The sparse bits are by definition sparsely populated. And the size of the country is for most trips irrelevant. You don't drive across half your country just because. You drive to work, and the shops and you only have the same 24 hours in a day the same as everyone else, so its the same upper bound. This[1] suggests the US has fairly short commutes, so it would seem country size is irrelevant for that at least.

[1] https://transportgeography.org/?page_id=5219