Okay, but now cars do exist. Do you think it's reasonable for cars capable of traveling 40mph should be stuck behind pedestrians at walking speed? Our modern transportation infrastructure exists for a reason, and that reason is speed/effeciency. Most areas with heavy pedestrian traffic have sidewalks. If I'm going 60mph down the highway then the presumption that I should be subjected to pedestrians casually strolling across is absurd.
If you try to compromise, you end up with stroads... which turn out to be just as bad as highways for pedestrians in practice, thanks in no small part to people like you who believe that all of the compromises of street design and urban planning should be borne not by vehicular drivers but by pedestrians.
(I'll add a side note here: a lot of efficiency depends on what you're trying to make efficient. Many people--including you, apparently--define efficiency as moving cars with as high throughput as possible. If you instead want to move people with high throughput, usually step one is to ditch cars, since single-occupant vehicles occupy vastly more space than any other form of transportation.)
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.