Up to 20 per cent of the flexible safety posts installed on State Highway 1 are expected to be skittled during the settling-in period.

About 410 fluorescent flexible posts have been placed on a 10km stretch of State Highway 1 between Springfield Rd and Toetoe Rd, south of Whangārei.

The poles are in the middle of a half-metre-wide centre line with a yellow no passing line on both sides and the poles also have reflectors.

The posts are at 20m intervals but there are gaps left at driveways, intersections, bridges and right hand-turn bays.

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NZ Transport Agency Northland system manager Jacqui Hori-Hoult said the agency had worked with residents who have expressed concern about the placement of posts outside their properties.

Hori-Hoult expected about 20 per cent of the poles to become dislodged in a three-month, initial settling-in period. The poles have been in place for about a month.

Last week the agency was aware of about 30 poles that had to be replaced which was to be done at night as soon as the work could be scheduled.

Hori-Hoult said the flexible safety posts had a permanent fitting on the road and were designed to flex or come off if hit.

"It is a concern that the posts are being hit. It tells us that vehicles are continuing to cross the centre line and put themselves and other road users at risk," Hori-Hoult said.

"There are 16,000 vehicles on this road every day, 12 per cent of them heavy vehicles. Almost all of them are keeping well to the left, driving with care and not overtaking. We have had good feedback from a range of stakeholders and road user groups since the safety improvements have been installed."

In the five-year period between 2012-16 head-on crashes accounted for seven of the 14 deaths and serious injuries on the stretch of road.

In November 2017, three people died in a crash when a car crossed the centre line and there was a fatal head on crash in May.

The safety improvements were to deter drivers from crossing the centre line, either in error or to overtake other vehicles, and would reduce the chances of a serious crash and make the road more forgiving of human error.