Perhaps 600k was the highest recorded traffic flow in one day.
Seriously, the 401 in the summertime is madness! Commuters, JIT inventory trucking routes, and a recreational route all in one! I was stuck at the 400 interchange one day for over 2 hours on my way to Canada's Wonderland (a theme park in the region). Amazingly busy road!
Doesn't help that its the only "true" east-west highway in Toronto. (Gardiner ends at DVP, 407 outside city limits.)
Yep, the 401's a monster. The 407 was supposed to help take some traffic off it, but with high tolls to use the 407 and exceptional growth in the region, the 401 remains the busiest highway in North America, perhaps the world.
Here's a pic of one of the busiest sections of the highway:
Crazy interchanges too!