At 1pm Thursday traffic lights will be switched on to manage traffic, one way at a time, on the Iron Bridge.
Minster for Regional Transport and Roads Paul Toole said the lights would support the construction of the new Camp Street Bridge.
"Switching on this set of traffic lights is a milestone for Forbes and a major milestone as part of the Camp Street Bridge replacement project," Mr Toole said.
"Iron Bridge was built in 1893 and will be used as a heavy vehicle detour during the replacement of the Camp Street Bridge.
"To prepare the bridge to be used as a detour, repairs to the deck, timber walkway and resurfacing the bridge deck were completed.
"Once the traffic lights are switched on, the Iron Bridge will be permanently reduced to one-way traffic for the safety of all road users."
The historic Camp Street Bridge will be closed on Sunday August 25 for construction of the new bridge to start on August 26.
Trucks travelling into Forbes on the Eugowra Road will be diverted via the Iron Bridge during construction.
Vehicles over 22.5 tonne (excluding buses) are advised to allow about 22 minutes additional travel time, the detour is about 15.5km via Wirrinya Road.
"It is an important milestone in preparation for the $12.8 million project to replace the existing Camp Street Bridge with a brand new bridge," Mr Toole said.
The new bridge will have two 3.5m lanes with a 1.5m shoulder on each side.
In addition, it will have cyclist / pedestrian footpaths.
Work on the Camp Street Bridge is expected to be completed by August 2020, weather permitting.
For more information on the project visit https://www.rms.nsw.gov.au/projects/western-nsw/forbes-bridge/index.html