What you are looking for is the Japanese KEI class of vehicle. The reason they are not street legal in the US is due to crash survivability. Even if their top

What you are looking for is the Japanese KEI class of vehicle. The reason they are not street legal in the US is due to crash survivability. Even if their top speed is 45, they share the road with vehicles that are heavier or faster than them and it is not socially tenable to create a vehicle class that is very similar to an existing one but limit it's access to certain roadways. In order to get survivability up to where we want it, you have to increase vehicle weight. Once you do that, you have to increase the engine in order to handle the weight i.e. acceptable acceleration