I suppose this depends on where you live, but going above 60mph is normal usage for me. In fact, a minimum speed limit of 45mph on freeways is fairly common (unless you can't due to traffic, of course).

They're called bicycles, motorcycles, buses, and trains. The first two give up protection from the elements, the second two give up independence. Keeping both the constraints nets you a single-person car-like device with say a 30hp motor - somewhat more efficient, but not enough to drive adoption. Gasoline (and energy in general) just isn't expensive enough for any of the alternatives to be taken seriously (infrastructure-wise) anywhere but dense urban areas.

My commute is mostly highway, at 65 Mph. The most common use case for my vehicle is thus very different from yours. Your vehicle would add extra time to my commute, and would not be useful for me in any situations where I am travelling long distance (120 km/h is the norm in Europe, that is about 75 Mph) I want to go faster, I don't want my trip to take even longer.

Fuel economy can only improve by a factor of order unity (typically, less than 2) by changing speeds. And as I said, vehicle size is also incapable of reaching the orders of magnitude needed.

When visiting family in SC and FL I usually gag when I see people driving around (even running errands) in golf carts. Maybe I should give them a high-five instead.