Now granted, I feel that the majority of people can use a regular sized automobile for their daily tasks. Why is it we need a 10,000 lb car to carry around 200 lbs of person and 20 lbs of groceries? It's a bit absurd isn't it?
That's why I am proposing that cars come with mandatory keys that weigh 1% of the vehicle's gross weight. Now, if you want to drive a hummer, you have to carry around a 10 pound weight with you everywhere you go. Heading to the movies in your BMW X5? Make sure you don't forget that 60 pound key under your seat else it'll be a long walk home.
Now I'm sure the airlines will balk a bit. "Sir, you'll need to check in your car keys."
Granted, that even the average car driving citizen will have to lug around 10 to 20 pounds with them everywhere, but hey, it might mean we all get into better shape. Either that or think about once and for all ditching that car and heading for the metro.