The RTA is advising motorists that the speed limits on the Wardell and Harwood bridges will be changed from August 3 to improve safety.

A spokesperson said the new 80km-h speed limit at both bridges was the result of an RTA speed zone review.

The review was undertaken with the new NSW Speed Zoning Guidelines and the new Traffic Signal Design Manual.

The bridges have 100km-h speed limits.

At the Harwood Bridge the 80km speed reduction will start about 1km south of the bridge, extending to 1km north of the bridge.

At the Wardell Bridge the 80km-h speed limit will start about 350m south of the bridge, extending to 500m north of the bridge.

The RTA says large signs will be put in place notifying motorists of the change prior to new speed zone signs being installed at each location.