As of 2017, where is the busiest stretch of highway (number of vehicles) in North America?

The MacDonald-Cartier Freeway or Highway 401 has the distinction of being North America's busiest road.

The controlled access highway goes from Windsor Ontario (in the west) and through Southern Ontario to Alexandria Ontario (in the east) ending at the Quebec border but continues as a Quebec highway. This connects the state of Michigan to the province of Quebec over 400 miles or 600 km. It passes through mostly rural lands but also goes through several large cities like London, Mississauga, Toronto and Kingston Ontario. This is the main trucking route from Detroit Michigan to Montreal Quebec or all the way to the Atlantic Ocean with connecting highways.

The section of the 401 from Mississauga at Mavis Road to the towns of Pickering and Oshawa just east of Toronto have up to twenty lanes of traffic, including on and off ramps. Lanes called "Collectors" are used for accessing or exiting the highway. The "Express" Lanes are used to drive through the Toronto section.This route handles on average 400,000 or more vehicles every 24 hours. Congestion or accidents can slow down or stop traffic for many hours.There is even a television show on the Discovery Channel called "Heavy Rescue 401" devoted to tow truck companies and police dealing with these transit snarls.The 401 passes right beside Pearson International Airport and it is interesting to watch the airplanes landing and taking off close to the highway.

The speed limit on the entire highway is 60 mph. or 100 kph.