[rgbrenner]

Yes, it's by (federal) law... specifically Federal Motor Vehicle Safety Standard 500

http://www.nhtsa.gov/cars/rules/rulings/lsv/lsv.html

Vehicles under 3000lbs and with a max speed of 20-25mph (like golf carts) do not have to meet the safety requirements of passenger vehicles.

This wikipedia page sums up the situation pretty well:

https://en.wikipedia.org/wiki/Neighborhood_Electric_Vehicle

[evgen]

Nice circular reasoning there. Google chooses not to meet the safety requirements that are imposed on regular vehicles. Because Google makes this choice you posit that the law is forcing it to limit the speed. I think you have causality backwards here.

There are similar laws preventing mopeds and other motor-driven bicycles from entering highways; the laws prevent the vehicle from maintaining highway speeds and therefore they are forbidden from entering the highway.