You can't use possession of cars as a proxy for sedentaryness. If you look at the number of people on sidewalks in the US (approximately none) versus the number of people walking in Japan or Thailand, you'll see a stark contrast. In the US, people (myself included) will take a car to the grocery store a ten minute walk away. In most cities around the world you get used to walking places, but since it's rarely feasible to walk in the US, you get used to driving everywhere.