Transport Minister Simeon Brown told a Budget scrutiny select committee on Thursday afternoon it was "unacceptable" that NZTA did not know these costs.
It was "one of the reasons why we've been in a situation where traffic management expenditure has become, in many cases, the number one cost of doing business on our roads", in the way it impacted on the wide range of uses of roads.
On top of making the agency start to measure them, a new less prescriptive code was coming in that should be more efficient, he told MPs.
"I've received advice that in some cases where traffic management's been proscribed, it's actually more dangerous putting out the road cones than it is actually doing the work," Brown said.
"So we actually just have to take a safety at a reasonable cost approach, rather than a safety at any cost approach."
The new code that was more like Australia's risk-based one "will make a significant difference".
He added a recent Waka Kotahi study found that, at many sites lots of road cones were being left out unnecessarily, frustrating motorists.
"There's been an infestation."
The agency would be much more rigorous about cones, Brown said.
Waka Kotahi has said that traffic management costs were included in tenders for an overall job, and not able to be separated out. They varied a lot but tended to account from between three and 10 percent of the capital cost of a project, it said.
Brown said contractors and councils would also be expected to cut traffic management costs.
Auckland Council said last year it spent at least $145m on temporary traffic management a year.
Contractors assess risk at each site - NZTA
NZTA chief executive Nicole Rosie later told the committee the current approach was compliance based - if contractors "tick all the boxes, they think they've met their health and safety standard" - which could lead to either too much, or even too little, traffic management.
Risk-based meant to a contractor gauging risk at a specific site, then making traffic management to suit, for instance on a big project like the Brynderwyns, dispensing with cones and closing the whole road to make for a quicker, safer job.
The traffic management cone shift was "very difficult for the industry" and a big change for the supply chain. which demanded NZTA leadership, Rosie said.
"We're all over it."
