Commenter RobM1981 on small cars:
We have experimental data here: the Smart Car.
Here was a car that was large enough for two people, or one person and a fair amount of groceries. It was driveable, if just, at highway speeds.
And it got relatively horrible mileage because physics is physics.
The Honda Fit seats four and has room - even with 4 people in it - for some decent cargo. Yet it gets much better mileage than the Smart, and is generally considered more fun to drive. Why?
Because it's large enough to have crumple zones. The human cargo is surrounded by some degree of space, and that space can be used to dissipate energy in a collision.
A Smart Car is built like a tank, because it has to be. It is essentially a roll cage on wheels. Even with the best air bag technologies available, merc had to build this thing with a relatively large amount of steel.
That means weight.
You can have a light one seat car that gets crazy-good gas mileage, or you can have a heavy one seat car that gets mediocre (for it's size) gas mileage.
But you can't have both. And given our regulations, you really aren't allowed to sell the first one.
Motorcycles would never, EVER be approved by NHTSA if they were invented today.
At some point all of the fun things will be illegal, and we can all go on to live dreary Orwellian lives, free of fear...