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 Post subject: Braking distances
PostPosted: Mon Jul 26, 2004 22:34 
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How many years ago were the braking distances measured?

the same time as speed limits were implemented?

yes with draconian cars with very little safety in the car itself


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PostPosted: Mon Jul 26, 2004 23:23 
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They're based on a 1960-something Ford Anglia with drum brakes all round. Yes, most cars these days will stop in shorter distances - the advent of disc brakes, ABS, etc, has helped that. But it doesn't necessarily make the roads safer if people drive faster to compensate. Perhaps some kind of study into the correlation between brake effectiveness and accident likelihood would be useful?

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PostPosted: Tue Jul 27, 2004 10:32 
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At the end of the day, the braking distances are there to paint a worst case scenario of braking distance. If some people are still driving round in Ford Anglias (there's probably a few) they need to be aware of their braking distance.

However, as braking distance AND reaction times are different for every vehicle and every driver, individuals need to take responsibility to drive appropriately, taking into account road conditions, and be aware of how their car performs.

This doesn't mean the police should be given the freedom to start prossecuting motorists who fail to maintain what they view to be a safe distance between them and the car infront.

If someone causes an accident because they are driving without due care and attention though, (even if no-one is seriously hurt), then that motorist should be dealt with accordingly.


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PostPosted: Tue Jul 27, 2004 20:59 
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Hmmmm, the last Ford Anglia I saw was crashing into the Whomping Willow in the Harry Potter film :D :D

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 Post subject: Harry Potter
PostPosted: Wed Jul 28, 2004 15:18 
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As the man from the scamera partnerships says "I see, driving too fast, flying low, underaged, not insured, probably not taxed, Harry Potter you're nicked !!"

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PostPosted: Wed Jul 28, 2004 15:24 
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it was Ron driving not Harry but then again, as it was Ron's dad's car, then he'll be the one getting the NIP


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PostPosted: Wed Jul 28, 2004 17:20 
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DevilsAdvocate, I'm not sure the HC braking distances were ever worst case. I think in the 60's they were based on an Anglia as it was felt that it was fairly representative. At the time it certainly wouldn't have been the worst at stopping. I'm sure an old series type Land Rover would have taken longer to stop, and I'm pretty sure that even today a series IIa or III is a more common sight than an Anglia. So the HC distances still aren't worst case, and to keep things simple they have to assume level dry road too. Road conditions, gradients and tyre types are not accounted for. Hitting the anchors in a IIa fitted with mudpluggers on a wet road while going downhill... :shock: HC distances are out the window, you'd probably stop in the next county. Still, it'd keep the speed kills brigade happy by driving such a slow car, albeit one that weighs the thick end of two tons, is shaped like a wall, and stops like a duck landing on a frozen pond. :lol:
I think the HC distances have never been changed through inertia as much as anything else. Quite simply, nobody has got round to updating them. Still, although apparently outdated, I think they do serve a purpose. For one thing they're a guide for new drivers. With experience they'll become better at judging the safe distance from the car in front, but while they're still inexperieced it's a good idea to make them memorize a set of stopping distances with a big safety margin. It's also easy for learners to work these out in their heads. Thinking distance is 10 feet per 10mph of speed, easy to remember. Braking distance is a little more complicated - at 20mph it's 20 feet, at 30mph times speed by 1.5 to get 45 feet, at 40 times speed by 2 to get 80 feet, at 50 times speed by 2.5 to get 75 feet, and so on. Then add 'em together - 70mph = 70feet + (3.5 x 70) = 315 feet. Bit harder to do in meters, but probably still easier than if the distances were actually representative of modern cars.

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PostPosted: Wed Jul 28, 2004 17:49 
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See what you mean about the landrover. Perhaps if we all drove them , then no-one would get hurt. We wouldn't be able to get above 40mph, we'd be wrapped in steel and nicely protected(ish) in an accident. Just there's be pedestrians lining the road when it took 4 miles to stop at a pedestrian crossing!

Seriously though, the Ford Anglia was an off-the-top-of-my-head example. I wasn't even born in the sixties so I wouldn't have a clue what they're really based on.

With reference to new drivers though, how many learner drivers can honestly say how far the vehicle in front is. Especially when you're not advised on it and testers don't question you on it.

Furthermore, if drivers have those kind of guidelines, the first time someone has an accident because they didn't stop within the HC published distances (because their brakes are shot or whatever) they're likely to sue!! Another wonderful part to todays culture!!!


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PostPosted: Thu Jul 29, 2004 02:36 
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DevilsAdvocate wrote:
See what you mean about the landrover. Perhaps if we all drove them , then no-one would get hurt. We wouldn't be able to get above 40mph, we'd be wrapped in steel and nicely protected(ish) in an accident. Just there's be pedestrians lining the road when it took 4 miles to stop at a pedestrian crossing!
That was just me being unable to resist a dig at the 'speed kills' crew. Old Landies are a good example of how it depends on the car too. Still, if we all drove Landies that stopped like supertankers with a full load I imagine pedestrians would soon learn to be more careful on the road.

DevilsAdvocate wrote:
Seriously though, the Ford Anglia was an off-the-top-of-my-head example. I wasn't even born in the sixties so I wouldn't have a clue what they're really based on.
Well, to be honest I was only around for the last few months of the 60's, but I'm pretty sure you're right about it being an Anglia. At least, popular wisdom says so, though we all know the caveats implied there. :)

DevilsAdvocate wrote:
With reference to new drivers though, how many learner drivers can honestly say how far the vehicle in front is. Especially when you're not advised on it and testers don't question you on it.
Fair point. Knowing the stopping distances isn't really enough without the ability to judge distances too. Think I got asked about stopping distances on my test, but certainly didn't asked to estimate how far away anything else was. That was 15ish years ago of course, but I don't think much has changed apart from the theory test. Anybody who's passed recently know any different?

DevilsAdvocate wrote:
Furthermore, if drivers have those kind of guidelines, the first time someone has an accident because they didn't stop within the HC published distances (because their brakes are shot or whatever) they're likely to sue!! Another wonderful part to todays culture!!!
That's what due care & attention and construction and use regs are for, but sooner or later someone will find the right lawyer and sue. And up goes everyone's insurance once again. Fine where genune, but don't get me started on the compo culture clowns. These ****wits are the reason my electric kettle has "WARNING: DO NOT IMMERSE THIS APPLIANCE IN ANY LIQUID" written across the bottom of it. :roll: :evil:

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 Post subject:
PostPosted: Thu Jul 29, 2004 08:15 
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Quote:
Still, if we all drove Landies that stopped like supertankers with a full load I imagine pedestrians would soon learn to be more careful on the road.


I'm liking that! Can they bring back bullbars as well? ;o)


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 Post subject:
PostPosted: Thu Jul 29, 2004 19:53 
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Gatsobait wrote:
DevilsAdvocate wrote:
With reference to new drivers though, how many learner drivers can honestly say how far the vehicle in front is. Especially when you're not advised on it and testers don't question you on it.
Fair point. Knowing the stopping distances isn't really enough without the ability to judge distances too. Think I got asked about stopping distances on my test, but certainly didn't asked to estimate how far away anything else was. That was 15ish years ago of course, but I don't think much has changed apart from the theory test. Anybody who's passed recently know any different?


Why do they persist in trying to teach people braking distances?
Distance is notoriously difficult for drivers to judge, with their foreshortened viewpoint, and judging braking distances is further compounded by the fact that they do not have a linear relationship with speed.
It's far more intuitive to judge braking distance in terms of time.
If you know that your car, under hard braking, decelerates at 20mph per second, then you know that - after allowing for your reaction time - it'll take you one second to stop from 20mph, one and a half seconds to stop from 30, two seconds from 40, and so on. A nice linear relationship. And, because you cover the equivalent of your braking distance in half that time if you don't brake, it simplifies the formula to 40mph = 1 second.
So, if you're travelling at 60mph it works out as 60/40 x 1 = one and a half seconds. Adding, say, 1 second for reaction time and an extra half-second for a safety margin makes 1.5 + 1 + 0.5 = 3 seconds. This all means that if you see an object at the side of the road which you judge will take you 3 seconds to reach at your current speed, you'll be able to stop before you reach that object.

I've calculated a table from the above assumptions:

10mph = 1.75 seconds
20mph = 2 seconds
30mph = 2.25 seconds
40mph = 2.5 seconds
50mph = 2.75 seconds
60mph = 3 seconds
70mph = 3.25 seconds
80mph = 3.5 seconds
90mph = 3.75 seconds, and
100mph = 4 seconds.

Easy, innit?


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 Post subject:
PostPosted: Thu Jul 29, 2004 20:30 
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My copy of HC - rule 105 gives it in feet, metres and car lengths and says car length is around 4 metres.

Not perfect - but at least it gives something for mind's eye to latch onto :roll: Would say this is makes it little easier to judge the distance.

But they say it takes 0.7 seconds for average person to see hazard, move foot from throttle and hit the brake pedal. Based on my medical knowledge - would say that is fair enough. Some are faster - but general most seem to fall within this!

Also little factor of knowing and being used to the vehicle. Since I drive lot of cars of various ages on track days (other hobby besides golf ;)) - would say brakes are unique to each car too! You get used to a vehicle's "feel" and that also influences driving style and when to hit the brakes for smooth stop.

HC is just guideline based on averages and generally - braking systems have improved on aggregate - and common sense, knowledge of your car and not over-estimating your ability and capability is part and parcel. Comes with experience as well.

As for learner drivers and testing vehicle in front - yes it is a shame they are not asked this out of blue.

"Only a fool ignores the two second rule" takes about two seconds to say in normal speak. And this is easy to check - when car in front passes sign - your say this aloud and if you pass same sign on utterance of rule - you have cracked it! :wink:

So simple! :wink:


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 Post subject:
PostPosted: Fri Jul 30, 2004 02:30 
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Pete317 wrote:
Why do they persist in trying to teach people braking distances? ...It's far more intuitive to judge braking distance in terms of time.
Fair point. I don't know if they go into the 2 second rule much now, but I don't remember it getting much coverage when I took my test. That's not to say there's no value in the braking distances. If nothing else the table shows that you can't just leave a constant gap, and that the thinking distance grows at the same rate while the braking distances grows at an increasing rate. FWIW I think the old style table was better where the graphic was car lengths instead of these meaningless red and blue arrows. Perhaps it would be better to use braking distance for the theory test only, and switch to teaching it in time for the practical.

Mad Moggie wrote:
"Only a fool ignores the two second rule" takes about two seconds to say in normal speak. And this is easy to check - when car in front passes sign - your say this aloud and if you pass same sign on utterance of rule - you have cracked it! :wink:
"One kangaroo, two kangaroo" does it for me. Picked that one up some years ago from a TV series on safe driving by Alexei Sayle (which would be worth repeating). Takes about the same amount of time to say, though passengers think you're a bit odd if you say it out load. :)

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PostPosted: Fri Jul 30, 2004 07:53 
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Gatsobait wrote:
That's not to say there's no value in the braking distances. If nothing else the table shows that you can't just leave a constant gap.


Why not? That would only be true if the vehicle in front of you stopped absolutely dead - which is hardly likely to happen. When you're two seconds further on, so is the car ahead, and, if you have equal brakes, you will both decelerate at the same rate.


Last edited by Pete317 on Fri Jul 30, 2004 17:49, edited 2 times in total.

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 Post subject:
PostPosted: Fri Jul 30, 2004 13:44 
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Think of how many times a car has pulled out from a junction in front of you in such a way as to cause you to have to break.

If other motorists can't judge whether there is enough room to pull out infront of an oncoming car, why should they be capable of judging breaking distance?


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 Post subject:
PostPosted: Fri Jul 30, 2004 14:58 
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Pete317 wrote:
Gatsobait wrote:
That's not to say there's no value in the braking distances. If nothing else the table shows that you can't just leave a constant gap.[/quoue]

Why not? That would only be true if the vehicle in front of you stopped absolutely dead - which is hardly likely to happen. When you're two seconds further on, so is the car ahead, and, if you have equal brakes, you will both decelerate at the same rate.
Sorry. I worded that very poorly. I wasn't thinking about the sort of situation you decribe. I meant that it shows learners that more speed needs a bigger gap than they might think. That is, the distance doesn't just increase proportionately with speed.

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 Post subject:
PostPosted: Mon Aug 02, 2004 17:01 
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One thing that one is rarely aware of, as far as braking distances are concerned, is that, quite often, one can stop in much shorter distance that we even think. The actual stopping distance of a modern car on a dry road is very short indeed, and we almost never use that capacity in normal driving (which is a good thing of course).

I mean, for example, suppose you find yourself, by inadvertance, with an obstacle in front of you (eg a car maneuvering at an crossing), closer thant the distance you are used to start braking at.
You will probably think you are too close to stop, while actually there is more than enough distance for maximum braking.

Now you think "God I'm gonna hit"!, and that is the beginning of panic. I imagine several consequences can happen:

- slamming the brakes and locking the wheels: now ABS prevents that, but there are still some cars around that do not have ABS. I almost rear-ended a car some years ago, in such a situation (the traffic on the motorway was so calm I let my attention drift, getting a bit close to the car in front and reacting late when, suddenly, everybody braked to a halt...), and I only avoided the accident because I had recently practiced emergency braking, and realized at the critical moment, after locking the wheels, that I had to unlock them to try stopping in time - with the wheels locked I would have hit :?

- getting paralyzed with fear and doing nothing. Everybody realizes the consequences... (I've often heard that many accidents happen with drivers under-reacting...)


Of course, in a descent on a wet road, the stopping distances will go up and will surprise many people.


For all these reasons, I think that just practicing from time to time stopping in a short distance in various situations can only be beneficial and transform many "emergency situations" that often end up badly into situations that simply demand quick action (that is, without even changing the timing, speeds, and positions of the vehicles involved).

What do you think?


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 Post subject:
PostPosted: Mon Aug 02, 2004 17:07 
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A prime example of the veriety of stopping distances was shown on BBC's Top Gear recently.

They took one of the new Mercedes SLR's and Jeremy Clarkson demonstrated how the ceramic brakes work on it. They marked a line on the road and put out cones the same distance away from the line as it takes to stop from 60mph (in HC). He then approached the line at 120mph in the SLR and started braking on the line. This machine stopped in the same distance from 120mph with room to spare.

This should not be used as a benchmark or tried by anyone on the roads!

However, if we compared this to the "rusty old Landy" used in previous replies, you start getting an idea of the shear veriety of stopping distance dependent on ojust vehicle type and quality! Not to mention environmental factors!

The only copmuters that could deal with forming a judgement on this kind of information are cooled by nitrogen and are very expensive indeed!


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 Post subject:
PostPosted: Tue Aug 03, 2004 11:22 
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Mad Moggie wrote:
"Only a fool ignores the two second rule" takes about two seconds to say in normal speak. And this is easy to check - when car in front passes sign - your say this aloud and if you pass same sign on utterance of rule - you have cracked it! :wink:

So simple! :wink:


Yep, this is a subject for the press today, apprntly there are only two two secondf chevrons painted anywhere in the country.. What they keep forgetting to tell people is you dont need chevrons, just pick a spot & count .

The reason many people dont use the two second rule, is someone pulls in in front of them on a busy motorway.
This makes for very very dangerous conditions and driving.


That all comes back to, not enough space on the motorway and instead of trying to tax us off the roads, they should build some new ones and v v v swiftly.

IMHO this govt has a lot to answer for.. & a lot of blood on their hands.

rgds
bill


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 Post subject:
PostPosted: Sat Aug 21, 2004 10:05 
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DevilsAdvocate wrote:
Think of how many times a car has pulled out from a junction in front of you in such a way as to cause you to have to break.

If other motorists can't judge whether there is enough room to pull out infront of an oncoming car, why should they be capable of judging breaking distance?


Of coarse you judge if you have enough time to pull out :shock: , this is what makes you a safe driver. You should also know the braking performance of your vehicle.

1st thing i do, when a change motors take a long drive to get a good feel for it, and complete a few emergency stops at differant speeds.

It would be interesting to know how many drivers have actually completed an emergency stop in their own car that they use every day.

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