ADUK Archive

Hi Folks,
A friend who manages a driver improvement scheme asked if I knew how the stopping/braking distance figures in the Highway Code were first compiled.He was asked this question by a victim, I mean a client, oops! He is aware of the square root formula, 3x speed = 9 braking distance etc, and that reaction time depends upon state of mind, concentration, attitude, noticing a problem, planning a response and only then being able to get the right foot onto the middle pedal, all of which makes the current figures still valid because reaction times can be slowed by lack of concentration thus affecting total stopping distance.

I read somewhere that a Ford Anglia was used in about 1960 to check the figures but the question is ''How were they first compiled and when?'' Was it by actual testing or by pure maths. I am hoping that some senior forum member might know the answer.

Kind regards,
Mike B.

Don't know about 'Senior' but...

As you know, the overall stopping distance is made up of 'thinking distance' and 'braking distance'.

Thinking distance is based on a 0.7 second reaction time...this coincidentally means it works out to be the same as the speed but in feet i.e. 30mph = 30 feet thinking distance. This is because 3600 (seconds in an hour) is close to 0.7 of 5280 (feet in a mile). Coincidence? Well probably not.

The braking distance can also be 'worked out'...
Take first digit of the speed...i.e. 30mph = 3
square it (3 x 3) = 9
multply by 10 = 90
then halve it = 45 feet braking distance

Add the thinking distance 30 + 45 = 75 feet - bingo...as per highway code

Coincidence? I don't think so either...I suspect the braking distances might have been approximately right sometime ago and someone came up with the method above to make it easy to work out (and remember) on the fly.

Of course in practice overall stopping distances will vary hugely - it's only a rough guide and helps to emphasise how the distance lengthens hugely with small increments in speed.

It was taught to me as "halve the first digit, multiply it by the speed, and add the speed"

so 30mph gives 3/2 (1.5) * 30 (45) + 30 (75)

avoids any mental "squaring".

A little tinkering with Excel suggests the original figures were derived using a deceleration of 25 fps^2 (about 0.77g) and rounded up to the nearest foot. You get:

30 = 44.86 ft
40 = 79.75 ft
50 = 124.61 ft
60 = 179.44 ft

... and so on

I think the braking performance used is akin to a Ford Anglia with cross-ply budget tyres and all round drum brakes without power servo assistance.

Car braking performance is much better now, just a pity that driver reaction isn't (in many cases).

Big Err wrote:I think the braking performance used is akin to a Ford Anglia with cross-ply budget tyres and all round drum brakes without power servo assistance.

Car braking performance is much better now, just a pity that driver reaction isn't (in many cases).

Indeed that's why I always argue with those that claim the stopping figures in the Highway Code are irrelevent. They are only a guide and I for one would be the first to admit I wouldn't always be on the brakes in 0.7secs.

I find if something serious is developing on front I often don't react until I'm sure (which is wrong I know) therefore often taking significantly longer before pressing that pedal.

Who saw this very thing demonstrated by Clarkson on Top Gear where he used various vehicles in the B licence band - just the stopping distances though as there was a marked point in which to apply the brakes.

Mr Cholmondeley-Warner wrote:It was taught to me as "halve the first digit, multiply it by the speed, and add the speed"

so 30mph gives 3/2 (1.5) * 30 (45) + 30 (75)

avoids any mental "squaring".

A little tinkering with Excel suggests the original figures were derived using a deceleration of 25 fps^2 (about 0.77g) and rounded up to the nearest foot. You get:

30 = 44.86 ft
40 = 79.75 ft
50 = 124.61 ft
60 = 179.44 ft

... and so on

You'll have noticed in the latest edition of HC that the Imperial value for stopping distance at 60 m.p.h. has been reduced from the "old" value of 120' to a new value of 118'.
Something to do with converting the metric distance back to Imperial.

crr003 wrote:You'll have noticed in the latest edition of HC that the Imperial value for stopping distance at 60 m.p.h. has been reduced from the "old" value of 120' to a new value of 118'.
Something to do with converting the metric distance back to Imperial.

er ... 40 mph?

Mr Cholmondeley-Warner wrote:

crr003 wrote:You'll have noticed in the latest edition of HC that the Imperial value for stopping distance at 60 m.p.h. has been reduced from the "old" value of 120' to a new value of 118'.
Something to do with converting the metric distance back to Imperial.

er ... 40 mph?

Well spotted. I've had a busy day.

It's still 2' less!

As someone has said better modern cars stop well but since alot of people on the road don't concetrate on driving we can't look at lowering them. I dare to say that in the 60's with a drum braked anglia alot fo people drove safer with better planning and observation as if the didnt they had little or no safety features and the brakes where not amazing.

I have to say in a modern car with good observation there is no reason to think that your car won't pull up quicker although Nigel always says if you have to use safety features (abs etc) on a vehicle you have got it wrong as you should be able to pull up undrematically at whatever speed you are traveling.

I had a so called advanced motorcyclist on a speed awareness course a few years ago tell me that even a modern brand new car with lighting fast reactions would not pull up shorted than the highway code stated. I am not sure if she truely believed that or that she was being paid by the local safety camera partnership to mislead low end speeders as we where classed.

jbsportstech wrote:I dare to say that in the 60's with a drum braked anglia alot fo people drove safer with better planning and observation as if the didnt they had little or no safety features and the brakes where not amazing.

I think one of the other key points is that there was significantly less traffic around in the 60's which may mean that while planning and observation were little different from today, the likelihood of needing to take evasive action was far less. I'm not sure there was the concept that the cars being driven were unsafe .. they were certainly far safer than those from 10 years earlier

Chris

Hi All,
Thanks for your contributions and interesting comments even though we did not get to the 'end of the road' so far as the original question is concerned.

Cheers,
Mike B.

MikeB wrote:Thanks for your contributions and interesting comments even though we did not get to the 'end of the road' so far as the original question is concerned.

The last contributor on this page says it's just maths:

"They use a reaction time of 0.67 seconds (people had to push a button when a light came on…) and a deceleration of 0.66 g (21.23 ft/sec2)

They use the equation: D = S2 / 20 + S. (315 ft = 70 mph squared / 20 + 70)"

http://something-interesting.co.uk/2008 ... distances/

I have always suggested to my pupils they work out teh stopping distances as....

20mph X 2
30mph X 2 1/2
40mph X 3
50mph X 3 1/2
60mph X 4
70mph X 4 1/2

The multiplier simply increases by 1/2 each time. The thinking distance is simply the speed in feet and braking distance teh difference between the two.

Personally I think that is simpler than the methods suggested above, but then that may be because I have done it this way for a long time and so more familiar with it.

The only problem with any of these calculations is that they rely on the question & answer using feet. How long will the DSA include the measurements in feet for the theory test?

Nigel

Isn't worrying about figures all a bit academic anyway? How many people have been out and walked the stopping distances to see what they look like, then compared that against what their car is capable of? (it's a bit harder to figure in thinking distances as if you've got a known point to start braking at for this test it won't show much about your reactions)

It's all well and good knowing that you should be able to stop within 72m, but unless you know what that looks like from your drivers seat is it really that useful?