CDL · General Knowledge · 8 min read
Choosing Speed and Keeping Space
- Explain how speed and vehicle weight determine total stopping distance, impact force, and braking requirements.
- Identify hazardous traction conditions, including wet pavement, ice, and standing water, and apply proper speed adjustments and hydroplaning recovery techniques.
- Calculate time-based following distance and apply correct space management procedures for tailgaters, overhead clearances, and intersection turns.
Speed, Stopping Distance, and Vehicle Weight
Driving too fast is a major cause of fatal crashes. As a commercial driver, you must constantly adjust your speed depending on traction, curves, visibility, traffic, and hills. Safely managing your speed starts with understanding total stopping distance. Total stopping distance is calculated by adding three distinct elements together: perception distance, reaction distance, and braking distance. Perception distance is the distance your vehicle travels under ideal conditions from the time your eyes see a hazard until your brain recognizes it. For an alert driver, the average perception time is 1 3/4 seconds. At 55 mph, that perception distance alone accounts for 142 feet traveled before you even begin to react.
Speed dramatically multiplies both braking distance and destructive energy. When you double your speed from 20 to 40 mph, the impact is 4 times greater and your braking distance is 4 times longer. If you triple your speed from 20 to 60 mph, the impact and braking distance become 9 times greater. In fact, at 60 mph, your stopping distance is greater than the length of a football field. Pushing your speed further from 20 to 80 mph makes the impact and braking distance 16 times greater. Because these forces scale so sharply with speed, small increases in velocity create massive demands on your vehicle's stopping capabilities.
Vehicle weight also plays a critical and counterintuitive role in how heavy vehicles stop. The brakes, tires, springs, and shock absorbers on heavy commercial vehicles are designed to work best when the vehicle is fully loaded. Traction is the friction between the tires and the road. Because an empty vehicle has less traction on the roadway surface, empty trucks require greater stopping distances than fully loaded trucks. Commercial drivers must never assume an unloaded truck will stop more quickly than a loaded one.
| Speed Increase | Impact Multiplier | Braking Distance Multiplier | Real-World Stopping Benchmark |
|---|---|---|---|
| Doubling speed (20 to 40 mph) | 4 times greater | 4 times longer | Significant increase in required braking area |
| Tripling speed (20 to 60 mph) | 9 times greater | 9 times longer | Stopping distance is greater than a football field |
| Quadrupling speed (20 to 80 mph) | 16 times greater | 16 times longer | Severe impact forces and extreme braking room required |
Common mistakes
- Assuming an empty truck stops faster than a loaded truck, forgetting that empty trucks have less traction and require greater stopping distance.
- Stepping on the brake pedal when hydroplaning occurs instead of releasing the accelerator and pushing in the clutch to allow the wheels to turn freely.
- Swinging wide to the left at the start of a right turn may lead a following driver to try to pass on the right.
- Assuming posted bridge clearance signs remain accurate after road crews repave the road or when snow becomes packed on the roadway.
Key points
- Total stopping distance equals perception distance plus reaction distance plus braking distance; for an alert driver under ideal conditions, average perception time is 1 3/4 seconds, or 142 feet traveled at 55 mph.
- Speed multiplies danger: doubling speed quadruples braking distance, while tripling speed increases impact and braking distance by 9 times.
- Empty trucks have less traction than fully loaded trucks and therefore require greater stopping distances.
- Reduce speed by about one-third on wet roads, by half or more on packed snow, and to a crawl on ice before stopping safely.
- Hydroplaning can occur at speeds as low as 30 mph; recover by releasing the accelerator and depressing the clutch without braking.
- Calculate following distance by allowing 1 second per 10 feet of vehicle length below 40 mph, and add 1 additional second at speeds over 40 mph.
- For right turns, turn wide as you complete the turn while keeping the rear close to the curb, and never swing left at the start.
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