A Waka Kotahi NZ Transport Agency claim that a speed drop of 20 kilometres an hour on an 83km stretch of highway would delay commuters by an average of one minute has been labelled as "junk maths" and "disingenuous" by a traffic engineering specialist.
But as anger in Hawke's Bay grows about the claim, the agency is standing by it.
State Highway 5 from Esk Valley in Hawke's Bay to Rangitaiki, near Taupō, is one of New Zealand's most dangerous highways.
After 250 injury crashes, and 16 deaths, Waka Kotahi NZTA decided to propose a blanket speed limit of 80km/h on the 100km/h stretch of road. Submissions on the proposed changes can be made up until 11pm on May 9.
In a Hawke's Bay Today Talking Point on Saturday Waka Kotahi's Emma Speight wrote:
"Our technical assessment of the road found that although the posted speed limit on SH5 between Rangitaiki and Esk Valley is 100km/h, the mean speed that people travel at is 81km/h.
"This means lowering the speed to 80km/h will increase the average travel time by less than a minute."
Dr Urie Bezuidenhout, director of Da Vinci Transport Planning Ltd, said this claim was disingenuous, and the correct calculation was closer to 10 minutes each, because the new average speed on the road was highly likely to be 70km/h or lower.
"The NZTA arithmetic is junk maths as the average speed means that half travel above the average and half travel below the average - that's how averages work.
"Therefore, there will be a distribution of speeds, similar to the original spread around the average each vehicle can travel, and the new average could quite easily be between 10 to 20 km lower."
Waka Kotahi director of regional relationships Emma Speight responded by saying the increase in travel time was measured in two ways.
The first is how it will affect the mean speed, and the second, against the posted speed limit.
She said because of the winding and hilly nature of parts of the road there was a "significant difference" between the posted speed limit and the speed most people travel at.
"We have found that most people travel the consultation section of SH5 [a section of about 84 kilometres] at 81km/h.
"This means lowering the speed to 80km/h will increase the mean travel time by less than a minute.
"If you measure against the posted speed, this change could add just over 11 minutes for some vehicles."
Waka Kotahi said while recent discussions had focussed on the impact on journey times, it was important to remember that the purpose of the speed review was to save lives and prevent serious injuries.
"Too many people have already been killed or seriously injured on this road, and lowering the speed is something we can do now that will have an immediate impact in reducing the risk of further deaths and serious injuries."
Bezuidenhout said lowering the speed limit could also reduce opportunities for drivers to overtake, meaning longer and therefore riskier overtakes at 80km/h.
Bezuidenhout also worried about the financial impacts of the reduced speed.
Drivers could face economic punishments, in the form of fines, for exceeding the new posted speed, despite trying to mitigate a safety concern, he said.
Bezuidenhout's concerns have been echoed across many in the community in recent days.
Mohaka ward councillor and Hastings district deputy mayor Tania Kerr said after a community meeting that Te Pohue wanted Waka Kotahi to prove its claims.
"Many have recently done the trip at 80 km/h and they say the increase in travel time will have a huge impact on their wages," she said.
"One truck driver who addressed the meeting said that the lower speed limit will mean he has to make one less trip a day, potentially losing a quarter of his income."
She said the road was used by a significant number of workers including logging crews, harvest crews and shearing gangs.
The extra 15 minutes added to the journey might mean those like shearers need to leave earlier for their 6.30am start or could see the cost of tradesmen working on properties along SH5 increase, she said.
Truck driver Tony Alexander travels the road five to six times a week and described Waka Kotahi's calculations as "flawed".
"I used to have a truck that enabled me to get up the hills about 45km/h, now I have one that is 10kmh slower uphill and I have lost about 10 minutes over the entire road each way."
A regular driver along SH5 for more than 40 years, Havelock North resident David Elliott was equally sceptical.
He said the increase to the average journey time was irrelevant.
"I can take three hours to do the journey, my friend can take two hours and 30 minutes.
"Journey time is only specific to an individual person and their driving."