The signs flash a "Slow Down" warning to fast-moving traffic on the main road at Terrington St Clement if motorists are detected waiting to cross the junction.
It is not only a boost to traffic safety but another feather in the cap of family-run Norfolk company Westcotec which specialises in vehicle-activated signs.
Business is booming for them as the recession forces councils to put high-cost road improvement schemes on ice and look instead at cheaper safety measures like illuminated signs.
Phil Hayton, managing director of Dereham-based Westcotec, said that the company had saved Norfolk County Council £40,000 - more than the value of the contract itself - by using radio links between the signs and six detector loops. Otherwise the project would have needed 300m of trench and cabling work, along with disruptive roadworks.
"The council asked us to look at this particular junction because of concerns over its record of serious accidents," said Mr Hayton.
"One of the problems is that it comes at the end of a long straight on the A17 and that some drivers need a ‘wake-up' call to the possible dangers ahead."
The signs flash their warning only if traffic is detected waiting to turn at the central island or to emerge from the side roads, one of which is on a slight slope.
If there is no waiting traffic or vehicles on the A17 are travelling at less than 45mph the signs remain blank.
"The sign only goes on if there is an obvious risk of danger," said Mr Hayton. "I think there is a great future for intelligent signs. I fear that many drivers do not even look at static signs now; they are just part of the clutter at the roadside.
"This system is one of the first of its kind in the country and certainly the first in this region - and we have two more planned near Horsford, which will be solar powered, and another in Derbyshire. It's not the technology which is new but the way we have adapted it."