Driving Combination Vehicles: Turning, Braking, Speed, and Space
The Physics of a Long Vehicle
A combination vehicle is longer, heavier, and less maneuverable than a single unit. It has a larger turning radius, and the rear wheels of the trailer track inside the path of the tractor's front wheels. This off-tracking means you must swing wide on right turns and watch the trailer's rear wheels in the mirrors. Never turn so sharply that the trailer tires climb a curb or strike a pedestrian.
Braking and Speed
Stopping distance grows with weight and speed, and air brakes have a lag time before the brakes apply. Keep a following distance of at least one second for every ten feet of vehicle length at speeds below 40 mph, and add one second at higher speeds. On downgrades, select a lower gear before you start down and use engine braking; do not ride the brakes, which can fade or overheat them. Traps: questions about whether a heavier load shortens or lengthens stopping distance, and about the correct following distance formula.
Space Management
You need more room to change lanes, merge, and pass. Look far ahead, scan mirrors frequently, and leave an escape route. When turning, complete the turn in the proper lane rather than cutting the corner. When backing, use a spotter and get out to check the area if unsure.
Worked Example
A 60-foot combination traveling at 55 mph should keep at least a 6-second following distance (one second per ten feet). If a car cuts in, drop back to restore the gap rather than tailgating.