Commercial Driver Handbook ( PDF ) - California Department of ...

perception time for an alert driver is about 3/4
of a second. At 55 mph you travel 60 feet in
3/4 of a 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 of a 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. (About
4 and 3/4 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).
Refer to Page 85 for stopping distances with air
brakes.
Control and stopping requirements. The service
brake must hold the vehicle or combination of
vehicles stationary on any grade on which it is
operated under all conditions of loading or unloading
(CVC §26454).
The service brakes of every motor vehicle or
combination of vehicles must be capable of stopping
from an initial speed of 20 mph as follows
Maximum Stopping Distance in feet (MSD):
• Passenger vehicle—25 MSD
• Single motor vehicle with a manufacturer’s
GVWR of less than 10,000 pounds—30 MSD
• Single motor vehicle with a manufacturer’s
GVWR of 10,000 pounds or more, or any
bus—40 MSD
• Combination of vehicles consisting of a
passenger vehicle or any motor vehicle with
a manufacturer’s GVWR of less than 10,000
pounds in combination with any trailer,
semitrailer, or trailer coach—40 MSD
• All other combinations of vehicles—50 MSD
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The effect of speed on stopping distance. The
faster you drive, the greater the impact or striking
power of your vehicle. When you double your speed
from 20 to 40 mph the impact is 4 times greater.
The stopping distance is also 4 times longer. Triple
the speed from 20 to 60 mph and the impact and
stopping distance is 9 times greater. At 60 mph,
your stopping distance is greater than that of a
football field. Increase the speed to 80 mph and the
impact and stopping distance is 16 times greater
than at 20 mph. High speeds greatly increase the
severity of crashes and stopping distances. By
slowing down, you can reduce stopping distance.
The effect of vehicle weight on stopping distance.
If a vehicle is heavier, brakes have to work harder
(and absorb more heat) to stop. The brakes, tires,
springs, and shock absorbers on heavy vehicles
are designed to work best when the vehicle is fully
loaded. Generally, 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 cannot steer or brake a vehicle unless you have
traction. Traction is friction between the tires and
the road. These are some of the road conditions
which 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
the stopping distance. Reduce
speed by about one third (e.g., slow
from 55 mph to about 35 mph) on
a wet road. On packed snow, reduce
speed by half, or more. If the surface is icy, reduce
speed to a crawl and stop driving as soon as you
can safely do so to install chains, if necessary.
Sometimes it is difficult to know if the road is
slippery. Here are some examples of slippery roads:
• Shaded areas. Shady parts of the road will
remain icy and slippery long after open areas
have melted.