Kei cars are called kei jidousha/軽自動車 which means “lightweight car”.
They are specifically designed to fit the Japanese car regulations and private kei cars all have yellow licence plates.
They have a 3 cylinder engine no larger than 660cc.
They are cheap, inexpensive to register and get you through tolls cheaper.
They are thought to be too small to be popular worldwide – but they are one of the most common type of cars in Japan.
Daihatsu, Honda, Mitsubishi, and Suzuki all manufacture kei cars.