Is not in California. It's the King's Highway 401 in Ontario, specifically the part that travels through Toronto. Almost half a million vehicles per day pass through the busiest part of the route along sixteen lanes of traffic.
The volume of cars is because the "four oh one" is (in American terms) an interstate and a regional highway for much of its length. It is the main route from the Windsor border at Detroit to Quebec City at the entrance to the St. Lawrence River, a route of vital strategic and economic importance to Canada and the US. Canada is the US's biggest trading partner, and much of that is cars, actually: Toyota, Chrysler and GM all have plants along this highway (and Honda has one that accesses it via Highway 400).
The four oh one uses an interesting collector-express system stolen from the Americans, one that sometimes bedevils travelers, tourists and newcomers. It's well illustrated in the aerial picture here, taken near Pearson International Airport. The inside lanes are Express. The outside lanes are Collectors. The Express lanes are meant to be the interstate (or inter-provincial, in this case), designed to carry traffic that is bypassing the busy Toronto section on and through to the other side and, eventually, the Quebec and American borders. The Collectors are meant for local traffic, and as their name suggests, collect travelers from local feeder roads and linked highways.
Not to put too fine a point on it, but take trucks: OTR (Over the Road) long-haul trucks would, in theory, want to stick to the Express, while local delivery trucks (parts of the 401 are lined with distribution warehouses and light industry) might use the Collectors.
What confuses some people is which they should use. In part because they don't understand the system. Many pour souls have missed their exit because they were in the Express lanes, which only have a certain number of interchanges with the Collectors - about 1 for every 3 or 4 Collector on and off ramps. Those who miss their exits are fated to spend eternity wandering the Express lanes like some automotive Flying Dutchman. Or at least until both merge back into a regular 8-lane divided highway outside the Toronto corridor.
Parts of the system can be tricky, not only due to multiple spaghetti junctions but because some interchanges don't seem to have been well-thought out. The eastbound exit to Kennedy Road from the Express requires you to thread the needle and dodge across four lanes in a very short space of time. It's almost impossible during rush hour, even for BMW drivers, taxis, bro trucks and luxury SUVs. So it pays to plan ahead and know where the exits are, and if you don't, to stick to the Collectors and play it safe.
Helpfully, two types of signage are posted: green for Express (the normal highway system colour in Ontario), and blue for Collectors, and signage in the Express lanes does alert you to approaching exits and what they link up to.
The Collectors may live up to their name, but the Express sometimes doesn't. If an accident snarls traffic ahead, you're stuck for quite a while before you can exit. The overflow makes the Collectors handle all the load, with predictable results. But generally speaking (by that I mean, anytime but rush-hour) traffic does travel faster in the Express lanes. There can be quite a speed difference between the furthest extremes of lane sometimes. The merge lane of the Collectors could be moving at 80-100 km/h, while the passing lane of the Express could be going 140 km/h or more.
The 401 is patrolled by the Highway Safety Division of the Ontario Provincial Police. You do not want to get down with OPP. They're our version of the CHP, and still like to kick it old school with their pursuit vehicles.
None of this "toothpaste tube" nonsense here. A classic Black and White. Like everybody else, they're going to eventually retire the Panther Interceptor, but they're still around in great numbers (though the Tahoe is well represented). Unless they're on some speeding blitz, they will generally leave you alone unless you're a menace to yourself and society, but if you are, watch out, they can be pretty hardcore (in other words, just as a highway patrol should be). They have aircraft and can call on the assistance of like a zillion other cop cars from local jurisdictions if need be. Like highway patrols everywhere, they're attracted to fast (or fast looking) cars going fast - another reason to want a sleeper.
They caught a Porsche driver doing 233 km/h (144 mph) late one December night. What kind of Porsche, you ask? A Cayenne (unknown if it was the Turbo S, not that it matters). The driver was drunk off his ass. Well, you did ask.
In summary, the 401 is busy and complicated in sections, don't aggro the OPP, don't drive drunk, and don't buy a Cayenne.
Photo Credits: Wikipedia Commons, Flikr pool. Sources: Wikipedia, Ministry of Transportation, author's experience.