The Automobile Association is hoping for a second straight Queen's Birthday weekend without any deaths on the roads.
Last Queen's Birthday weekend had no fatal crashes - for the first time since records began in 1956.
The 4km/h tolerance will be enforced by police this weekend.
AA Motoring Affairs general manager Mike Noon said safety basics are the focus, with a recent climb in road deaths.
About one in three of the people killed in cars in the first four months of the year were not wearing seat belts.