Robert, I disagree with some of your design criteria for vacuums. We use canister vacuums, we have our parking on an angle, with the vacuum in front of the car rather than on the traditional island style location, we add to the length of the hose to compensate for the distance. As to the width, our spaces are narrower than what you recommend. If people are too comfortable, they will spend all day, especially on a busy weekend. Another point, parking on an angle still requires 18' of depth, not 14'. You can skimp on the distance between if necessary, but I agree that 28' is the best to allow ease of exiting.
Having said all this, around the country, I have seen just about everything that you would think of that would hinder usage, yet the people figure it out, and are happy as bugs in a rug.