The signs have been successfully used in other parts of the country, including Kaiapoi and at Himatangi, he said.
The signs will activate if a vehicle is waiting at, or approaching, the intersection from the side road.
The warning sign will then flash and illuminate, reducing the speed for vehicles on State Highway 2 from 100km/h to 70km/h, a legally enforceable speed limit. If there are no vehicles waiting or approaching on the side roads, then the open road speed limit of 100km/h will not change.
Masterton Acting Traffic Sergeant Shayne Nolan said police would be enforcing the new reduced speed limit, although there would be times when they would need to use judgment.
"We will know when that sign changes and we will be able to see and judge how far away they [drivers] are and if they would have a reasonable amount of time to react.
"Obviously, there will be some judgment calls on it, because we've got to give people a reasonable amount of time to react to the sign but people can't ignore it and not expect enforcement action to be taken."
A light on the back of the sign would be illuminated when the sign was activated, which would mean police would know when drivers should be slowing down.
The intersection was a high-crash area, with a crash there as recently as last weekend, Mr Nolan said.
"I can only see it having a positive effect on the number of crashes, as long as the public takes on board the reasons for it."
As well as reducing crashes, it was expected the signs would improve the flow of traffic, he said.
"It's not just so that people can slow down and have more time, it's so that if the traffic slows down it gives more time for turning traffic to fit in."
No changes were expected to be made to the passing lane.
Wairarapa Road Safety Council manager Bruce Pauling said the initiative was a good idea.
"It has always been an intersection of concern and we do know any turning accident can often result in serious injuries or death because of the nature of the impact and the speed of the vehicles."
Preliminary data showed the signs were successful in reducing speed, he said.