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Confusing stopping distances

Thursday 2nd February, 2012 at 17:02 COMMENTS (0)

Do you remember the days prior to taking your driving test when the mass of information in the Highway Code became such a blur?

Desperately trying to remember the information contained within regarding road signs and stopping distances. 

More recently those taking a driving theory test are given options to choose from but then they are in metres, feet or car lengths. 

Knowing the actual number doesn't actually prove that drivers can relate this in the real world, i.e.  following another vehicle. 

The Highway Code has published the same figures since 1946 which has given the driver plenty of time to memorise the numbers but as modern cars are capable of stopping in much shorter distances, do those numbers still have any relevance?

A number of factors affect the vehicles ability to be able to stop quickly such as tyres, brakes, the road surface, the cars load and weather conditions.  But the one single common factor in any and every slowing vehicle is the driver. 

Driving in 2012 is busy, fast and filled with a mass of hazards that present themselves at the most inconvenient times which all add to the drivers overload and reduced ability to stop promptly. 

Over the years we have seen a variety of public information adverts in the media that have been enlightening and filled with graphic detail that has sometimes neared shock level.  During which we've been informed that when travelling at just 5mph over the 30 mph speed limit, we travel an extra 7m or 21ft further. 

Again, those numbers are great but in my experience, drivers rarely can tell the distance accurately and it doesn't matter if it's in imperial or metric measurement. 

Back to the Highway Code which shows a typical stopping distance table which highlights clearly those distances.  Simple calculations allow the driver to work out the stopping distance at a certain speed. 

Maths was never my favourite subject at school and I certainly didn't expect it to be useful as a driver but if you can have the time, there is a method but only in feet. 

Ready, here goes.  The overall stopping distance is made up of two parts, thinking distance and braking distance.  Overall Stopping Distance
Firstly the easy bit, the thinking distance is simply the same as the speed.  e.g. 
30 mph = 30 feet
40 mph =40 feet and so on

The braking distance is a little more complicated and goes like this,
Starting at 20 mph (speed) multiply by 1 = 20 feet
Then 30 mph multiply by 1.5 = 45 feet
And 40 mph multiply by 2 = 80 feet and so on. 

To calculate the overall distance add the appropriate thinking distance to the braking distance, e.g. 
At 30 mph the thinking distance is 30 feet plus braking distance is 45 feet= an overall stopping distance of 75 feet.  Phew

Did that make sense? I would suggest a lot less when your actually travelling at 70 mph on a busy dual carriageway whilst overtaking another car and a Land Rover pushing you from behind. 

Rule 126 of the Highway Code simply suggests;

Drive at a speed that will allow you to stop well within the distance you can see to be clear. 

Allow a two second gap between you and the vehicle in front.  When the road is wet, the distance should be doubled and further on an icy road.


In short, wouldn't it be easier to apply the 2 second rule when travelling at speeds in excess of 30mph? Two second rule

In this case, there are no complicated calculations or searching of the memory, just count 2 seconds from the car in front passing a stationary object to when you pass the same object.  A useful way to remember this is to apply the following saying, 'Only a fool breaks the 2 second rule' , done. 

Note this applies on a dry road but must be said twice on a wet road which leads me to remember a pupil once applied this on a wet road and positively said, 'only a fool breaks the 4 second rule', oops. 

The Highways Agency in certain areas such as the A13 stretch of road between Basildon and Corringham in Essex are making this even easier by placing arrow on the road.  In this case the driver should visibly see 2 arrows between the vehicle they're following. 

If this helps the driver to pay more attention to what's happening, this should have a more positive effect on being able to stop promptly despite the outdated numbers shown in the Highway Code. 

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Further reading:
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