Speed and Space Management
Speed
Drive at a speed that lets you stop within the distance you can see. Stopping distance equals perception distance plus reaction distance plus brake lag plus braking distance. At 55 mph a loaded truck needs much more room than a car.
Following distance
Use at least one second per 10 feet of vehicle length at speeds under 40 mph; add one second above 40 mph. A 60-foot truck at 55 mph needs 7 seconds; a 70-foot truck needs 8. Add more for rain, ice, darkness, or a heavy load.
Space
Keep a cushion ahead, beside, and behind. Pass promptly; don't hang beside another truck. On grades, downshift before the descent and leave room for a runaway truck.
Worked example
A 70-foot combination travels at 55 mph. Rule: 70/10 = 7 seconds; above 40 mph add 1 = 8 seconds. At 55 mph, 8 seconds is about 645 feet. Reasoning: the driver should open a gap of roughly two football fields, more in rain.
Traps
Using a car's 3-4 second gap; counting seconds from the wrong landmark; tailgating to prevent cut-ins; braking late on a downgrade.