"Traffic issues and constraints in this area will only be exaggerated once the Transmission Gully expressway is opened next year," Whanganui Mayor Hamish McDouall said.
Palmerston North Mayor Grant Smith has suggested the group engages with central government on the issue.
The mayors say the route is important to the entire central North Island, and that landowners on the route have put plans on hold and have been waiting for certainty for years.
Horizons Regional Council chairman Bruce Gordon said he feels sorry for the people of Levin.
"It can take 40 minutes to get through Levin on a Friday."
There have been many accidents on SH1 between Ōtaki and Levin, he said, with narrow overbridges over the railway especially dangerous on a wet night. The mayors and chairs were hoping for four lanes of expressway as far as the Waitarere curves north of Levin.
If development is allowed to happen along the likely route, it will be much harder to build more lanes later, Gordon said.
The mayors and chairs would like the route decided, even if they have to wait for the expressway to be built.