it's North America's busiest highway too.
Ontario provincial highway 401, The King's Highway 401, or the Macdonald-Cartier Freeway, which is normally referred to simply as Highway 401 (pronounced "Four-O-One"), is a freeway that extends across Southern Ontario, Canada. It is (by far) the longest 400-Series Highway in Ontario and one of the busiest highways in the world. Together with Quebec Autoroute 20, it is the road transportation backbone of the Quebec City-Windsor Corridor, containing over half of Canada's population.
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The 401 is widely considered to be North America's busiest highway, with an estimated Annual Average Daily Traffic (AADT) of over 425,000 in 2004, near the interchange with Highway 400. This surpasses the Santa Monica Freeway in Los Angeles, CA, and several Interstate freeways in Houston, TX. Due to its triple use as the main trade, commuting and recreational corridor in Ontario, the AADT rises to well beyond the 500,000 level on some days. The just-in-time inventory systems of the highly integrated auto industry in Michigan and Ontario have made the highway the busiest truck route in North America. The 401 also includes the continent's busiest multi-structure bridge at Hogg's Hollow in Toronto (four structures for the highway's four roadway beds).
The 401 is one of the most important highways in Canada, as it connects the populous Southern Ontario region with Quebec and Michigan, while also connecting to most other major highways in the province. The highway also serves as the principal connection to Montreal and points east, becoming Autoroute 20 at the Quebec border. The border crossing at Windsor and Detroit is the busiest trade crossing in the world, and although the 401 itself does not physically extend the last few kilometres into Detroit, it is the only route from Toronto to Windsor and on to Interstate 75. A future expansion of the Windsor-Detroit border crossing, which will include a freeway bypass of the existing Highway 3, may result in Highway 401 having a direct freeway link to the border. Some 40 percent of Canada-US trade travels the highway, which is one-third of Canada's foreign trade, and 4 percent of the all US foreign trade.
http://en.wikipedia.org/wiki/Highway_401