Traffic refers to the cars and buses that can be seen in several tracks throughout the Mario Kart series. On the tracks, the traffic serves as an obstacle to add more a challenge to the track. If one hits one of these vehicles, he/she either spins out, flies into the air, or becomes squished. Traffic travels slower than the speed of the racers, though karts that are shrunken (as a result of a Thunderbolt) needs to be careful since they move at about the same speed. Running into a car while invincible, such as while using a Starman or Bullet Bill, causes the car to flip upwards into the air.
In newer games, traffic can be made up of different kinds of vehicles, such as Bob-omb Cars which explode when touched by a racer or item, and Wigglers, which are long and don't travel in a straight line. In Mario Kart: Double Dash!!, there are cars that look similar to Mushrooms. When one bumps into one of these, he/she does not spin out, and a Mushroom flies out of the car.