ADUK Archive

I use the same method as adiNigel to calculate HC stopping distances. I use kerb stones to demonstrate the distance to pupils, although I find that I do not relate to stopping distances in ft or mtrs myself.

Stopping distances are only a very rough guide as many factors are involved. Driver reaction is a major factor. When you consider that at 40mph a car travels approximately 60ft per second, and if we say that the average reaction time is .75 of a second, the driver has travelled 45ft before he/she reacts. If the driver reacts in .5 of a second the driver will travel 30ft, and a slower reacting driver, say one that reacts in 1sec. will travel 60ft. You therefore have stopping distances that differ by 30ft at the same speed. Add to that the mechanical condition of the vehicle, ie tyres, brakes, suspension, etc and road surface condition, and the stopping distances will vary considerably.

Safe Speed produced a braking calculator based on brake pedal pressure. Comfortable braking at .45g of pressure at the pedal, and emergency braking at .9g. This can be found on http://www.safespeed.org.uk/braking.xls

Nigel ADI
IAM trainee observer

Stopping distances were introduced in the 1946 edition of the HC and they are remarkably similar as today's.

Have a look at page 35.

I love it. Number 65, advice to cyclists.
Never ride close behind a fast moving vehicle, it may slow down or stop suddenly.

Clearly cyclists in those days were fitter, or was their definition of "fast" 20mph?

Red Herring wrote:I love it. Number 65, advice to cyclists.
Never ride close behind a fast moving vehicle, it may slow down or stop suddenly.

Clearly cyclists in those days were fitter, or was their definition of "fast" 20mph?

Yes but fast is a relative term. Surely when giving advice to cyclists fast would be relative to the speeds that a cyclist might normally expect to reach. For an unfit cyclist 20mph going up a hill 20mph would probably qualify as "extremely fast".

Anyhow, 20 mph is not that fast for a fit cyclist. I have followed cyclists at speeds of up to 30mph on the flat. Just last week I was behind one going downhill at 50mph!

Hi All

The Stopping distance is based on a reaction time of approximately One and a half seconds and a deceleration of approximately 2/3 of g or roughly 6.6 metres per second per second (around mph 15 per second). The figure for thinking or reaction time is most likely based on statistics from laboratory tests of reaction times (which are much quicker than non laboratory tests).

The deceleration rate was at the top end of performance for tyres and brakes of typical fifties cars.

The learning of these figures or even those based on the performance of modern brakes and tyres for the purposes of passing a test is not relevant to the driving task, as people are generally very bad at estimating distance. Estimates can also vary depending on the time of day, familiarity of hazards and other factors.

Hope this helps.

Best regards

Martin A

Martin A wrote:<snip> ... as people are generally very bad at estimating distance. Estimates can also vary depending on the time of day, familiarity of hazards and other factors.

Does it make sense then to think of stopping "distance" in time rather than meters/feet/yards/inches or whatever units.

i.e. if I know that a normal car, with normal brakes on a normal surface that is dry and sound etc. will take ~1 sec per 10mph to stop then I know I need about 3 seconds to stop from 30 mph + my 1 second reaction time = 4 seconds....

I'm not sure if the 1s is technically correct but as a theory does it hold up? It's quite easy to use fixed markers to measure time while driving as anyone who's ever used "only a fool breaks the 2 second rule" as a guideline would know.

Am I the only person who really doesn't care what the HC says about stopping distances?

I really don't find it helpful or relevant. I find it an arcane exercise in memory or calculation (ooh ooh - this is how *I* remember it) which adds little or nothing to driver knowledge and action.

More interesting is to try it several times on a dry/wet/icy road.

Maybe I just have absurdly large safety margins that I could pare down to "terrifying" if I tried a little harder to estimate 134ft whilst doing 60mph into a blind bend.

Is that 134 feet of tarmac or view? I'm not very good at physics but I think you will find even those braking distances were based on braking in a straight line. Now trying to factor in a steering co-efficient as well might prove even more complicated. I wonder just how much tarmac you really do need to stop from 60mph in a left hand bend, in the rain, whist crossing two manhole covers and some overbanding....?

7db wrote:Am I the only person who really doesn't care what the HC says about stopping distances?

I really don't find it helpful or relevant. I find it an arcane exercise in memory or calculation (ooh ooh - this is how *I* remember it) which adds little or nothing to driver knowledge and action.

More interesting is to try it several times on a dry/wet/icy road.

Maybe I just have absurdly large safety margins that I could pare down to "terrifying" if I tried a little harder to estimate 134ft whilst doing 60mph into a blind bend.

No I don't take alot of notice other than learn them incase someone asks in a test scenario.

I use the old met police teachings of turning my speed across the ground into feet per second ie 30mph is 45 feet per second as that is more relevant in my mind than miles and hours. I normally observe a 3-4 second following distance and more if wet or icey etc or in a contact position in preparation for an overtake. This allows plenty of time to plan and gives and keeps your options open where as 2 secs on dry day gives you much less time and relies on quick reactions all of the time.