The new maximum speed limit of 100kph will start coming into effect in some but not all motorways on Friday and motorists must heed the signs or face fines, police have said.

The official changeover is not until 6am on Monday but because of the sheer volume of work for the transport department, some stretches of motorway have their new signs – and maximum speeds – from Friday 13.

The department has to change over some 4,000 signs nationwide and tape over 120 and 130 kph notices on thousands more motorway kilometre posts.

‘The new speed limit comes into effect as soon as the new signs have been revealed,’ project leader Egbert-Jan van Hasselt said. ‘There can be no two speed regimes on one road so what it says on the sign is binding. If motorist exceed the speed limit on one of these roads at the weekend they will be fined.’

Although the measure is meant to ease pollution, Van Hasselt said it will also improve road safety. ‘We are happy with this measure. A slower speed gives people more time to react if they have to brake or swerve. And that means fewer accidents as a result of speeding.’

The last time the maximum speed was limited to 100kph was in 1974 under prime minister Joop den Uyl. A unpopular measure then as now, the speed limit was meant to reduce the spiralling number of traffic deaths and save on petrol in the wake of the 1973 oil crisis.