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Highway code stopping distances.
I was just watching this video of 5th gear reviewing the new Mazda 2 on youtube, and its got me thinking about stopping distances.
Now, in the clip Jason Plato is testing the new Mazda’s acceleration, handling and stopping in several tests. The test that interests me is the last test of braking. Jason accelerates to 70mph heading towards a designated point, and when at that point hits the brakes hard to bring the little hatch to a stop as quickly as possible. This is a simple test, and no means a scientific one, but is effective at showing the little cars stopping performance.
Now, the Mazda 2 manages to stop dead in just 25 metres. That’s with just Jason in the car. Then he continues to do the test again with the extra weight of 200kg in the car (in the safe of strong man Terry Holland) and the Mazda 2 manages to come to a stop in 41 metres. This in itself shows the benefit of shedding weight when it comes to performance, but that’s not my point.
My point is this, below is a chat showing the ‘typical stopping distances’ stated in today’s Highway Code:
As you can see, the ‘typical’ stopping distance stated by the highway code is (excluding the thinking time) 75 metres. But, as you can see from the Mazda 2 test, new cars can stop in a third of that distance. and even with an extra 200kg of weight it stopped in almost half that time.
I know that there are still some old cars on the road in the UK (I drive a 17 year old car), but I doubt even these cars would be that much worse then the Mazda (the Mazda even has drum brakes on the rear).
So, when were the current highway code stopping distance last checked, and how accurate are they?? Do we even need to know them, as every car will be different depending on the wear on the drakes, tyres etc, and the amount of people in the car, and even the luggage in the boot.
I’d say that having stopping times in the theory test is a massive waste of time, given that the information will never be used by those taking the test, and are not even accurate.
05/02/2008 at 02:06 pm
thing is you get someone who dont have abs and dont know what they are doing and they`ll be exceeding even the highway code distances,
i think its a good thing how long they say they are in the book, its there to make people think and slow down,,
some people just dont appreciate how long it can take to stop sometimes!!
06/02/2008 at 09:36 pm
Who actually pays any attention to the highway code and the many laws and procedures that one has to learn to pass the theory test - it’s just a memory game to put more cash in the DSA coffers.
Stopping distances are flexible for all drivers and should ideally contain a ’stupidity factor’ which is determined by a series of profiling exercises prior to the theory test. Once in the test, the question will be:
“Taking into account your level of personal arrogance and general stupidity, how long will it take you to stop at NN mph?”
07/02/2008 at 09:42 am
haha, i like that idea Jason.
thats kind of my point though, after the theory noone will use or even remember the distances.
20/02/2008 at 02:31 pm
The highway code is all well and good. It has to exist because the laws of the road need to be written down somewhere! I do agree, however, that it is highly innaccurate. The “thinking distance” is almost as big a variable as the “stopping distance, many drivers (members of my own family included - not mo of course) drive along with their eyes either fixed on the car in front or two foot in front of their bonnet!
Good drivers are looking much much further ahead (and behind!) and so can anticipate hazards that will require them to stop and can even hover over the break pedal in readiness reducing the thinking distance substantially! Also some people never practice braking and so have no idea how quickly their car can stop if they need it to!
If the highway code encouraged people to look further ahead and practice emergency stops there would be a lot less accidents!
Where I really disagree with the highway code’s stopping distances is for Motorbikes. A standard yamaha R1 can get from zero to sixty to zero in under 7 seconds and can go from 100 - 0 in under 70 feet - Not an official figure but from my own experiments!
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People spend an awful lot of time driving on “autopilot” not even thinking about the potential hazards. By upping the observation you enjoy transporting yourself more, you don’t hit things and touch wood you don’t get caught speeding because you see the blighters before you steam through them!
06/03/2008 at 08:07 pm
I believe that the highway codes are not merely a reflection of how quickly a car could stop in the best of conditions, but also take into account reaction times of drivers, etc.
Rob is correct to point out that different driving behaviors will make individuals better or worse at quick stopping. However, all people (no matter their driving skill) have some limitations in reaction time. At higher speeds, these reaction time lags lead to longer distances before a stop… which is reflected in the safety guidelines.
In any case, the safety codes are primarily supposed to serve as a sort of rule of thumb under average driving conditions with a typical driver. Generally, I think its wise to take these kinds of suggestions under advisement even if an individual feels that s/he falls into a group of unusually diligent, skilled, and observant drivers. Everyone’s attention slips at some time or another- at the wrong time that kind of lapse can be deadly. (And, of course, self-perceptions of skill have the potential to be biased- so an individual may not really be able to accurately report how good of a driver they are anyway.)
27/03/2008 at 06:08 pm
If your vehicle has just passed the MOT braking test, it will take considerably longer to stop than the distances in the highway code (these require approx 70% brake efficiency, while passing the MOT requires just 50%). the numbers are just a guide.
You might argue that both the code and MOT test are well out of date, as with modern tires and brakes efficiencies of > 100% can easily be achieved up to around 200% without the use of clever aerodynamics and racing car technology
However unless I’ve done the maths wrong, Rob’s stopping distance (or starting speed) is a trifle in error as I calculate his deccelleration to be around 4.8g (480% braking efficiency!) which I believe even to be a challenge for a formula 1 car. 70 metres maybe…….as compared to the highway code distance (including the 0.68 second reaction time) of 176 metres
All in all its far better to keep ones eyes open, and watch out for things happening rather than worry about the numbers as the other respondents have already said.
By the way ABS doesn’t necessarily improve your stopping distance - it lets you steer while braking hard!
11/06/2008 at 11:56 am
Dear all,
Nobody’s answered the question yet.
The stopping distances on page 42 of the Highway code were done in the 60’s with a Ford Anglia. 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)
01/04/2009 at 06:20 pm
bob you have got the maths wrong mate
70mph is approx. 31.1 m/s. The acceleration (deceleration) is (v^2 - u^2) / 2s where v is the final velocity, u the initial velocity, and s the stopping distance.
(967.9 - 0) / 2*25 works out to be about 2g which is very good, but not quite F1 territory. I think the supposed average braking ability of a normal hatch is meant to be around 1g so 2g is questionable…but possible.
Surprising given that its a non sporty hatch with drums at the back…
01/04/2009 at 06:21 pm
…and I just realised that was march last year, not last week
22/04/2009 at 04:03 pm
Ok so brake efficiency is 100% when a decelleration of 1g is achived.
That means that your brake and tyre combiation are expected to apply a braking force equal to the weight of your car. Or put another way, your car shoud stick to a vertical wall. if it is also pulled towards the wall by the same force. Does that seem reasonable?
At that point the total force applied by the road surface to your tyres is 1.4 times the car weight, directed at an angle of 45 degrees backwards (braking reaction force) and upwards (weight reaction). This force however is not evenly distrubuted as the point of balance shifts by an amount depending on it’s height above the road. (Draw a line from the GG alonf the force vector, that’s the 45 degree line for 1g. Where it hits the road that’s your point of blance. If that get’s in front of the front wheels the car will somersault.)
In any case it affects the stability of the vehicle. The car will lose accurare steering long before is somersaults. Low slung sports cars ar more stable, and can remain stable for much higher braking forces. A vehicle with a much higher CG will not. An SUV for example will be much less controllable under heavy braking.
The maximum braking force your vehicle can apply is limited by that stability factor.
In formula 1, where speeds in excess of 100mph are normal, there is considerable aerodymamic downforce which increases the stability enourmoulsy. The angle of the braking vector is much steeper. Closer to the vertical. So stability is maintained at high speeds.
However, reduce the speed and the downforce is also reduced. So a F1 car is less stable under braking at lower speeds.
Also the downforce increases the maximum horizontal force a tyre can cope with before it loses hold. Tyres are not resisted by simple friction, as that is a dynamic force. Ie one resulting from the movement of one surface against another. Tyre grip is static friction, that’s different. That’s a maximum limit.
Static friction is the result of the surfaces interlocking by their bumps and irregularities. Both surfaces ust be rough and at about the same scale to maximize this. For an ideal think of meshed gears! In practice, if one surface is rough and the other is not (and is hard) then it will be close to a minimum.
Rubber is a good material to use when you cannot predict actual surface roughness, and cannot arrange a perfectly syncronised mesh. It conforms to the surface under pressure. The more pressure the better the conformance. Hence soft compounds use in F1 stick better, but are less mechanically strong. When they really lose grip the rubber becomes a lubricant!
Some things to note.
First understeer/oversteer/drift are not caused by the tyres sliding. It is cused by ‘creep’. Tyres are flexible. They deform. Apply a side force, and the part of the tyre just about to hit the road is a little off to one side. Also the more forward force is applied (ie from a driveshaft) then the tyre deforms circumferantially. Effectively the back of the tyre is stretched compared with the front. (This is why steel belts are necesary to help distribute the force reducing the effect.) The net effect increases creep, which is why rear wheel drive tends to oversteer. Both driving forces and steering forces combine to increase creep compares with the front wheels.
Second, rubber bounces. Or at least most do. At room temperature. It changes with temperature. When the front of your tyre hits the road surface you do not want it to bounce. That would reduce the tread area and grip. Rubbers are availblae which hardly bounce at all, but they also waste energy, which means more petrol, and absorb power. Not good for racing, and not really good for fuel economy in domestic cars either. Other rubbers bounce a lot. So it’s possible to make a compromise.
But this changes considerably with temperature.
The non bouncy rubber is very bouncy at 100 centigrade, and the bouncy rubber loses it’s bounce at higher temeratures. So you would think it’s possible to make a perfect balance, but not so. Firts the bounce curves don’t match, but more importantly they have other properties which are different, two important ones are tack, and tear strength. Tack is a measure of how ’sticky’ the rubber is, tear strength relates to wear. YOu want both to be high. Unfortunately they rubber with both these properties in aces is the one which is not bouncy at room temperature.
In F1 to get grip they use a lot more of the bouncy rubber wich is not so bouncy at high temeratures. These run at much higher temepratures than ordinary car tyres. This is what they want as it maximises grip, by minimising traead bounce. They can afford to lose a little power, which is no god if the tyre is not gripping anyway. On the downside this is the rubber which wears a lot more. One reason why F1 tyres wear so quickly. It’s not just the sheer hammering they get.
This leaves you with the tyres on your car which have to grip at lower temperatures, and operate efficiently without the benefit of a ton of added downforce, last a long time, and not waste too much petrol. Depending on how you set your priorities.
The setting of stopping disctances is based on an economy tyre running on a cheap domestic car without ABS or other shenannigins. On the basis of this the actual stopping distances are barely adequate. Especially under less than ideal conditions.
It is true that a car fitted with ABS, with a low CG, and a competent driver will under test conditions pull up in less than this distance, but that is not a typical situation. The Ford Anglia will only just pull-up safely under ideal conditions with an expert driver behind the wheel who is expecting the emergency stop. Most people could not manage that after a week of tutoring, certainly not anyone used to a modern sports-car.
The imprtant factor is control. The distances set are mimimum safe stopping distances, not just the stopping distance. They take into account the ability and reactions of real drivers, in real cars. That includes granny in her old Ford Anglia.
If you find you can comfortably stop your car using your tyres in much less than that distance on an ordinarly road surface than great. You have a good margin for safety, but the bloke behind you may not be simialrly equiped, and it’s no good arguing about tailgating after the event, the insurance claim will still leave you out of pocket!
So it’s not a matter of whether your car will do it, or if your reaction times are better. (In fact studies have shown that racing drivers reaction times are at best marginally better than the average. The big difference is anticipation. They read the road better, and besides have had plenty of practice laps!) It’s to do with a safe minumum so that granny has plenty of time to stop, in controll of the car, and not for instance stuff it into the traffic lights.
In any case, consider this.
Traffic lights are set for a 3 second amber phase for 40mph but could be as short as 2 seconds. At 40mph with 100% brake efficiency and allowing 0.65 seconds reaction time, you can stop in 2.5 seconds. This gives a stopping distance of 36m which is just shy of the official figure. So half a second margin right.
With the minumum brake efficiency allowed by law of 50% stopping time is 3.8 seconds, and you’ve jumped the light despite braking as hard as is safe in that car. Tell that to the judge!
That’s of course assuming that you are not momentarily distracted. I for one prefer to monitor the traffic, and look out for pedestriand when approaching lights. On more than one occassion I have had people step out in front without warning. Haven’t hit one yet.
Given that at 40mph it will take 2 seconds to reach the light if you are the official ’safe’ stopping distance from the light should you carry on to avoid the possibilty of a fine? I would say not, but then I would not like to have to make an unnecesary and possibly unsafe emergency stop either (Which can also be an offence!) in order to slavishly pull up behind a light that might just flick red just before I cross the line!
Sooner or later it seems we will all get caught on a red-light camera unless we all make sure that we have brakes of at least 100% efficiency (and who knows without having the brakes tested?) watch the lights to the exclusion of all else as we approach the braking limit, and are prepared of course to risk a possible shunt in order to stop behind the line. (I know that you are allowed to cross the line in order to avoid an accident, but if you are watching the light, not the daft git behind you…)
Stopping distances, fine the less I have to break hard the happier I am, I’m ok with that. The bloke in front may not agree but then I’m not sitting on his tailpipe so what do I care. It’s those darn traffic lights that assume that I can and will mak an emergeny stop every time I see amber that bother me. I would rather a longer amber phase to allow for the minumm allowable braking efficiency, and time to scan the road properly on approach than the current timings which may not be possibe.
In fact I would like to see much fewer traffic lights in the first place. Many don’t do the job they are supposed to, but that’s another story for another day.
27/04/2009 at 12:15 pm
I read all of ur txt Gordon, very interesting thoughts. I’d rather get caught on a traffic light camera than do an unneccessary emergency stop! my point is, if u don’t hesitate whether to go or stop, u will b fine!! knew sum1 who used to think out loud whether to go or not, by the time he decided it would b too late to do either! LOL
28/04/2009 at 08:06 pm
Deceleration…
diggurl = ‘http://something-interesting.co.uk/2008/02/05/highway-code-stopping-distance’; I was jus […]…
28/05/2009 at 02:20 pm
I’m just reading the Autocar results for the new Nissan GTR, which is a very high performance car with very good brakes.
It is heavier than a mazda 2 but should be have very good braking figures. A small citroen C3 Picasso managed 48 metres
The nissan managed to stop in 41 meteres from 70 MPH - I very much doubt that a Mazda 2 could stop from that speed in 25 metres.
But the point still remains that the Highway codes figures are misleading and innacurate, as they are based on cars that are around 40 years old.
Nvertheless I would suggest that all drivers allow themselves that 50 % margin of error just in case the conditions, weight of the car or iother factors conspire to increase your stopping distance. Only a fool would say that they were relying on being able to stop more quickly than they were ultimately actually capable of doing, having hit the back of another car and possibly injured themselves or other road users. Always allow a healthy braking distance and work on the worst case scenario, it will keep you safe and alive.