Driving Hazards, Stability, and Special Situations
Domain 5 tests judgement: recognising the situations where a long, top-heavy combination behaves differently, and acting before the rig is in trouble.
Stability limits
- Rollover: triple trailers are tall and carry a high centre of gravity. Speed must come down before the curve, not in it; braking in a curve unsettles the set.
- Wind: crosswinds and the bow wave of a passing truck hit the last trailer hardest. Slow down and hold a steady position with small corrections.
- Empty or light trailers: they bounce, their brakes lock easily, and they amplify every steer. A light rear trailer behind a heavy load is the classic jackknife setup.
- Slippery pavement: throttle, brake, and steering inputs all need to be gentle. A drive tire that loses traction under a retarder can start a jackknife.
Things that only happen with a dolly
- The dolly is a separate vehicle behind you. If its pintle hook or chains fail, it becomes a runaway, which is why the safety pin and crossed chains matter in the inspection.
- Separated air lines: a breakaway sets trailer spring brakes through the emergency line, which is why the red line must be connected, not just the blue.
- Rear trailer tire failure: hold the wheel steady, ease off the throttle, avoid hard braking, and slow gradually to the shoulder.
Worked example
An empty set of doubles on a windy overpass is overtaken by a tractor-trailer. The driver feels the last trailer push sideways, brakes, then steers back sharply. That combination of braking and a sharp steer is what puts a light trailer into a skid or a swing. Better: slow before the overpass, keep light steady throttle, and correct with one small smooth steering input as the bow wave passes.
Special situations
- Passing takes more time and clear distance. Do not return to the right lane until the last trailer is past the vehicle and visible in the mirror.
- Railroad crossings and low bridges: know your height and length, and never stop on the tracks.
- Never uncouple on a grade without chocks, and never leave a set standing on the tractor's parking brake alone.
Traps that cost marks
- Braking hard in a curve to fix a speed problem.
- Over-steering after a wind gust or a passing truck.
- Assuming a passing vehicle will not disturb the last trailer.
- Forgetting that the red emergency line is what releases the trailer spring brakes.