The bus has a bomb and you are on it. Fortunately, you are able to communicate with some bomb specialize units outside the bus. They told you that you should keep the bus speed to 50 miles. if the bus speed goes under or over that then the bomb explodes.
The game is about keeping the speed to 5 miles per hour. If you can't keep it that way then the game is over and you will be scolded by the bomb specialize unit. There are special items on the road that will help you keep the 50 miles speed of the bus.