What Is the Stopping Distance of Tractor Trailers?

Inertia is a prime factor in the difference in braking distance between a car and a heavy truck. A fully loaded semi truck has the gross vehicle weight, depending on its cargo, of up to 80,000 pounds. (Compare this to an average car's weight of 4,000 pounds.) At a speed of 55 m.p.h., a semi truck's stopping distance is 100 yards--the length of a football field. But that doesn't take into account reaction time and or the time for the brakes to effectively engage. All told, from when a dangerous situation is spotted until the truck is completely stopped, a truck needs about a tenth of a mile to stop when fully loaded! A mid-size automobile traveling at the same speed can stop within half of that distance.

The compressed-air brake system of a semi truck is very different than that of an auto. Air brakes have a lag time after the driver steps on the pedal. A second of travel time will occur before the boost of the air brakes activates the friction parts of the braking system. Semi trucks primarily use drum brakes, which can be problematic on mountain roads. The brake drums can get hot and expand if the brakes are used to control the speed of the truck for an extended period of time. The result is brake fade and reduced braking power.

Cars use a hydraulic brake system. Brake engagement is almost instantaneous with a car's system. Most modern automobiles also incorporate disc brakes, which are more efficient than drum brakes, for automotive applications.

The U.S. Department of Transportation report, "Large Truck and Bus Crash Facts 2007" states that collisions involving semi trucks accounted for 5,235 fatalities in 2004. 1995 had 4,918 fatality collisions involving semi trucks. The same report says that the number of large trucks involved in injury crashes has decreased by 21 percent over the past 10 years, and the vehicle involvement rate for large trucks in injury crashes has declined by 33 percent.

An engine compression brake system is installed on many large trucks; it supplements the friction brakes to help keep the semi at a steady speed while descending hills. The engine compression brake releases trapped air from the engine's exhaust valves, thus using the engine's intake compression to slow the vehicle. This type of brake system is also known as an engine retarder or Jake Brake.

Bendix Commercial Vehicle Systems has developed a full stability system which helps prevent roll-over accidents by controling the yaw of a skidding truck. This will mitigate the loss of control in a panic situation. The Bendix ESP System is capable of recognizing and helping to prevent oversteer, understeer and loss of traction due to bad weather.

The American Trucking Association developed defensive driving guidelines for motorists driving in the vicinity of semi trucks. These techniques are designed to make driving safer for truck drivers and motorist alike:

Never cut in front of a semi for any reason. Fully loaded trucks can take over 500 feet to come to a complete stop from when the driver first notices danger. Be sure to maintain a distance of at least five car lengths when maneuvering in front of a semi.

Never linger alongside a semi because they have huge blind spots in which cars can disappear from the view of the truck driver. The blind spot on the right hand side of the semi runs the length of the trailer and extends three lanes. Pass on the left because the blind spots are smaller.

Keep a safe distance behind a semi truck--about 20 to 25 car lengths. If you travel any closer you run the risk of being in the blind spot.

Commerce relies on semi trucks to bring goods to market. The AAA Foundation for Traffic Safety says car drivers cause most of the truck-car collisions. Semi trucks require longer stopping distances and larger turning circles. A wide degree of separation is needed when driving in the vicinity of these large vehicles.