Doubles and Triples: Dollies and Rearward Amplification
Maryland's Doubles and Triples domain tests coupling, air-line routing, and the handling of multiple trailers. More trailers mean more pivot points, more clearance problems, and stronger rearward amplification.
Coupling and inspection
A converter dolly supports the front of the second trailer. Inspect the dolly, pintle hook, safety chains, air lines, and shut-off valves. Connect the red emergency line and blue service line correctly, open the shut-off valves, and check for leaks. Secure the dolly with the locking pins, attach safety chains in a crossing pattern if required, and test the trailer brakes. Make sure the rear trailer's lights work. After coupling, do a pull test and check the clearance between the trailers.
Handling and braking
Doubles and triples off-track more than a single trailer, and the rear trailer can swing wide during a lane change. Slow down, increase following distance, and avoid quick steering. Brake smoothly and early; a hard stop can cause the trailers to jackknife or swing. Check clearance before turning or changing lanes, and remember that the last trailer clears last.
Worked example
A triple with 28-foot trailers changes lanes at highway speed. The driver feels the rear trailer sway. The correct response is to ease off the accelerator and steer gently, not to brake hard or jerk the wheel. The sway is rearward amplification, and the last trailer is the most affected.
Traps
Crossing the red and blue lines; leaving shut-off valves closed; forgetting safety chains; skipping the pull test; turning too sharply; and braking hard to correct sway.