Waka Kotahi NZ Transport Agency has extended its consultation period for the State Highway 5 and SH51 speed review by four weeks.
It's also apologised for its calculation that suggested the average travel time on Napier-Taupō Rd would go up by less than a minute.
Waka Kotahi NZ Transport Agency last month proposed lowering the speed along SH5 from 100km/h to 80km/h from Eskdale to just south of Rangitaiki, a distance of about 83km.
State Highway 51, between Waipatu in Hastings and Marine Parade in Napier, would also be lowered.
But the consultation and engagement period for both speed reviews have now been extended by four weeks following concerns about the calculation and presentation of likely travel time increases.
Waka Kotahi claimed a speed drop of 20km/h on an 83km stretch of highway would delay commuters by an average of one minute.
But traffic engineering specialist Urie Bezuidenhout labelled the notion as "junk maths" and "disingenuous".
Bezuidenhout, director of Da Vinci Transport Planning Ltd, said the correct calculation was closer to 10 minutes each way.
NZTA regional relationships director Emma Speight said "full, reliable and trustworthy information" must be provided to inform the community's feedback about our proposals.
"We have heard that many people who travel SH5 regularly have concerns about how we calculated and presented the likely travel time increases as part of the speed review consultation, and we apologise for the uncertainty this has created," she said.
Speight said to ensure the public participate in the consultation with full and accurate information, the consultation must be extended.
The community will now have until June 6 to provide feedback and make their submissions.
"We are reviewing the travel time calculations to ensure that the information we provide reflects the average experience of people who travel on SH5 regularly," Speights added.
"We will provide an update with further information next week, once we have completed this work."
People who have already submitted a form may resubmit.
Between 2010 and 2019, there were 250 injury crashes on SH5, leaving 16 people dead and 75 seriously injured.