Already today, cars do not travel at 40mph in those areas with heavy pedestrian traffic--the typical speed limit on a city street is 25mph, and even that is probably above typical speeds. If you want cars to travel more quickly, you need to resort to highways that completely cut off pedestrian access, which has seriously deleterious effects on the neighborhoods it travels through.

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.)