So Highway 427 may have become the second highway in the world to hit average daily volumes of 400,000 vehicles per day (the other being the 401)
http://www.skyscrapercity.com/showthread.php?t=489565&page=217
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Source: http://www.raqsa.mto.gov.on.ca/techpubs/TrafficVolumes.nsf/fa027808647879788525708a004b5df8/f51986ea499a13b08525745f006dd30b/$FILE/Provincial%20Highways%20Traffic%20Volumes%202016%20AADT%20Only.pdf
401's highest count is 416,500 between Highway 400 and Weston Road
427's highest count is 400,700 between Burnhampopre and Rathburn roads.
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For comparison with the USA, here's some 300,000+ roads
Source is from 2012
https://www.fhwa.dot.gov/interstatebrief2012/
I-405 in LA - 377,500
I-75 / I-85 in Atlanta (Downtown Connector) - 358,390
I-90 / I-94 in Chicago (Dan Ryan Expressway) - 309,700
I-10 in Houston - 305,630
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It's hard to claim the 401 as the busiest on Earth, but based on the AADT (Average Annual Daily Traffic) counts, only the 401 and 427 have volumes over 400,000 in Canada / USA.
Other countries may publish their information differently, or not at all.