The new Royalmount shopping centre project will make traffic in a busy area even more congested, despite planned mitigation measures, according to the City of Montreal.

A presentation prepared by the city's economic and urban development commission says the $1.7-billion project will have a "major impact" on traffic once it opens in 2022.

The report concludes that between 5 p.m. and 7 p.m., drivers could be in traffic for an additional:

20 to 30 minutes on Highway 40 east between Cavendish Boulevard and Highway 15.

10 to 15 minutes on Highway 520 east between Cavendish Boulevard and Highway 15.

10 to 15 minutes to get from Cavendish Boulevard on Highway 520 to Jean-Talon Boulevard off the Décarie Expressway.

When the project was unveiled, the promoter, Carbonleo, said it would only add "five per cent increased vehicle traffic" to Highways 15 and 40, but it never specified what that actually meant.

In a statement issued Tuesday evening, Carbonleo said the results of the study have been misinterpreted and that on average, the project would only add two minutes to travel time.

An estimated 360,000 vehicles drive through the nearby Décarie Interchange every day. An estimated 70,000 cars will travel to and from the mall every day.

The 10 proposed mitigation measures include reprogramming street lights, widening roads and a pedestrian bridge linking the mall to De la Savane Metro station.

The 2.5-million-square-foot shopping centre was announced in 2015. It will be built in the Town of Mount Royal, near the intersection of Highways 15 and 40.

It will feature restaurants, entertainment venues — including an aquarium and water park — office space, 1,000 hotel rooms, plenty of stores and 8,000 parking spaces.

Éric Alan Caldwell, the executive committee member in charge of city planning and transport, said since the project is in the Town of Mount Royal, and the highways are the responsibility of the province, there is little Montreal can do.

But as part of its role as a member of the agglomeration council, Montreal will host public consultations on the project, which now includes a proposal to build residences. The first session is set for Tuesday, Nov. 27 at 7 p.m. at city hall.

Figures are outdated, promoter says

Carbonleo hired consulting firm WSP Global to conduct a traffic impact study. It filed a preliminary report in August 2017 and a final report in January.

The city presentation cites numbers from that study. But the promoter says they are outdated, and that WSP is still working on the file.

A spokesperson for Carbonleo, the same promoter who is behind the Dix30 in Brossard, said other mitigation measures will be proposed, and the company is open to negotiating with government partners.

Carbonleo has already began work preparing the site, with the permission of the Town of Mount Royal. (Carbonleo)

Saint-Laurent borough Mayor Alan DeSousa says he is extremely concerned because the traffic will affect everyone travelling through the area, not just residents of his borough.

"This is what I would call planned traffic chaos that would be permanent," he told CBC Montreal's Daybreak.

He called on both the city and the provincial government, which has jurisdiction over the highways, to act.

DeSousa wants the promoters and Town of Mount Royal to go back to the drawing board and rethink the project.

Carbonleo has already began work preparing the site, with the permission of the Town of Mount Royal. Construction is slated to start in the spring.

With files from Steve Rukavina and Radio-Canada