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.
total stopping distance of a vehicle is made up of 4 components.
Vehicle Braking Time/Distance
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).
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).
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.
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
- the type of braking
- 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
technique applied by the driver.
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
- 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
- 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.
- 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
- 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
- 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.
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.
- 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,
- 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 firstname.lastname@example.org.
Total Stopping Distance