Traffic refers to the cars and buses that can be seen in several tracks throughout the Mario Kart series. On these tracks, traffic serves as an obstacle to add more of a challenge to the track. If players hit one of these vehicles they will usually flip over, but depending on the game they can spin out, fly into the air, or even be squished. Traffic travels slower than the speed of the racers, though karts that are shrunken (as a result of a Thunderbolt) will need 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 will cause it to fly up 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 Wiggler Busses, which are long and don't travel in a straight line. In Mario Kart: Double Dash!!, there are also the Mushroom Cars, when a player bumps intot one of these, they will not spin out, and a Mushroom will fly out of the car. This can be helpful if the Mushroom lands in a spot that is easy for the player to get to, and in a direction for the boost to not cause the player to fall off of the edge or into a harmful car. In the reappearance of Toad's Turnpike in Mario Kart 8, some vehicles even allowed the racers to do tricks or glide off of them.