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

• Human Perception time/distance is the distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it. Perception time for an alert driver is about ¾ second. At 55 mph, you travel 60 feet in ¾ second (A good way to calculate this is to take 1.1 times the speed [in miles per hour] = perception distance in feet).
• Human Reaction time/distance is the distance traveled from the time your brain tells your foot to move from the accelerator until the time your foot pushes the brake pedal. An average driver reacts within ¾ second. This adds an additional 60 feet to the distance traveled at 55 mph (A good way to calculate this is to take 1.1 times the speed [in miles per hour] = reaction time in feet).
• Vehicle Reaction time/distance.  Once the brake pedal is applied there is the vehicles reaction time which depends on the brake pedal free-play, hydraulic properties of the brake fluid and working order of the braking system.  This time usually is from 0-¼ second (for purposes of this discussion, the vehicle reaction time/distance will be calculated as zero, however, it can be up to ¼ second). This is why the tailgating car usually cannot stop, when the brake light came on in the car in front, this driver had already completed the perception, human and vehicle reaction periods.
• Vehicle Braking distance is the distance it takes the vehicle to stop once you hit the brakes. At 55 mph on dry pavement, it takes a vehicle with good brakes about 4 ½ seconds to stop. Within that time, the vehicle will travel another 182 feet (braking distance = 0.06 times the speed squared).  This last factor is relating to the vehicle’s braking capability which depends on factors such as;
• the type of braking system,
• brake pad material,
• brake alignment,
• tire pressures,
• tire tread and grip,
• vehicle weight,
• suspension system,
• the co-efficient of friction of the road surface,
• wind speed,
• slope of road,
• surface smoothness
• braking technique applied by the driver.
• Total stopping distance; traveling at 55 mph, it will take about 6 seconds to stop your vehicle. The vehicle will travel approximately 302 feet before coming to a stop. That is longer than the length of a football field.

Rules of Thumb

• When you double your speed, it takes four times as much distance to stop your vehicle.
• Your vehicle will have four times the destructive power in a crash.
• You can not steer or brake a vehicle unless you have traction. Traction is the friction between the tires and the road. Reduce your speed on wet and slippery roads.
• Wet roads can double stopping distance. Reduce your speed by about 1/3 on a wet road. For example slow down from 55 mph to 35 mph.
• On packed snow, reduce your speed by ½ or more.
• If the road is icy, reduce your speed to a crawl. Stop driving as soon as you can.
• Empty trucks require greater stopping distance. An empty vehicle has less traction. The brakes are designed to control the maximum weight of the unit; therefore, the brakes lock up more readily when the trailer is empty or lightly loaded. This can cause skidding and loss of control.

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.

• Slow to a safe speed before you enter a curve.
• Braking in a curve is dangerous because you can lock the wheels and cause a skid.
• Never exceed the posted speed limit for a curve.

Speed and Distance Ahead

• You should always be able to stop within the distance you can see ahead.
• Fog, rain or other conditions may require you to slow down.
• At night, you can't see as far with low beams as you can with high beams. When you use low beams, slow down.

Speed on Downgrades

• As you go downhill, your vehicle's speed increases.
• Never exceed the maximum safe speed on a downgrade.
• Downshift to a low gear before staring down a grade.
• You must use the braking effect of the engine to control your speed on downgrades. The engine's braking effect is greatest when it is near the governed RPMs and the transmission is in a low gear.
• Save your brakes so that you can slow or stop as required by road and traffic conditions.

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 safety@siu.edu.

	20 mph	30 mph	55 mph	70 mph
Human Perception Distance
Human Reaction Distance
Vehicle Reaction Distance
Vehicle Braking Distance
Total Stopping Distance