I think doors. Most people on Earth reside in a domicile, and many domiciles on Earth have doors. Far fewer people have vehicles and wheels for other purposes have limited use in environments with lots of inclines. I'm caught up in the proportion of housing which has doors versus other housing with doorways or otherwise no room separation.

Edit: Come to think of it, most vehicles on the road have four wheels and four doors. Two door sedans are generally luxury and semi-trucks with many wheels and only two doors are far less common proportionally than standard passenger vehicles.