In the American society of automotive engineers standards SAEJ 687 c for automobile is defined as: power driven by itself, with a driving device, can in a fixed orbit outside the road or on regional transport passenger and cargo or vehicle traction vehicles. Cars are divided into bicycles and trains according to the overall structure. Cycling is a basic form. 4 x 2, 6 x 4, 4 x 4 and 6 x 6 are used to represent the driving characteristics. The previous number represents the total number of wheels (double tire and still count a wheel), and the latter number indicates the number of driving wheels. If all wheels are driven wheels are called all-wheel drive vehicles. The train is made up of a tractor or a bicycle trailer or semi-trailer. Car generally can be divided into the following categories, according to the use cars, buses, freight cars (commonly known as the truck), off-road vehicle, traction motor, dump truck, special vehicles, semi-trailer and special semi-trailer. Last year, labour market economist John Philpott found that more than one in five workers, around 7.1 million people, were in precarious employment, up from 5.3 million in 2006.