And there are fewer cars per km in rural areas.

You said sentence that has no clear meaning. Per km of what? Per average distance between houses? Per average distance those cars travel? Or you want to say rural areas require more car infrastructure per car? If so, then this is close to what I was trying to say.

I reread entire convo. This started from

If you live in rural areas with really low density it is often cheaper and greener to not build mass transit systems there.

And if you are not the only person living in that area, then public transport WILL be greener. One car for two people is more efficient than two cars for two people, one car for four people is more efficient and one minivan for eight people is more efficient than two cars for four people. And minivan is just few steps awa from bus.

And again, less total amount of cars means less car infrastructure needs to be built and maintained, which means less money spent.