The move is part of a suite of changes, including traffic lights at the intersection of Agnes Street and Lagmhor Road, for the Tinwald corridor.
Waka Kotahi NZ Transport Agency is currently consulting on the plan and looking for feedback.
The traffic lights are seen as a main and reliable way to cross or access State Highway 1, while cyclists will be encouraged to use Melcombe Street and the viaduct will be for north-bound traffic, left-hand turn only.
Traffic will not be able to turn into the viaduct from the highway.
Transport planning manager Mike Blyleven said the traffic lights were the anchor improvement, though the agency had been looking at the whole corridor. South-bound traffic would have to travel further south to turn on to Compton or Lagmhor to access the western side of Tinwald.
He said the project was about safe community access. The signals would cause traffic on the highway to stop but signal optimisation would look to minimise that.
Mr Blyleven updated Ashburton District Councillors on the work recently and said money for the project was already in the bank, thanks to a covid stimulus package. The project would be complete by the end of 2023, consenting and weather willing.
Cr Carolyn Cameron said it would be a disruptive period for traffic through Ashburton, with the Tinwald lights and the installation of lights at the intersection of SH1 and Walnut Avenue. The latter project will also take 18 months.
She said pedestrians on the highway north of Agnes Street still needed a safe place to cross the highway and she suggested a pedestrian refuge between Carters Terrace and Wilkins Road.
“But all of it is irrelevant if you can’t cross the bridge.”
Mr Blyleven said the traffic lights would provide safe access for children crossing the highway to go to Tinwald School and for people going to the Tinwald shops.
People coming into Ashburton from Lake Hood will be encouraged to turn right on to McMurdo Street and then left on to Agnes to use the new lights. That route had fewer residential properties than others, he said.
Submissions close on the transport plan on April 16.
