Quebec Autoroute 20

A-20 begins at the Ontario-Quebec border near Rivière-Beaudette as the continuation of Ontario Highway 401. The westernmost section of A-20 was named the Autoroute du Souvenir (Remembrance Highway) in 2007 to honour Canadian veterans. Road marker signs on this stretch of the autoroute feature a poppy (a traditional symbol of Remembrance in Canada).

At km 29, A-20 crosses A-30 (former A-540) before becoming an urban boulevard for approximately eight kilometres (km 30 to 38) in Vaudreuil-Dorion and L'Île-Perrot. This stretch of highway takes A-20 across the Ottawa River. The speed limit is 50 km/h (30 mph) in Vaudreuil-Dorion and 70 km/h (45 mph) in L'Île-Perrot. A-20 once again becomes a limited-access highway at km 38, just before crossing the Sainte-Anne-de-Bellevue Canal onto the Island of Montreal.

A-20 then traverses the West Island (in French, l'Ouest de l'Île) along the north shore of Lac Saint-Louis to an interchange with A-520. Commonly called the Dorval Interchange, this exit is the main access to Montreal's Trudeau International Airport. Further east, A-20 crosses the A-13 at its southern terminus, and then, at the St. Pierre Interchange, Route 138 west towards the Mercier Bridge.

Just west of downtown Montreal, A-20, A-15, and Route 136 meet at the Turcot Interchange. In 2020, the completion of a major project involving the rebuilding of the Turcot Interchange has shifted the autoroute north between Route 138 and the interchange. From there, A-20 east is multiplexed with A-15 south on the approach to the Samuel-de-Champlain Bridge.

Multiplexed with A-10 and A-15, all three autoroutes cross the Saint Lawrence River via the Samuel-de-Champlain Bridge to Longueuil. The multiplex splits south of the bridge. The A-20 parallels the south shore of the river through suburban Longueuil. The junction with A-25 affords a direct connection to the Louis Hippolyte Lafontaine Bridge-Tunnel and Montreal's East End. The Trans-Canada Highway joins A-20 at this junction.

Autoroute 20 in Saint-Michel-de-Bellechasse with the Saint Lawrence River and the Laurentian Mountains at the background

The longest section of A-20 (from its junction with A-25 to its easternmost terminus) is named after Jean Lesage, who served as Premier of Quebec from 1960 to 1966, during the Quiet Revolution. Autoroute Jean-Lesage currently exists as two discontinuous sections separated by about 55 kilometres:

• The main section between the Louis-Hippolyte-Lafontaine Bridge-Tunnel and Trois-Pistoles in the Bas-Saint-Laurent region.
• A shorter section that serves as a bypass of Rimouski and extends east to a final terminus at Route 132 in Mont-Joli.

From the junction with A-25, A-20 travels east, away from the St. Lawrence River. At kilometre 98, A-20 intersects A-30 near Boucherville, crossing the Richelieu River (km 112) just north of Mont-Saint-Hilaire. Bypassing Saint-Hyacinthe, A-20 forms a multiplex with Route 116 for six kilometres between exits 141 and 147. This section of A-20 in Centre-du-Québec is located the furthest from the St. Lawrence River (approximately 45 kilometres). Between Drummondville and Sainte-Eulalie, A-20 forms a multiplex with A-55 for 37 kilometres.

A-20 continues across the Great Lakes–St. Lawrence Lowlands and Quebec's agricultural heartland. The autoroute once again parallels the river as it approaches metropolitan Quebec City. From this point eastward, A-20 is never more than five kilometres from the river. At km 312, A-20 crosses the A-73, a north–south link between Saint-Georges and Quebec City via the Pierre Laporte Bridge. While the control city on A-20 is listed as "Québec", the autoroute never enters the city proper. Before departing the region, the freeway bypasses suburban Lévis.

Autoroute 20 westbound in Montreal, ~km 66

As it continues eastward, A-20 passes the regional centres of Montmagny and La Pocatière before approaching Rivière-du-Loup and the junction with A-85 at km 499. The Trans-Canada Highway departs A-20 at this interchange and travels southeast on A-85 toward Edmundston, New Brunswick and the Maritime Provinces. At km 518, the highway becomes a super two for the remainder of its main section. The eastern end of the main section of the A-20 is located in Trois-Pistoles, approximately 40 kilometres east of Rivière-du-Loup.

The second section of Autoroute Jean-Lesage (km 597 to km 641) connects Rimouski to Mont-Joli. It begins at a junction with Route 132 in Rimouski, approximately 55 km from the current terminus of A-20's main section. Like its larger counterpart, the Rimouski section of A-20 parallels the St. Lawrence, providing a southern bypass of Rimouski before ending (at another junction with Route 132) at the western approach to Mont-Joli.

This section of the A-20 is currently a single carriageway with occasional passing lanes. It was built to accommodate an eventual expansion to four lanes and most overpasses include the clearance needed to accommodate the additional lanes. The high accident rate along this section of A-20 have prompted many politicians to call for the highway to be expanded to two lanes in each direction.

An electric fence has been installed on both sides of the highway between km 621 and 628 to prevent moose from crossing the road, while at km 624 an underpass has been constructed to allow wildlife to pass under the highway safely.