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Combination Vehicles · Driving Combination Vehicles Safely

Why must drivers use extra caution and allow more following distance when driving an empty or lightly loaded combination vehicle?

1.It can take longer to stop than a loaded rig, since a light load gives too little tire grip, so wheels lock up easilyCorrect answer
2.It stops much faster than a fully loaded rig simply because there is much less total weight that needs to be slowed down
3.Its air brakes automatically and continuously reduce the stopping distance to compensate for the lighter load
4.Its parking brake activates fully automatically at highway speed in order to help prevent a rollover

Explanation

An empty or lightly loaded combination vehicle can actually take longer to stop than a fully loaded one. With little weight pressing the tires down, grip is scarce, so the stiff springs and powerful brakes lock a wheel easily, and the trailer can then swing out or the tractor jackknife. A bobtail tractor is the extreme case: smooth stops are genuinely difficult, and its stopping distance can exceed that of a fully loaded tractor-semitrailer at its maximum gross weight. This is a manual-only technique, topic-scoped to 49 CFR 383.111(c)(3)(i), since the paragraph names combination-vehicle operation as required knowledge without stating this braking fact itself. The intuitive belief that less weight means a shorter stop is exactly backward here, since traction, not just mass, governs stopping distance. The other two options invent automatic brake behaviors that don't exist; no air brake or parking brake system self-adjusts for load this way.

Source: 49 CFR 383.111(c)(3)(i)

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