New Zealand's first weather-activated speed signs have been fine-tuned to stop fog at the Kaimai summit of State Highway 29 slowing traffic further down where driving conditions were normal.

The New Zealand Transport Agency has responded to frustrations from motorists by increasing the number of adjustable speed limit signs to 26.

Chief safety adviser Colin Brodie said the trial started with 22 signs but in September following feedback from police and the community, the number was increased so that the signs could be activated at different speeds on three zones of SH29 over the Kaimai Range.

The zones were the steeply descending Waikato side, the summit section and the gentler Tauranga side.

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''This means in very localised conditions, for example when fog is only over a small summit section, the speed reduction can be put in place only for the summit zone.''

The improvements have been welcomed by McLaren Falls Rd resident Brian Patterson whose frequent trips to the Waikato had been frustrated by weather conditions at the summit dictating speeds further down the road.

''It happened quite a lot. There were very few times when the speed restriction was justified,'' he told the Bay of Plenty Times.

It might by a sunny day until he hit cloud at the summit, but the whole section of SH29 from Soldiers Rd to the top was 80km/h, with speed cameras catching those driving at 100km/h.

''You got done for being 20km/h over the limit shown on the signs...I wondered at the logic.''

He has had a four month break from driving over to the Waikato for his job, so was pleased that the issue had been recognised by the agency.

Mr Brodie said the weather-activated signs had been live for one year, with the very early results showing promise. The aim of the trial was to reduce the crash rate on the road.

Results for the first seven months of the trial showed speeds reduced when the signs were on.

''There has been a migration away from serious injury crashes towards minor injury crashes, and an overall small reduction in the total number of crashes,'' he said.

Mr Brodie said the input of locals and others who travelled the route on a daily basis was vital to the success of the trial.

''We need to hear from people so that we can make adjustments and improvements to the trial, like adding the extra zone at the top of the Kaimais. We will continue to monitor the trial, particularly around how we determine when the signs are turned on and off.''

The trial has won two awards for safety and innovation.

Kaimai road's weather-activated speed signs
- Monitored by a weather station and sensors and cameras near the signs
- Speeds outside summit section reduced in poor weather to 60km/h or 80km/h
- Speeds reduced to 30km/h for crashes or other circumstances