Driving Combination Vehicles Safely (Turns, Braking, Speed, Space)
Why a combination handles differently
A tractor-trailer is two or more vehicles hinged together. The trailer's rear wheels track inside the tractor's path (off-tracking), and the last trailer in a set swings out more than the first (rearward amplification). The exam tests turns, following distance, downgrades, and stopping.
Turns and lane position
- Start turns slightly wider, then watch the trailer tires in the mirrors; do not let them clip the curb.
- Do not swing wide before the turn unless you need room-a buttonhook turn from the outside lane can trap a car alongside.
- Complete the turn in the same lane; avoid lane changes mid-intersection.
Speed and stopping
- Brake lag adds distance: air pressure must travel to the trailer before its brakes apply.
- Below 40 mph, use one second per 10 feet of vehicle length; above 40 mph add one second. A 65-foot combination needs 7 seconds at 55 mph.
- Before a downgrade, slow to a crawl at the top, select a gear that holds the speed, and use the same gear all the way down. Do not ride the brakes.
Worked example
At 55 mph a 65-foot double needs about 7 seconds of following distance. One second at 55 mph covers about 81 feet, so 7 seconds is roughly 565 feet-over a tenth of a mile. A passenger-car three-second gap is far too short.
Traps
Using car following distances; braking hard in a curve; entering a downgrade in a gear that is too high; assuming an empty trailer stops quickly.