What factors affect braking distance?
Again, speed is the first obvious factor
whatever the road vehicle - car, lorry, wagon, bus etc.
you are going the more kinetic energy has to removed from the car's kinetic
energy store. At a constant rate of braking force, it will take longer the
greater the speed, because more kinetic energy has to be converted to heat
energy in the brake pad and disc system.
This is shown on the right (brake pads P in contact with
All of the factors discussed here become particularly
crucial in an emergency braking situation or you suddenly find
yourself too close to the car in front.
The greater your speed, the greater the stopping
distance and the greater distance you should allow between one vehicle
and another e.g. the two chevron distance for 70 mph you see on some
sections of a motorway.
However good your brakes are, its no
good being too close to another vehicle i.e. well within the stopping
distance, if you are to avoid an accident if the vehicle in front does
an emergency brake or the traffic head rapidly comes to halt!
Speed limits aren't simply about speed reduction,
they are also about reducing both the stopping distance where higher
speeds are considered hazardous for a particular section of road. This
for the safety of road users and pedestrians e.g. 20 mph in narrow
streets in built up areas where there are likely to be many people
walking and crossing roads.
condition and the weather: The adverse condition of a road has already
been mentioned. With a dry road (and tyres in good condition) you will get
the maximum friction grip from the tyre-road surface contact on braking,
giving you the minimum distance travelled - the minimum thinking distance.
If the road is wet from rain, coated in snow or ice, the friction grip is
reduced (ice > snow >> standing water all leading to skidding on
braking). Modern tyres do very well on braking if the road is just a bit wet
and no obvious standing water - where can get 'aquaplaning'/'hydroplaning'
as you skid over a layer of water on the road surface. Leaves and split oil
also reduce the friction grip between tyre and road. All of these conditions
reduce the friction grip of tyre on road and increase the braking time and
The condition of the tyres: Tyres are designed to
give the maximum road grip and expel water from under the tyres on wet
roads. If the tyres are worn (bald or little tread left), there is less grip
and the vital friction and water expelling function to slow the vehicle down
are reduced and so increasing the braking distance and the chance of
skidding. Also, the tyres should contain enough air to give the correct
The effectiveness of the brakes: If the brakes are
not well maintained, braking function may be impaired. Brake pads might be
worn or a leak in the hydraulic brake system can be a source braking
impairment. Are the brakes balanced so that you slow down in a straight line
- this is point applies to the condition of the tyres too.
INDEX of physics notes on
reaction times, stopping distances of road vehicles, Newton's 2nd Law,
braking friction force, KE calculations
Keywords, phrases and learning objectives for
the physics of road vehicles -
braking distance factors
Be able to describe and explain factors that affect braking distance of road vehicle
including speed, road conditions -
ice snow wet slippery roads all reduce friction, as do worn tyres or condition of
brake pads, lack effectiveness of braking system, all of these can increase
your braking distance, which ultimately affects a driver's overall