15mph is still faster than walking speed. I'm not saying cars are the most optimally efficient mode of transportation, just that they are much more efficient that walking.
In the few places with high 100% walking commutes, I don't see cars being more efficient either. Do you want every college student to drive from their dorm to class? How about poor countries without the money to maintain good road networks?
This entire discussion started as whether pedestrians should/shouldn't be on roads. No, they shouldn't. They should be on a sidewalk, best of both worlds.