Stopping a vehicle


Concepts and Research by Dale O. Ritzel, Ph.D., Safety Center, Southern Illinois University, Carbondale, IL 62901-6731

9 September 2003


Driving too fast is a major cause of crashes, injuries, and fatalities.  You must adjust your speed to suit weather conditions, the road (such as hills and curves), visibility and traffic.  Many persons drive in a false belief that if the car in front suddenly started braking, they would react and brake and end up stopped the same distance apart.

The total stopping distance of a vehicle is made up of 4 components.

·         Human Perception Time/Distance

·         Human Reaction Time/Distance

·         Vehicle Reaction Time/Distance

·         Vehicle Braking Time/Distance

Rules of Thumb

Slippery when wet

  • Shady parts of a road will remain icy and slippery long after open areas have melted.
  • Bridges freeze before the road freezes. Be careful when the temperature is around 32 degrees F.
  • Slight melting makes ice wet. Wet ice is more slippery than ice that is not wet.
  • Black ice is a thin layer that is clear enough that you can see the road underneath. It makes the road look wet. When the temperature is below freezing and the road looks wet, watch for black ice.
  • If ice is on the front of your mirror, mirror support or antenna, the road surface is probably starting to ice up.
  • Roads are very slippery when rain first begins. Just after rain begins, water mixes with oil on the road making it unusually slippery.

Hydroplaning - In some weather, water or slush collects on the road. When water forms a layer between the pavement surface and the tire while the vehicle is operating.  When this happens, your vehicle can hydroplane. The tires lose contact with the road and have little or no traction. You may not be able to steer or brake. Hydroplaning can occur at speeds as low as 30 mph. Hydroplaning is more likely if tire pressure is low or the tread is worn.

  • Take your foot off the accelerator.
  • This will slow your vehicle and let the wheels turn freely.
  • Do not use the brakes to slow down.
  • If the drive wheels begin to skid, steer in the direction you want to go.

Speed and Curves

If you take a curve too fast, your tires can lose traction with the road. This could cause your vehicle to skid off the road or roll over. Tests show that trucks with a high center of gravity can roll over at the posted speed limit for a curve.

Speed and Distance Ahead

Speed on Downgrades

Assignment:  For the following speeds indicated in the table, determine the human perception distance in feet, the human reaction distance in feet, the vehicle reaction distance in feet (use 0 seconds), and the vehicle braking distance in feet.  Finally, calculate the total stopping distance for 20 mph, 30 mph, 55 mph, and 70 mph.  Send the results of your calculations to your instructor at


20 mph

30 mph

55 mph

70 mph

Human Perception Distance





Human Reaction Distance





Vehicle Reaction Distance





Vehicle Braking Distance





Total Stopping Distance






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