It's been a while since we've had a good chart around here, but this Paul Krugman post got me wondering about the relationship between national population density and national auto dependence. So I took his data on which percentage of trips are undertaken in a car, and put it together with population density information. The relationship winds up looking real but also fairly vague -- speaking English seems to lead to driving a lot in a more clear-cut way than does high population density.
But of course overall density data can be misleading here since some countries contain large empty wilderness areas that reduce density but contain so few people that they don't have a noteworthy causal impact on overall commuting patterns. If you lopped the Yukon Territory, the Northwest Territory, and Nunavit off of Canada, their density would be way higher but the total population would be very similar. One might get a more enlightening comparison by comparing some of these European countries to specific states. New Jersey, for example, is slightly denser than the Netherlands but seems to feature far more driving.