for exiting or some other purpose. With all lanes travelling at the same speed, all are all fully utilised, allowing more traffic on the road.
In the UK, the right-hand lane is commandeered by small numbers of businessmen in speeding BMWs. If undertaking were allowed, combined with a rigorous imposition of the speed limit that is essential for its safe working, existing motorway capacity would be increased. This would avoid much of the rationale for the Government's continuing environmentally-disastrous road-building programme, while reducing CO2 emissions and making for more relaxed and safer driving.
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