The term “street legal” will usually refer to vehicles that are considered to be properly equipped to be driven in public roads. This concept not only applies to car and automobiles, but also to motorcycles and certain kinds of trucks. Generally speaking, a street legal vehicle will have certain kinds of safety equipment within the vehicle, such as seat belts for example.
Other aspects that are to make a street legal vehicle are the use or implementation of certain kinds of lights, turning signal lights, and certain kinds of emissions regulators. Typically speaking, a street legal vehicle would not include anything that is designed to be used off-road, such as a dirt bike or atv, or a race car, which is designed to be driven on a race or closed track. These types of vehicles will usually contain the necessary safety equipment to be considered street legal.
In the United States, each state will determine what makes a vehicle street legal. There are variety of different laws that can be implemented by the states in order to consider a vehicle street legal. Generally speaking, certain things that almost always are necessary will be seat belts, lighting system, turning signals, and emission regulators.
However, there certain things that can make a vehicle not street legal, regardless of having all of the necessary safety devices. An example can be modifications to a car to provide for undercarriage lights, which may be typical in tuner cars. Many jurisdictions will not allow these lights to be legally used for they cause a distraction to other motorists.