An independent report of the policy that has seen speed limits lowered on particularly dangerous stretches of roads has found reducing a 100km/h speed limit to 80km/h on a 76km stretch of road prevented 34 crashes in 12 months.

The report reckoned the lower speed limit added 0.5 to 2.8 seconds per km travelled, increasing journey times of between 36 seconds and 3.6 minutes across a single journey.

A cost-benefit model reckons the avoided crashes created $93 million worth of benefits, while the slower journey times created costs worth $1.3m that year. A sensitivity analysis showed benefits range from $65m to as high as $120m.

The review was conducted by Ernst and Young for Waka Kotahi NZ Transport Agency. It looked at a 76km stretch of State Highway 5 between Rangitāiki and Esk Valley and took in the period between March 2022 to February 2023.

The particular stretch of road is particularly dangerous, with 16 fatalities and 75 serious injuries occurring there between 2010 and 2019. Crashes during that period led to 250 injuries.

The benefits comprised not just crashes avoided, which were worth $0.9m each, based on a Waka Kotahi model, but also a reduction in the severity of crashes that did occur, which was worth $3.2m a crash or $62m overall.

Slower speeds are an election issue. Labour significantly scaled back the speed limit programme earlier this year, but some roads are still having speed limits lowered. National wants to scrap the policy altogether, and has said it would do so if it wins the election.

Transport Spokesman Simeon Brown would change the rules Waka Kotahi uses for setting speed limits in such a way that puts most state highways limits back to 100km/h and local roads to 50km/h.

He said there would be an exception for areas where these speed limits were obviously unsafe.

Brown told the Herald that the review was “only over one year so it’s important to take that context.

“I think it’s important to take that in its context. I note that a number of justifications for reducing speed limits are using reports that have only one or two years of data to try and assert the evidence,” Brown said.

He said the route was “critical” for freight operators in and out of Hawke’s Bay.

Labour’s transport spokesman and the current minister, David Parker, said his party recognised “speed reductions shouldn’t be widespread, so we have limited the approach to the most dangerous one per cent of our roads”.

Parker said National preferred an “open-slather approach, allowing excessive speeds in areas with proven safety concerns, such as this section of State Highway 5.

“National needs to explain why they’re closing off a road safety option, even when it’s proven to save lives on our most dangerous roads,” he said.

The review said that the speed limit change did not “significantly affect” whether drivers chose to go above the limit. Twenty-five per cent of reported crashes were above the speed limit in 2020, when it was 100km/h and in 2022, when it had dropped to 80km/h.

The median speed was lower in 2022 than in any previous year.

Thomas Coughlan is Deputy Political Editor and covers politics from Parliament. He has worked for the Herald since 2021 and has worked in the press gallery since 2018.