Braking Distance
Discussion
Hi,
I was wondering whether the highway code braking distances are still valid for today's modern cars.
I checked the other day to see how far my car would travel after I stopped quickly at 30mph. My test was basic and at a particular lamp post (with no other road users around me) I applied the brakes very hard,
the ABS did not trigger and the tyres did not skid. I was surprised I had stopped with the lamp post in-line with my drivers seat (approx 8 to 10 feet) past the point I braked. So I checked the recommended stopping distance on the Highway code website..
Interesting to find out,
Thinking distance @ 30 mph = 30 feet (9 m) and the Braking distance = 45 feet (14 m) Total = 75 feet. Why should my car's braking distance be so different, are they giving Wet road stopping distances??
Any thoughts or theories.
Cheers
G.
I was wondering whether the highway code braking distances are still valid for today's modern cars.
I checked the other day to see how far my car would travel after I stopped quickly at 30mph. My test was basic and at a particular lamp post (with no other road users around me) I applied the brakes very hard, the ABS did not trigger and the tyres did not skid. I was surprised I had stopped with the lamp post in-line with my drivers seat (approx 8 to 10 feet) past the point I braked. So I checked the recommended stopping distance on the Highway code website.. Interesting to find out, Thinking distance @ 30 mph = 30 feet (9 m) and the Braking distance = 45 feet (14 m) Total = 75 feet. Why should my car's braking distance be so different, are they giving Wet road stopping distances??
Any thoughts or theories.
Cheers
G.
A couple of points.
The HC figures are ancient and modern cars should beat the braking distance no problem.
Some heavily laden trucks and Range Rovers only just meet it though.
Also are you looking at 'braking distance' or 'stopping distance'?
Stopping distance includes reaction time which is constant ie approx 1 foot/mph
The HC figures are ancient and modern cars should beat the braking distance no problem.
Some heavily laden trucks and Range Rovers only just meet it though.
Also are you looking at 'braking distance' or 'stopping distance'?
Stopping distance includes reaction time which is constant ie approx 1 foot/mph
30 mph, call it 15 m/s, assume you achieve 1G decelleration (which is just about possible under ideal conditions) it will take you 1.5 seconds to stop, you will have travelled roughly 10 meters (32 feet ish) while you were braking.
If you managed to stop ten feet after your braking point, assuming you didn't run into something, then either you weren't doing 30 mph to start with or you braked earlier than you thought.
If you managed to stop ten feet after your braking point, assuming you didn't run into something, then either you weren't doing 30 mph to start with or you braked earlier than you thought.
This’ll bore you!
Highway Code stopping distances act as a reasonable guide to your average driver about how far it will take him to stop from a given speed. The OVERALL stopping distances comprise the REACTION (thinking) distance and the BRAKING distance.
Generally, because stopping distances are aimed at the 'average' driver (read as 'crap driver'), they are considered fairly optimistic based on the fact that very few drivers will ever achieve a reaction time of 0.67 seconds. Most take well over a second and quite often as long as 2 seconds or more to react. For sure, under test conditions reaction times much lower than that can be achieved, but for your average driver on the road in the real world with his head up some dark, damp place, reaction times are considerably slower.
As far as BRAKING distances are concerned, it's not so simple. It seems highly likely that the figures quoted by the Highway Code were the results of tests done by a vehicle NOT fitted with ABS, SKIDDING to a stop. The deceleration rate used to calculate their distances is generally about 0.67g. In my experience of investigating fatal road crashes for 10 years and performing hundreds of skid tests on roads in all kinds of vehicles, this value is fairly typical for non-ABS'd vehicles on a dry road, thereby giving credibility to the Highway Codes figures. To quote shorter distances based on ABS test results would not be safe and to start printing tables depending on the type of vehicle or braking system would cause many a nervous breakdown.
Now whether these tests are performed with a Ford Anglia or a Porsche (with no ABS) is totally irrelevant, as the tests involve lock-up and the only thing that really matters is the type of road surface. (The effects of weight, tyre compound, speed, etc are almost entirely irrelevant and have negligible effect on the vehicle's overall ability to stop in these circumstances, and so are not worthy of discussion).
There's no doubt in my mind that the introduction of ABS is a good thing and that generally, cars fitted with ABS will stop in shorter distances than those without. This is especially the case in the wet and although still the case in the dry, less significantly so. In the tests I’ve done on the ROAD with ABS’d vehicles, I cannot recall achieving a deceleration rate in excess of about 0.8g and this in itself is rare. What difference would this make to the HWC’s braking distance? Braking at 0.67g from 60 mph takes 55 metres. At 0.8g, it would stop in 46 metres.
So you want the HWC to be re-printed to take modern braking systems into account? Well here’s why they can’t:- they quote that from 60 mph you can BRAKE to rest in 46 metres. So you’re driving along at 60 mph approaching some traffic lights. Now because you’ve got a really sporty red car with wide tyres and yer bird’s sat next to you, you leaving the braking until 46 metres before the stop line. Unfortunately you forgot that the HWC figures were based on ABS test results and that your car isn’t fitted with it. So you only achieve 0.67g. You then find yourself crossing the stop line at 24 mph and venture 9 metres out into the junction. As your vehicle grinds to a halt 16 metres to the right, having been T boned to your nearside by an artic, killing yer bird in the process, you are then told that the pedestrian you hit crossing by the stop line is also dead. Oooer. Who’s fault? Yours? High Way Codes? Both? Get me drift?
Please don’t listen to all this b*llo* on forums with some dude telling you he can stop his RX8 from 90 mph in 5 car lengths (4.7g!!). And as far as those of you who have done tests on airfields, don’t think that what you achieve there can be applied to a real road. Can’t remember how many times I’ve done tests on airfields, but they are nearly always considerably grippier than a real road. 70 mph to 0 in 55 metres in a TVR Chimaera. That’s 0.91g. You’d be extremely lucky to get that out on the road, so be careful.
So surely it’s right for HWC to err on the side of caution, whilst being generally quite realistic with their figures. Remember this; they are a SAFETY GUIDE. You can’t start trimming figures when you’re talking safety. In any case, if you’re out there doing this sort of braking all the time, you’re cruisin’ for a bruisin’ and if you kill someone, you will probably end up in prison.
Highway Code stopping distances act as a reasonable guide to your average driver about how far it will take him to stop from a given speed. The OVERALL stopping distances comprise the REACTION (thinking) distance and the BRAKING distance.
Generally, because stopping distances are aimed at the 'average' driver (read as 'crap driver'), they are considered fairly optimistic based on the fact that very few drivers will ever achieve a reaction time of 0.67 seconds. Most take well over a second and quite often as long as 2 seconds or more to react. For sure, under test conditions reaction times much lower than that can be achieved, but for your average driver on the road in the real world with his head up some dark, damp place, reaction times are considerably slower.
As far as BRAKING distances are concerned, it's not so simple. It seems highly likely that the figures quoted by the Highway Code were the results of tests done by a vehicle NOT fitted with ABS, SKIDDING to a stop. The deceleration rate used to calculate their distances is generally about 0.67g. In my experience of investigating fatal road crashes for 10 years and performing hundreds of skid tests on roads in all kinds of vehicles, this value is fairly typical for non-ABS'd vehicles on a dry road, thereby giving credibility to the Highway Codes figures. To quote shorter distances based on ABS test results would not be safe and to start printing tables depending on the type of vehicle or braking system would cause many a nervous breakdown.
Now whether these tests are performed with a Ford Anglia or a Porsche (with no ABS) is totally irrelevant, as the tests involve lock-up and the only thing that really matters is the type of road surface. (The effects of weight, tyre compound, speed, etc are almost entirely irrelevant and have negligible effect on the vehicle's overall ability to stop in these circumstances, and so are not worthy of discussion).
There's no doubt in my mind that the introduction of ABS is a good thing and that generally, cars fitted with ABS will stop in shorter distances than those without. This is especially the case in the wet and although still the case in the dry, less significantly so. In the tests I’ve done on the ROAD with ABS’d vehicles, I cannot recall achieving a deceleration rate in excess of about 0.8g and this in itself is rare. What difference would this make to the HWC’s braking distance? Braking at 0.67g from 60 mph takes 55 metres. At 0.8g, it would stop in 46 metres.
So you want the HWC to be re-printed to take modern braking systems into account? Well here’s why they can’t:- they quote that from 60 mph you can BRAKE to rest in 46 metres. So you’re driving along at 60 mph approaching some traffic lights. Now because you’ve got a really sporty red car with wide tyres and yer bird’s sat next to you, you leaving the braking until 46 metres before the stop line. Unfortunately you forgot that the HWC figures were based on ABS test results and that your car isn’t fitted with it. So you only achieve 0.67g. You then find yourself crossing the stop line at 24 mph and venture 9 metres out into the junction. As your vehicle grinds to a halt 16 metres to the right, having been T boned to your nearside by an artic, killing yer bird in the process, you are then told that the pedestrian you hit crossing by the stop line is also dead. Oooer. Who’s fault? Yours? High Way Codes? Both? Get me drift?
Please don’t listen to all this b*llo* on forums with some dude telling you he can stop his RX8 from 90 mph in 5 car lengths (4.7g!!). And as far as those of you who have done tests on airfields, don’t think that what you achieve there can be applied to a real road. Can’t remember how many times I’ve done tests on airfields, but they are nearly always considerably grippier than a real road. 70 mph to 0 in 55 metres in a TVR Chimaera. That’s 0.91g. You’d be extremely lucky to get that out on the road, so be careful.
So surely it’s right for HWC to err on the side of caution, whilst being generally quite realistic with their figures. Remember this; they are a SAFETY GUIDE. You can’t start trimming figures when you’re talking safety. In any case, if you’re out there doing this sort of braking all the time, you’re cruisin’ for a bruisin’ and if you kill someone, you will probably end up in prison.
Prancing:- 30 mph to 0 in 10 feet!! Don't reckon - unless there was brick wall involved somewhere along the line. That's a 3g deceleration rate. Perhaps Mr. G's tests should be a little more scientific! Fat tyres, thin tyres – make’s no difference.
Mat_t16:- Ford Anglia, Ford Mondeo, Vectra, Clio, blab la, makes no difference. Drums/discs, makes no difference. Generally, you’ll only beat the HWC braking distances if you have ABS, and then not by much.
Greenv8s:- Hope you’re talking about track or airfield. 1g on a road? Don’t think so.
Mat_t16:- Ford Anglia, Ford Mondeo, Vectra, Clio, blab la, makes no difference. Drums/discs, makes no difference. Generally, you’ll only beat the HWC braking distances if you have ABS, and then not by much.
Greenv8s:- Hope you’re talking about track or airfield. 1g on a road? Don’t think so.
Definately correct about airfields, my car would pull 0.9g odd on their grippy surface but maybe only manage 0.8g odd on the road.
With a wet greasy road only 0.69g
Ice covered road 0.18g on road tyres!!!! & 0.3g same road on winter tyres.
So the HWC is really just a guide, as said many times before drive according to the conditions, I bet any motorcyclist will be more than able to tell you more about that one.
I'm off for a test with the data logger, hot summer tarmac here we come.
>> Edited by cptsideways on Sunday 16th May 11:14
With a wet greasy road only 0.69g
Ice covered road 0.18g on road tyres!!!! & 0.3g same road on winter tyres.
So the HWC is really just a guide, as said many times before drive according to the conditions, I bet any motorcyclist will be more than able to tell you more about that one.
I'm off for a test with the data logger, hot summer tarmac here we come.
>> Edited by cptsideways on Sunday 16th May 11:14
Wotabloke said:
cptsideways:- How are you measuring your g forces? Is the kit calibrated? Pulling 0.8g on the road!! Assuming uniform acceleration, that’s 0-60 mph in 3.4 seconds. You sure? What tyres you got? What friggin car you got?!
AP22 Data logger, easy enough to calibrate, stick it on its end it measures 1.00g perfectly so yes accurate (manufacturers claim 0.01 accuracy). Obviously mounting is an issue but fairly used to that, it's trackday proof. But accurate enough for comparisons, used by lots of motorpsport types. Note the indication on the left says 0.01-3 g which is indicating it's vaguely level, adjustment for dive are taken into account in the settings.
Right test results:
Please note you can watch the g-guage rise and fall as you apply & brake. A bit like Julian64's pooh shape braking, these are peak G's so not uniform braking at all, i'd say up to 3 secs to achieve peak g. I could video it if you want? if someone can host an AVI movie about 8meg. I could plot it on graph if you'd like.
I see why coucils like the stick on cheapy tarmac!!!
Standard good quality looking tarmac =
Council budget stick on chippings =
The cars my wifes, a Rover 420 Turbo Diesel albeit with 205/40/17's on but nothing special in the tyre dept. They do a smoke a bit if they lock up though!
>> Edited by cptsideways on Sunday 16th May 12:01
Has anybody here got a brake assist car somewhere near north dorset?
I'm just off to do a g/time graph to show the "Pooh shape" braking graph.
Would be very interesting to do a comparison with a brake assist car as they have less pooh shaped graphs I think having driven one. Assuming you set it off that is, which can be done with your fingers only on the brake pedal honest!!!! I tried it.
I'm just off to do a g/time graph to show the "Pooh shape" braking graph.
Would be very interesting to do a comparison with a brake assist car as they have less pooh shaped graphs I think having driven one. Assuming you set it off that is, which can be done with your fingers only on the brake pedal honest!!!! I tried it.
forever_driving said:
cptsideways:- How are you measuring your g forces? Is the kit calibrated? Pulling 0.8g on the road!! Assuming uniform acceleration, that’s 0-60 mph in 3.4 seconds. You sure? What tyres you got? What friggin car you got?!
I don't think that's 0-60, it's 60-0
Ah, well you see when he said 'my car will PULL...', to me that meant accelerate (i.e.0-60), as in 'pull away'.
So CPTSideways, are you talking ACCeleration or DECeleration? My guess is you're talking ACCeleration as I doubt yer Rover can do 0-60 in 3.4!!
Yup, peak G's makes SO much more sense. Generally, I find that the difference between PEAK and MEAN can be about 20-30%. As the subject is about braking distances, we need to look at the MEAN and not the PEAK. However, when a car has got a good ABS system, it's MEAN will be much closer to it's PEAK than a car's without ABS. It's all aboot static and sliding friction, innit?
Can your device measure MEAN deceleration?
Is this the sort of kit you reckon car and bike mags use when they quote their stuff? Awefully misleading what!
>> Edited by Wotabloke on Sunday 16th May 13:10
Ok heres a brake test from 60mph, one wheel locked up on me & I cadenced braked just once
Clearly showing the pooh shape & it takes 2.4 seconds to reach peak G of approx .82g.
Now I'm fairly good at braking I'd say, yet it still took me 0.6 seconds to unlock & re apply the brakes back to almost the G before, ABS would do that in 0.2 seconds or less yet not lessen the brake force on those wheels not locked.
Look at the loss in G under skidding down to 0.58 g approx.
I came to a stop in 4.0 secs approx
>> Edited by cptsideways on Sunday 16th May 13:18
Clearly showing the pooh shape & it takes 2.4 seconds to reach peak G of approx .82g.
Now I'm fairly good at braking I'd say, yet it still took me 0.6 seconds to unlock & re apply the brakes back to almost the G before, ABS would do that in 0.2 seconds or less yet not lessen the brake force on those wheels not locked.
Look at the loss in G under skidding down to 0.58 g approx.
I came to a stop in 4.0 secs approx
>> Edited by cptsideways on Sunday 16th May 13:18
Interesting even if its not very scientific, clearly shows we all have room improvent or maybe we really should trust ABS after all. I'm a big ABS fan, even on trackday car.
A well trained right foot can be a very effective means of stopping quickly, however if your are that good then your ABS will very rarely kick in, if it does it simply meant it's beaten you.
This was a test, I was prepared for a lock up & yes I got a lock up, yet I dealt with it ok'ish. Many others would & do simply carry on at .58g. Brake assist takes care of this & of the ABS & will haul you up regardless of foot pressure at probably 0.8g or more consistently.
I noticed something very interesting, I tried this on some very newly laid super smooth tarmac & with no lock ups I left a hoofing great skid mark, this was the new layer of tar overheating I assume yet if it was an accident scenario I think that might have been calculated as a lock up? what the criteria do you look at the pattern stretch?
A well trained right foot can be a very effective means of stopping quickly, however if your are that good then your ABS will very rarely kick in, if it does it simply meant it's beaten you.
This was a test, I was prepared for a lock up & yes I got a lock up, yet I dealt with it ok'ish. Many others would & do simply carry on at .58g. Brake assist takes care of this & of the ABS & will haul you up regardless of foot pressure at probably 0.8g or more consistently.
I noticed something very interesting, I tried this on some very newly laid super smooth tarmac & with no lock ups I left a hoofing great skid mark, this was the new layer of tar overheating I assume yet if it was an accident scenario I think that might have been calculated as a lock up? what the criteria do you look at the pattern stretch?
cptsideways said:
Ok heres a brake test from 60mph, one wheel locked up on me & I cadenced braked just once
Clearly showing the pooh shape & it takes 2.4 seconds to reach peak G of approx .82g.
Now I'm fairly good at braking I'd say, yet it still took me 0.6 seconds to unlock & re apply the brakes back to almost the G before, ABS would do that in 0.2 seconds or less yet not lessen the brake force on those wheels not locked.
Look at the loss in G under skidding down to 0.58 g approx.
I came to a stop in 4.0 secs approx
>> Edited by cptsideways on Sunday 16th May 13:18
60 to 0 in 4 seconds = 0.68G. More like it. This would be a braking distance of 53.6 metres. The HWC braking distance from 60 mph to 0 is 55 metres. I rest my case.
I'm impressed with your braking ability. However, as you can see from yer graph, when you came off the brakes, G fell to aboot .58. It would be a highly skilled and trained driver that can cadence brake and achieve a deceleration rate equal or better than that achieved by the vehicle in four wheel lock up. SO I reckon yer average (cr*p) driver is best just slamming on the anchors and concentrating on a quick prayer. Unless of course he needs some steering!
Nice graph.
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