Tony King, site manager for agency contractors Fulton Hogan, said the speed of vehicles driving through the roadworks had increased lately despite more signs, pre-warnings, and other traffic-calming devices such as bollards being put in place.
"There are 16 signs that point to 30km an hour and if people stick to the posted speed limit they won't fall off the road and will negotiate the curves safely," he said.
Mr King said crews were at present working way below the walls but when their work joined the existing road, they could be at risk from speeding drivers.
Contractors work from 7am to 6pm daily and expect repairs to be completed in two months, barring bad weather.
Speeding is also a concern at the Brynderwyn Hills where contractor United Civil is working on a $16 million safety improvement.
Company managing director Andrew Campbell said speeding and vandalism of road signs have been a constant problem since work started in December last year.
The agency has been working with police to increase its presence in the area, as well as increasing the number of road cones and road signs through the work site to remind motorists of the reduced speed limit and that they should drive to the conditions.
"Speed controls are in place for a reason. It's difficult to understand why people speed... probably they don't recognise the risk to themselves and to other road users," Mr Campbell said.
Mr Campbell said the New Zealand Transport Agency spent hundreds and thousands of dollars on traffic control at major roadworks and it was disappointing motorists continued to ignore signs.
He said workers had noticed cones knocked over, wheel marks, and beer bottles strewn over road works and had reported the matters to police. Those acts were mostly done when the site was not managed, in the night or over the weekend, he said. Police vigilance on the Brynderwyn Hills was a deterrent.
The agency was to carry out maintenance on the south side of the Brynderwyn Hills last night. Work included repairing potholes, clearing drains, re-erecting road signs that had been damaged, as well as cleaning signs to improve their visibility.