NASCAR cars are not street legal today; while they can be modified and tuned to drive on the street, the stock racing cars are not legal for roads. The main reason why these cars can't drive on the street is because they do not have the same safety features that a normal car would have.
In the early days of auto racing, these cars were in fact "stock cars" and would be completely stock from the factory. Some racers would even drive their cars to the racing events. Today, the narrative is much different as racing cars aren't really stock anymore and only just resemble the name and shape of their showroom counterparts.
The Department of Transportation (DOT) has strict guidelines and regulations that all automobiles operating on the road must follow. NASCAR cars do not have any doors so that makes it hard for passengers to get in and out of the vehicle. There are no lighting systems in these cars meaning they have no visibility at night as well as an inability to use turn/brake signals. Instead of window glass, lightweight Lexan is used which is not nearly as protective as regular glass. Additionally, the engines do not have any mufflers meaning they emit an extremely loud and unpleasant noise, as well as not meeting emission standards in place. For these reasons, NASCAR cars do not meet any of the safety standards set by the DOT.
These cars are designed for racing and would be extremely impractical to drive around on the streets. Interior temperatures can reach up to one hundred thirty degrees fahrenheit and the first gear can go all the way up to sixty miles per hour. These cars are simply designed to race and it would take a lot of modifications to get one these NASCAR cars to become street legal.
In order to make one of these NASCARS cars street legal, it would take a lot of money, work, and dedication. However, there are two examples of NASCAR cars made to be street legal.
The first is a 2008 #5 Mark Martin Conquest car. This car was not built from a NASCAR chassis, but instead it was hand built based on a NASCAR style tube chassis. The car was estimated to cost around $100,000 to construct and took 2,000 man hours to build. The car has been modified to abide by all traffic laws and regulations and has doors so passengers can get it and out freely. This car was priced at $69,000 and set to be auctioned.
Another example of a street legal NASCAR car was actually built off an original NASCAR chassis. It was originally bodied as Chevrolet, but the owner attached the Ford body during the restoration process. The engine is the exact same as the original car, however, the clutch and gearing have been heavily modified to meet the rules of the road.