Eastbound 401 to southbound 427 in Toronto. Seven lanes neck down to three in the space of a kilometre: two coming in from the southbound 427, two from the eastbound 401, and three from the westbound 401. There are always jamups there, even at times when the rest of the highway is flowing freely.
Actually, that whole 427/401/Eglinton Avenue/Highway 27 interchange is rather badly designed. Originally, back in the late fifties when the 401 was two lanes each way and the whole area was farms, it was two separate interchanges: the Hwy 401/Hwy 27 interchange, and a kilometre further west, the Airport Expressway going to the expanding Malton Airport.
But they decided to change Hwy 27 into a “400-series” freeway, and plug it into the airport. So, south of the 401, it became a freeway, and then it kind of curved westward overtop of the 401 until it got to the Airport Expressway, and then continued north around the east side of the airport property until just before Airport Road, where it then went to the terminal.
The result was a set of bridges at about a 20-degree angle to the roads beneath, which required enough support pillars that the roads beneath could not be easily widened, and when the entire 401 across Toronto went to a 12-lane configuration, this part couldn’t. Result: permanent jamups in this part of the 401.
The interchange now, from Google Maps.
The interchange then: historical photographs of the 401 from http://www.thekingshighway.ca/ : photographs 1953 to 1960. This page has several photographs of the Hwy 401/Hwy 27 interchange. Note that the 401 was originally known as the Toronto Bypass and ended at Hwy 27, coming in from the northeast. It was later extended west.
People have taken to bypassing that whole section of the 401 by going up the 427 and across the 409 (the other airport expressway, coming fromm the east), then rejoining the 401. Zoomed-out view, showing the whole thing.