Speed and Space Management
Speed is a decision, not a limit
Posted limits describe ideal conditions. Your safe speed depends on load, grade, curve, surface, and visibility. Braking distance grows with the square of speed: going from 30 to 60 mph does not double stopping distance, it roughly quadruples it.
Stopping distance has parts
Total stopping distance is perception distance plus reaction distance plus brake lag plus actual braking distance. In an air-braked combination, brake lag alone can eat half a second. At 55 mph that is about 40 feet before the brakes even begin to work.
Space cushions
Keep a front cushion you can stop within, side cushions to allow for wind and errors, and a rear cushion you protect by slowing gradually rather than brake-checking. Position the vehicle to keep escape routes open, especially in construction zones and near stopped traffic.
Grades
Select a lower gear before a downgrade, not on it, and let the engine brake hold you back so the service brakes stay cool. Riding the brakes down a long grade causes fade and, eventually, loss of braking. On upgrades, downshift early rather than lugging the engine.
Traps
Questions assume ABS shortens stopping distance (it does not; it maintains steering control), or that the posted limit is automatically safe for a loaded truck. Another trap: thinking you can pass a slow vehicle on a short sight line because the road looks clear.
Rule of thumb
Pick a fixed object ahead and count seconds as the vehicle in front passes it. Below four seconds at highway speed, you are too close.