Controlling Speed: Limits, Curves, and Energy
Controlling Speed tests whether you treat the posted limit as a ceiling for ideal conditions in a passenger car, or as a target for a loaded truck.
The rules
- Never drive faster than is safe for your weight, the road surface, curve banking, visibility, and traffic.
- Advisory speeds on curves and ramps are written for cars; a loaded combination often needs to go slower.
- Brake before a curve and use gentle throttle through it. Braking hard mid-curve shifts weight forward and invites a jackknife or rollover.
- Kinetic energy rises with the square of speed, so 40 mph needs about four times the braking distance of 20 mph, not twice.
Worked example
A ramp carries a 25 mph advisory. Empty, your truck takes it comfortably at 30. Loaded, the center of gravity is higher and the trailer pushes the tractor through the curve, so you enter at 20 to 25 and hold steady throttle. Why not 30? Because the advisory assumes a passenger car's roll center and tire grip, and your load raises the rollover risk on the same pavement.
Traps that cost marks
- Treating the posted limit as achievable in rain, fog, or on a downgrade.
- Braking hard in the middle of a curve instead of before it.
- Assuming the advisory speed is mandatory for every vehicle, rather than a car-based warning you may need to undercut.
- Forgetting that speed too fast for conditions is a citation even under the posted limit.