Since many decades, route 125 (route 18 before the number swap of the 1970's) was used by Montreal people who have a cottage in the north, so the traffic jams on week-ends. As time goes by, those people retired, transform their cottage in a cozy home, and more Montrealers did the same. The problem is that in this kind of place, population density will never be sufficient enough to have any kind of mass transit. People take their cars to go here and there, and it's like this since cars exist. With the pandemics, even more people left the city to get countryside; that explains, at least in part, the 37% increase of population there.
There's another question about areas like that one; the more the villages grow, the more trucks will use the roads. An autoroute - or an Interstate in USA - helps to separate the truck traffic from the rest, as the transit traffic almost always take the fastest route. With many roundabouts, the future route 125 will be anything but fast.