mud flap

A mudflap or mud guard is used in combination with the vehicle fender to protect the vehicle, passengers, other vehicles, and pedestrians from mud and other flying debris thrown into the air by a rotating tire on a wheeled vehicle. A mudflap is typically made from a flexible material such as rubber that is not easily damaged by contact with flying debris, the tire, or the road surface.
On bicycles, a mudflap is called a "spoiler".
Mudflaps can be large rectangular sheets suspended behind the tires, or may be small molded lips below the rear of the vehicle's wheel wells. Mudflaps can be aerodynamically engineered, utilizing shaping, louvers, or vents to improve airflow and to lower drag.
While some flaps are plain, in the colour of rubber, many contain company logos, other art, or advertisements. One common feature is the mudflap girl, a woman's silhouette.
In the United States, mudflap regulations vary from state to state.

View More On Wikipedia.org