^ Autoroute 40 is only a couple of kilometres north of Autoroute 20 through the West Island, with a decent highway connection between the two via A-540, A-13 and the Décarie. Autoroute 20 has never been finished because it's never really needed to be finished. The section of A-40 that by-passes A-20 functions pretty adequately most of the time, so there hasn't really been the need.
A-20 is the former alignment of Quebec's Highway 2. It was built as a divided highway before the autoroute era, replacing the former alignment of Highway 2 that went through all of the communities. Quebec actually had a number of rural divided highways before the autoroute era, most notably Highway 9, which became Highway 116.
I have attached a couple of old maps of Montreal. The first is from 1957, which shows Highway 2 as a divided highway:
The second is from 1964, which shows Highway 2 as a divided highway, with the Metropolitaine Autoroute under construction to the north:
Some of the traffic signals on Montreal Island along A-20 weren't removed until relatively recently. I don't think that the traffic signals at Woodland Avenue were removed until 2000 or so.