UK: Population 59 million, Area 241,500 km^2, 371,603 km paved highways, 26 million cars.

US: Pop 280,500,000, Area 9,158,960 km^2, 5,733,028 km paved highways, 220 million cars.

Canada: Pop 32 million, Area 9,,220,970 km^2, 318,371 km paved highways, 16 million cars.

(Figures from the CIA factbook, I used the Western Europe average rate of car ownership for the UK, and an estimate of 500/1000 for Canada, based on some advocacy pages in Canada)

Here's how it breaks down: Cars/km^

Little surprise here, the USA and Canada are huge in relation to the UK, so
UK: about 107 cars per square km
US: about 24/km^2
Can: about 1.7/km^2.

So that's not a great estimate of traffic density, particularly for Canada that has a lot of empty space. Here's cars/km of paved highway.

UK: 70.18
US: 38.32
Can: 50.10

And km paved highway/km^2:
UK: 1.54
US: 0.63
Can: 0.03

Comments? I was very surprised that Canada appears on the surface to have more cars per km of highway than the States, despite our population being clustered around the border.

Given the problems with traffic currently in the UK (seriously, it sometimes feels like the entire country is going to get gridlocked together one of these days), it looks like we'll be next, even before the States, although admittedly building on slightly loose information.

Having posted the same thing to a UK cycling forum, I've now found out that Britain actually has fewer cars per capita, only about 400/1000 people where the average is something like 435/1000 for Western Europe. Go figure. I certainly never would have believed that if I hadn't had the figures in front of me, especially given the regular traffic jams I see in York, a city of 130,000-odd with a high proportion of cycle commuters.

I suspect the 50 cars per square kilometer may indeed be related to population density, since it's just an average figure and takes no account of the differential that exists between the Canadian south and the further north - so Vancouver, Regina, Winnipeg, and so on have fairly large "car populations" and thus also contribute to the overall "car per km^2" figure. I think if you disaggregated the data you would find the northern portions of Canada have barely any cars at all while the southern portions hit maybe 48 cars per km^2.