I think Ming raises an interesting topic.
However, instead of limiting where or how fast such car can go and creating a whole complex set of regulations for them, I would use far less restrictive but more advisory definitions for this class of vehicle. I would base it on contractual notification and consent.
Drivers and passengers should have free choice when they are fully advised of the facts.
As an option for any car that gets over 50mpg combined City/Highway cycle:
1/. It need not have any airbags, side intrusion protection or other add-on or built-in crash protection capability for the benefit of the passengers, but it must comply with all current pedestrian safety requirements.
2/. It can be of any size within current car size regulations but must be able to drive straight and safely for other road users at speeds up to and including national highway speed limits.
3/. It must only be driven by a driver over 18 years of age when carrying no passengers.
4/. It must only be driven by a driver over 25 years of age when carrying passengers and not carry any passenger under the age of 18 years.
5/. It most have a large 12 inch by 6 inch red plastic sign ahead of each passenger seating position clearly stating that the vehicle has no airbags or crash technology, is dangerous to travel in and is illegal for any passenger under 18 years of age.
Such simple clear regulation leaves the manufacturers free to give consenting adults what they want in a very frugal vehicle without other government limitation. And it prevents class action legal actions with its cigarette packet style warnings and stops dead complaints about innocent children. There would be a market for such vehicles for adults only; just as there is a market for high-powered road bikes that are effectively limited to adult use.
