The Commercial Drivers License
B-6 CONTROLLING SPEED
Driving too fast for conditions is a major cause of fatal crashes.
You must adjust your speed depending on driving conditions. These
include traction, curves, visibility, traffic, and hills. Speed
and Stopping Distance
There are three things that add up to total stopping distance:
Perception Distance + Reaction Distance +
Braking Distance = Total Stopping Distance
• Perception distance.
This is the distance your vehicle travels from the time your eyes
see a hazard until your brain recognizes it. The perception time
for an alert driver is about 3/4 second. At 55 mph, you travel 60
feet in 3/4 second.
• Reaction distance.
The distance traveled from the time your brain tells your foot
to move from the accelerator until your foot is actually pushing
the brake pedal. The average driver has a reaction time of 3/4 second.
This accounts for an additional 60 feet traveled at 55 mph.
• Braking distance.
The distance it takes to stop once the brakes are put on. At 55
mph on dry pavement with good brakes it can take a heavy vehicle
about 170 feet to stop. It takes about 4 1/2 seconds.
• Total stopping distance.
At 55 mph it will take about 6 seconds to stop and your vehicle
will travel about the distance of a football field. (60 + 60 + 170
= 290 feet).
The effect of speed on stopping distance.
Whenever you double your speed, it takes
about four times as much distance to stop and your vehicle will
have four times the destructive power in a collision. High
speeds increase stopping distances greatly. By slowing down a little,
you can gain a lot in reduced braking distance.
The effect of vehicle weight on stopping distance.
The heavier the vehicle, the more work the brakes must do to stop
it and the more heat they absorb. But the brakes, tires, springs,
and shock absorbers on heavy vehicles are designed to work best
when the vehicle is fully loaded.
Empty trucks require greater stopping distances,
because an empty vehicle has less traction. It can bounce and lock
up its wheels, giving much poorer braking. (This is not usually
the case with buses)
.Matching Speed to the Road Surface
You can’t steer or brake a vehicle unless you have traction. Traction
is friction between the tires and the road. There are some road
conditions that reduce traction and call for lower speeds.
Slippery Surfaces. It will take longer to stop and it will be harder
to turn without skidding when the road is slippery. You must drive
slower to be able to stop in the same distance as on a dry road.
Wet roads can double stopping distance. Reduce speed by about one
third (e.g., slow from 55 to about 35 mph) on a wet road. On packed
snow, reduce speed by a half, or more. If the surface is icy, reduce
speed to a crawl and stop driving as soon as you can safely do so.
Identifying Slippery Surfaces. Sometimes it’s hard to know if the
road is slippery. Here are some signs of slippery roads:
• Shaded areas. Shady parts of the road will remain icy and slippery
long after open areas have melted.
• Bridges. When the temperature drops, bridges will freeze before
the road will. Be especially careful when the temperature is close
to 32 degrees F.
• Melting ice. Slight melting will make ice wet. Wet ice is much
more slippery than ice that is not wet.
• Black ice. Black ice is a thin layer that
is clear enough that you can see the road underneath it.
It makes the road look wet. Any time the temperature is below freezing
and the road looks wet, watch out for black ice
• Vehicle icing. An easy way to check for ice is to open the window
and feel the front of the mirror, mirror support, or antenna. If
there’s ice on these, the road surface is probably starting to ice
• Just after rain begins. Right after it starts to rain, the water
mixes with oil left on the road by vehicles. This makes the road
very slippery. If the rain continues, it will wash the oil away
In some weather, water or slush collects on the road. When this
happens, your vehicle can hydroplane. It’s like water skiing: the
tires lose their contact with the road and have little or no traction.
You may not be able to steer or brake. You can regain control by
releasing the accelerator and pushing in the clutch. This will slow
your vehicle and let the wheels turn freely. If the vehicle is hydroplaning,
do not use the brakes to slow down. If the drive wheels start to
skid, push in the clutch to let them turn freely. It does not take
a lot of water to cause hydroplaning. Hydroplaning can occur at
speeds as low as 30 mph if there is a lot of water. Hydroplaning
is more likely if tire pressure is low or the tread is worn. (The
grooves in a tire carry away the water; if they aren’t deep they
don’t work well). Be especially careful driving through puddles.
The water is often deep enough to cause hydroplaning.
Speed and Curves
Drivers must adjust their speed for curves in the road. If you
take a curve too fast, two things can happen. The wheels can lose
their traction and continue straight ahead, so you skid off the
road. Or, the wheels may keep their traction and the vehicle rolls
over. Tests have shown 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 it is easier to lock the wheels and cause a skid. Slow down
as needed. Don’t ever exceed the posted speed limit for the curve.
Be in a gear that will let you accelerate slightly in the curve.
This will help you keep control.
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 that you slow down
to be able to stop in the distance you can see. At night, you can’t
see as far with low beams as you can with high beams. When you must
use low beams, slow down.
Speed and Traffic Flow
When you’re driving in heavy traffic, the safest speed is the speed
of other vehicles. Vehicles going the same direction at the same
speed are not likely to run into one another. Drive at the speed
of the traffic, if you can without going at an illegal or unsafe
speed. Keep a safe following distance. The main reason drivers exceed
speed limits is to save time. But anyone trying to drive faster
than the speed of traffic will not be able to save much time. The
risks involved are not worth it. If you go faster than the speed
of other traffic:
• You’ll have to keep passing other vehicles. This increases the
chance of an accident.
• It is tiring. Fatigue increases the chance of an accident. Going
with the flow of traffic is safer, and easier.
Speed on Downgrades
Going slow is the most important thing in going down long steep
hills safely. If you do not go slowly enough, your brakes can become
so hot they won’t slow you down. Shift your transmission to a low
gear before starting down the grade. Pay attention to signs warning
of long downhill grades, and check your brakes before starting down
the hill. Use snub braking. Going down steep hills safely is discussed
more in “Mountain Driving.” Read that section carefully.