General Knowledge: Speed and Space Management, Hazard Perception
Choosing speed
The posted limit is a ceiling, not a target. Speed must drop for curves, ramps, downgrades, rain, snow, fog, night, and heavy or shifting loads. Look 12 to 15 seconds ahead so speed changes are gradual and signaled.
Following distance and space
Use one second per 10 feet of vehicle length below 40 mph, and add one second above 40 mph. Add more time for poor traction, darkness, or a heavy load.
Total stopping distance has four parts: perception distance, reaction distance, brake lag, and braking distance. Air brakes add lag of roughly half a second, which is about 32 feet at 55 mph - the reason a fully loaded rig needs around 369 feet to stop from that speed.
Hazard perception
- Scan far ahead and read clues: brake lights down the line, warning signs, shadows, side roads, brake lights at dusk.
- Always keep an escape route - a shoulder, a gap, or a lane you can use.
- Communicate early with signals, four-way flashers, and the horn when needed.
- Handheld phone use and texting are prohibited for commercial drivers, and distraction erases following distance fast.
Worked example
A 65-foot double travels at 55 mph. That is 6.5 seconds of base gap, rounded to 7 seconds because you are above 40 mph. At 55 mph you cover about 81 feet each second, so 7 seconds is roughly 565 feet - nearly two football fields. In rain you would stretch that further, because the braking portion of stopping distance grows while perception and reaction time stay the same.
Traps
- Using a car-sized following gap behind or ahead of a rig.
- Forgetting that brake lag exists, so air-brake stopping distance is longer than a car's.
- Tailgated by another driver and braking to teach a lesson instead of opening space ahead.