Changeable weather on the Kaimai Range between Waikato and the Bay of Plenty is to start being reflected in the speed limit on State Highway 29.
Rain, fog and ice are a constant concern for motorists travelling over the hill.
A two-year trial by the New Zealand Transport Agency using New Zealand's first weather-activated road signs is designed to encourage people to drive at safe speeds.
The system involving 22 high-tech signs and with four web cameras will come into force tomorrow.
It is linked to a MetService weather station near the summit which detects adverse conditions and sends an alert to NZTA's Auckland traffic operating centre. The traffic centre checks the cameras and can activate a change to the speed limit on the signs.
NZTA said more than 70 percent of crashes on the Kaimai Range happened in wet weather and more than 40 percent were caused by drivers travelling too fast for the conditions.
Speeds can be reduced down to either 80 or 60 km/hr 30 km/h if there is a crash or road works.
NZTA chief advisor for road safety Colin Brodie said if the trial was successful it might be rolled out across similar sites.
The speed limits will be enforced by police.