MONTREAL -- It was an engineering marvel when it opened in 1967, but decades of heavy use are taking their toll on the Louis-Hippolyte Lafontaine Bridge-Tunnel.

The road needs to be replaced, the ventilation system has to be brought up to new norms, and the structure itself will need to be modernized.

A maintenance tunnel that separates the two spans will also be improved to intervene faster in case of emergencies.

It is not going to be cheap.

The cost of the project is over $1 billion, and traffic headaches are going to be unavoidable.

Until the end of the year, traffic disruptions will be relatively minor, as the focus of construction will be on reserved bus lanes and parking lots for commuters.

However, by 2022, things will get bad.

The southbound lanes will be reduced from three to two, and just one lane during nights and weekends.

Reserved bus lanes will be set up to by-pass traffic, but project director Stephane Audet said the bus lanes will not continue through the tunnels as there is not enough room.

In 2023 it will be the northbound lanes' turn where just two lanes will remain open, but, in the summer, the tunnel will be completely closed first on the southbound side and then on the northbound.

To compensate for the closure, the remaining tunnel will operate with lanes opened in both direction, but at a considerably reduced pace.

"We're optimistic that people will adapt," said Transport Ministry spokesperson Gilles Payer.