Cyclists are thrilled with a new solar-powered hazard sign Auckland City has installed for about $60,000 at the dangerous intersection of Tamaki Drive and Ngapipi Rd.

The prominent 5m sign, which began operating last week, flashes up in orange lights when approaching city-bound cyclists ride over loops in the road surface in Tamaki Drive, about 30m east of the intersection.

It warns motorists facing away from the city while queuing up to turn right at the intersection, into Ngapipi Rd, that cyclists are about to cross their bows from the opposite direction.

The council's transport committee decided to run the sign as a trial after 12 cyclists were injured in crashes at the intersection over five years, making it the city's second-worst black spot for pedallers.

Plans to install it in February were brought forward after a young woman drove into a pack of cyclists in September at the St Heliers end of Tamaki Drive, injuring four and prompting the council to set up a forum of interested groups to improve safety along the popular waterfront route.

The council has also erected 10 blue static signs along the route with messages to road users such as "Slow down and enjoy the journey" and "Take care, watch out for each other".

Council network performance manager Karen Hay said much of the cost of the flashing sign was for ducting laid beneath the road to carry "incredibly sensitive" electronic loops triggered by the light weight of a bicycle.

She compared the price of about $60,000 with an estimated social cost of about $1.9 million from the crashes which injured the 12 cyclists at the intersection, two seriously.

That sum was exceeded at only one other Auckland intersection, that of Mt Smart Rd and Rockfield Rd in Penrose, where the death of a cyclist in a crash last year carried a social cost which was estimated at about $3.5 million.

Although no cyclists have been killed on Tamaki Drive in the past five years, the council has calculated a cost of about $9.8 million for 39 injury crashes at 15 intersections along the way, of which Ngapipi Rd is the most notorious.

Ms Hay said the council also considered turning it into an intersection controlled by traffic lights, but there was not enough space without widening an approach bridge in Tamaki Drive, which would have pushed the cost to $5.4 million. Although the new sign is working well, Ms Hay urged cyclists to remain in a cycling lane when approaching the intersection, to ensure its sensor loops were triggered.

Cycle Action Auckland deputy chairwoman Barbara Cuthbert said cyclists were thrilled that the sign had been put in place for summer, when Tamaki Drive is at its busiest.

"It's a particularly hazardous intersection for cyclists and we are thrilled to bits," she said.

"I think the council has taken a very important initiative."

City transport committee chairman Ken Baguley will consider longer-term suggestions from the safety forum in February to "allow our world-class Tamaki Drive to be enjoyed by all users in a co-operative and mutually beneficial manner".