The Federal Bridge Gross Weight Formula is a mathematical formula used by truck drivers and United States Department of Transportation (DOT) officials to determine the appropriate maximum gross weight for a commercial motor vehicle based on axle spacing. The formula is part of federal weight and size regulations regarding interstate commercial traffic (intrastate traffic is subject to state limits). The formula is necessary to prevent heavy vehicles from damaging roads and bridges. Commercial motor vehicles are most often tractor-trailers or buses, but the formula is of most interest to truck drivers due to the heavy loads their vehicles often carry. Early 20th century weight limits were enacted to protect dirt and gravel roads from damage caused by the solid wheels of heavy trucks. As time progressed, truck weight limits focused primarily on gross weight limits (which had no prescribed limits on length). By 1974, bridges received special protection from increasing truck weight limits. The bridge formula law was enacted by the U.S. Congress to limit the weight-to-length ratio of heavy trucks, and to protect roads and bridges from the damage caused by the concentrated weight of shorter trucks. The formula effectively lowers the legal weight limit for shorter trucks, preventing them from causing premature deterioration of bridges and highway infrastructure. Compliance with the law is checked when vehicles pass through a weigh station, often located at the borders between states.