Truck & Car Guides / Road safety
Stopping distances explained
Why a loaded truck needs far more room to stop than a car, the four parts of total stopping distance, how speed and road conditions multiply it, and safe following distances.
The four parts of stopping
Total stopping distance is not just the distance the brakes need. It has several parts, and every one grows with speed.
| Part | What happens | Typical figure used in CDL training |
|---|---|---|
| Perception distance | The distance you travel between seeing a hazard and your brain recognizing it | About 1.75 seconds for an alert driver |
| Reaction distance | The distance you travel while moving your foot to the brake | About 0.75 seconds |
| Brake lag (air brakes) | Time for air to travel through the lines and apply the brakes | About half a second |
| Braking distance | The distance the vehicle travels once the brakes are applied | Depends on speed, weight, brakes and road |
At highway speed, a vehicle covers more than 80 feet every second. That means you travel well over 200 feet before the brakes even start to work, even if you react quickly.
Why trucks need so much room
A heavy truck has more brakes and tires than a car, but it also has vastly more weight to stop. Truck brakes can also heat up and lose effectiveness on long descents. FMCSA safety materials note that a fully loaded truck traveling at highway speed can need roughly the length of two football fields to stop. Air brakes add the brief brake lag described above. See air brakes.
Speed changes everything
Braking distance increases with the square of speed. Double your speed and you need roughly four times the braking distance. Even small speed increases add significant distance:
| Speed change | Approximate effect on braking distance |
|---|---|
| 50 mph to 60 mph | About 44 percent more |
| 50 mph to 70 mph | About double |
| 30 mph to 60 mph | About four times |
These ratios come from physics and apply to any vehicle, though actual distances vary with tires, brakes, load and surface.
Road conditions
Wet, snowy or icy roads reduce tire grip and greatly increase stopping distances. Commercial driver training commonly advises reducing speed by about one-third on wet roads, by about half or more on packed snow, and slowing to a crawl on ice. See winter driving.
Other factors that lengthen stopping:
- Worn tires or low tread
- Poorly adjusted or overheated brakes
- An empty truck, which has less grip and can be harder to stop smoothly than many people expect
- Downhill grades
- Driver fatigue or distraction, which lengthens perception and reaction time
Following distance
For trucks, the CDL manual guidance is to leave at least one second for each 10 feet of vehicle length at speeds below 40 mph, and to add one more second above 40 mph. For a 70-foot rig over 40 mph, that is about 8 seconds.
For cars, many state driver handbooks recommend at least a 3-second following distance in good conditions, more in bad weather.
How to count seconds
- Pick a fixed object ahead, such as a sign or bridge shadow.
- When the vehicle ahead passes it, count "one thousand one, one thousand two..."
- If you reach the object before finishing your count, drop back.
The space in front of a truck is its safety margin. Car drivers who cut in and brake take that margin away. Give trucks room, and truck drivers, keep your following distance even when others cut in.
