From the Super Mario Wiki
A car is an obstacle that frequently appears in the Mario Kart series. Cars appear on stages set on roads, such as Mushroom City, and will follow the road with the rest of the other traffic. The effect of hitting a Red Car has the same effect as hitting a Banana, causing the player to spin out. They cannot be destroyed or removed, but can be launched into the air by driving into them while using a Super Star, Bullet Bill, Chain Chomp or Mega Mushroom, or by throwing a Bowser Shell at them. Cars may also honk their horns if they hit an item or the player.
Mario Kart 64
Mario Kart: Double Dash!!
Cars reappear in Mario Kart: Double Dash!!, where they appear in two courses, Mushroom Bridge and Mushroom City. The game features normal red, yellow and blue cars, with "POW" on their license plates, as well as Bomb-cars and Mushroom Cars.
Mario Kart DS
Cars also appear in Mario Kart DS, in Shroom Ridge and the returning retro course GCN Mushroom Bridge. Both courses feature red, yellow and blue cars. However, in the returning GCN Mushroom Bridge, the Mushroom Cars and Bomb-cars have now been removed.
Mario Kart Wii
Cars appear again in Mario Kart Wii, where they only appear in the course Moonview Highway. The course features blue, red and yellow cars, with "POW" on their number plates, and are exactly the same as the cars from Mario Kart: Double Dash!! However, if the player hits a car head on, the player will be squashed by the car in the same way as being run over by someone with a Mega Mushroom, and will have a lower speed and drive flat for a short while, and hitting the back or side of a car will cause the player to flip over. Bomb-cars also return, now under the name "Bob-omb Cars."
Mario Kart 7
Mario Kart 8 / Mario Kart 8 Deluxe
Red and black cars appear on the retro track N64 Toad's Turnpike in Mario Kart 8 and Mario Kart 8 Deluxe. Yellow taxis with blue details and Green Shell Taxi branding also appear on this course and function identically to the cars, despite their different appearance.