Speed, yes, but efficiency is questionable. Efficiency depends both on how fast you're traveling, but also on how far you need to travel in order to reach a destination. With minimum parking requirements, the space required for a shop increases drastically. If there's one shop every 50 feet, you can efficiently visit them at low speeds. If there's one shop every quarter mile, walking past a row of shops is impossible. The car traveling 40 mph can go at a faster speed than a pedestrian, but it also requires the shops to be spaced out such that the car can park. Optimizing for speed reduces efficiency.