Stopping Distances for Cars and Lorries.
Car Stopping distances
Stopping distances in detail
Safe Following Distance - Trucks
Safe Following Distance - Cars
The average reaction time
from seeing an emergency situation to actually placing your foot
on the brake pedal is 0.7 seconds At 30mph, 44 feet per second, you will have travelled 30 feet
before you even take action, and a further 45 feet before the
brakes bring the car to a halt. On motorways, marker posts are
set up at 100 metre intervals - about the stopping distance when
driving at 70 mph. So if the car in front is passing a post, you
should not have passed the previous one.
STOPPING DISTANCE (Highway Code)
||Distance to Stop
||Feet per Second
Stopping distances for Cars and Lorries in detail.
In traffic, you should always
choose the lane that allows the maximum possible stopping distance.
It is possible to calculate stopping
distance mathematically using a graph if you know approximately
how much time it takes to stop at a certain speed. Lets say
you're going 60 mph.
How long does it take for your car or
truck to stop at 60 mph? The obvious way to test it is to look
at your watch and slam your brakes on, then count how many seconds
it takes to stop.
Lets pretend you are driving an eighteen-wheeler
and it takes six seconds to stop at 60 mph. That works out to
10 mph of slowing down for each second you apply the brakes.
At 10 mph. per second, every second you put the truck's brakes
on will slow you down by 10 mph. By knowing how much time it
takes to stop at a certain speed, you can calculate very precisely
what the stopping distance is going to be at any speed.
First let us calculate what the stopping
distance is at 10 mph. 10 mph is equal to about 14 feet per
second. There are 5,280 feet in a mile and 3,600 seconds in
an hour, so to convert miles per hour to feet per second you
just multiply your mph by 5,280 and divide by 3,600. You will
go fourteen feet in one second at ten miles per hour. Since
you're slowing down from ten mph and ending up at zero, you
will only average 5 mph or 7 ft. per second while you have your
brakes on. How many feet do you travel in one second at 7 feet
per second? Obviously, you travel seven feet, so your stopping
distance is seven feet.
To calculate what your stopping distance
is going to be from 20 mph you first have to calculate the distance
you are going to travel as you slow from 20 down to ten. Your
average speed is going to be 15 or 22 feet per second so the
total stopping distance is 22 feet plus the seven feet when
you slow from 10 down to zero or 29 feet.
With air brakes, that's not all there
is to it.
You also have to take into account the
air brake lag. It takes about a half-second for air brakes to
activate because pressure must build in the lines before the
actuators will start to move.
At 10 mph or 14 feet per second, you
will travel seven feet in a half second so the stopping distance
from the time you step on the brake pedal will be 7+7 or 14
ft. That is seven feet of air brake lag and seven feet of stopping
distance added together. At 20 mph the total distance will be
22+7+15 or 44 feet including 15 ft. of air brake lag. What you
can immediately see from this is that if you double your speed,
you increase your stopping distance considerably. 14 feet to
44 feet is more than three times.
So if you are going 10 mph and you increase
your speed to 20, your stopping distance is going to be three
times what it was a 10 mph. You can continue calculating and
find out what your stopping distance will be at 30 mph. At 30
mph you will add 38 feet, so your stopping distance will be
7+22+38 which works out to 88 feet when you include 22 feet
of air brake lag. If you increase your speed from 20 to 30,
you double your stopping distance.
Looking at these calculations its easy
to see why we have speed limits. If you increase your speed
from 10 mph to 20 mph, you triple your stopping distance. If
you increase your speed from 20 mph to 30 mph, your stopping
distance is six times greater.
TRUCK STOPPING DISTANCE @ 10 MPH. PER SECOND
|| STOPPING DIST
||SAFE FOLLOWING DIST
CAR STOPPING DISTANCE @ 20 MPH. PER SECOND
|| STOPPING DIST
||SAFE FOLLOWING DIST
This table shows stopping distances. (calculations are different
from the top table, fig. 1 and Fig. 2-3, you will notice the
distance to stop is shorter on fig. 2-3, Average speed reduction
time is different in the two calculations). It also shows the
minimum safe following distance.
A vehicle following another needs more
room to get stopped because it takes about two seconds longer
for the person following to get his foot off of the accelerator
and onto the brake, even if the two vehicles have identical
The safe following distance is equal
to the regular stopping distance plus the distance the vehicle
travels in the two seconds it takes for the driver to react
and move his foot at each given speed.
Following another vehicle at a distance
less than this would not be safe because if an object were to
fall off the vehicle in front, you would not be able to stop
in time to avoid colliding with it. The "seconds" column is
the number of seconds it takes to cover the distance in the
safe following distance column at each given speed. This makes
it easy to measure following distance while you are driving.
For instance, it takes three seconds
to go 44 feet at 10 mph and it takes 5.5 seconds to go 482 feet
at 60 mph. Measuring safe following distance in seconds is convenient
because you can easily count how much time elapses between the
time the vehicle in front passes a landmark such as a road sign
until you pass the same landmark yourself. Notice that the number
of seconds needed to be safe increases with speed from three
seconds for cars at 40 mph to four seconds at 80 mph.
Note that the table does not make a distinction
between single and multi-lane highways. The safe following distance
is 305 feet at sixty miles an hour regardless how many lanes
a highway may have or what lane the other traffic may be using.
This is because if an object falls off a long flatbed eighteen-wheeler,
it is more likely to fall to the side than to the rear.
The following table
shows typical stopping distances for Motorbikes
( 30 feet )
( 45 feet )
( 75 feet )
( 50 feet )
( 125feet )
( 175feet )
( 70 feet )
( 245feet )
( 315 feet )