Safe Speed Forums

This is exactly the situation which catches out the arrogant speeders. It's fine to zoom along when the going is good, but when the unexpected happens, and you have no margin to correct, that's your lot.

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Where did he say he was speeding?

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I borrowed the numbers off the post before. However from here the friction coefficent for tires on a dry road is given as 0.9 for static (ie to get it moving) however once it's moving it's 0.8. therefore maximum decelleration with just tires is 0.8g. this site says that "0 £ m £1" which is odd, as it should be 0<(mu)<1. however i guess this is so as to not piss off the html (and with a lack of greek charaters like me). I'm sure there is some physics as to why friction can't be more than weight, but i don't know it, and can't be bothered to trawl through google for it at the moment, but it is a maximum as far as i can see. It could be that A level mechanics is full of crap, but i doubt they would tell you an actual lie, instead they'd have a sentence like "for A level maths, the coefficent of friction is assumed to be between zero and one." or some other bull. and they'd have put it on the front of the papers too, or in the question, along with the stuff telling you to assume negliable air resistance and g=9.8 or even g=10.

you can get decellerations of more than 1g, however because you can have more than one frictional force acting on the car. I guess air resistance and downforce come into it, though my mechanics isn't that good to explain this.

Simon

Its mass not weight ( force downwards) that is considered in the calculation.

F1 cars corner and decelerate at > 1g because of the downforce applied to the vehicle through the aerodynamics. Modern, especially high performance, cars increasingly include aerodynamics in their design although at slow speeds the effect is negligible.

It's a long time since I did mechanics, but I think the confusion here is arising from the units involved. The coefficient of friction is not measured in "g" as has been hinted at here. It has no relation to gravity, it is just a measure of the amount of grip available, where 0 means no grip whatsoever (ie you would slide forever) and 1 means perfect grip.

If something has a co-efficient of friction of 1 it doesn't mean that it can achieve acceleration at 1g, it merely means it doesn't slip. Think of a rack and pinion railway.

In simple terms the rate of acceleration that can be achieved is proportional to the co-efficient of friction, and inversely proportional to it's mass. Nothing about this relationship limits the resulting rate of acceleration to 1g. If there is lots of grip and little weight you can achieve as many g as you want!

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True, but the relationship to g here is based around the comparison of the F=Ma and F=(mu)R equations. Since F is the same force in both equations, Ma = (mu)R, and if R is simply equal to Mg, then the mass cancels and we end up with a = (mu)g...

So I don't think anyone is suggesting that the coefficient of friction is measured in g, merely that in this example the deceleration of a braking vehicle is simply the product of mu and g, and thus expressing the deceleration as 0.7g is correct.


However, as Richard points out, this comparison falls apart if you remember that the R term in the F=(mu)R equation isn't necessarily equal to Mg, but is also affected by any aerodynamic effects causing the force applied by the vehicle to the road surface to be something other than Mg. If the vehicle in question isn't an aeroplane, then these aerodynamic effects should act to increase the applied force, thus increasing the effective braking force. However, since this increased braking force has been achieved without an increase in vehicle mass, you can't then simply cancel out the M terms on both sides of the equation.

Where did I say he was speeding?

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I want you to explain to me exactly how going at a different speed magically alters the passing of time, ie how does going slower give you more time to react?

Hypothetical situation: The unexpected happens in front of you such that you have 2 seconds to react. Now, had you started your journey 2 seconds later than you did, you'd arrive at the scene 2 seconds later, so you'd now have 4 seconds to react. Had you started out one second earlier then you'd only have one second to react. So, giving your wife that extra peck on the cheek as you're leaving may save your life.

On the other hand, going 5mph slower will only 'buy' you 0.125 seconds in any situation where the unexpected happens. That's less time than it takes to blink.

It goes even further than that.

Hypothetical situation 2:
At 30mph it takes you 120 seconds to travel one mile, 103 seconds at 35mph, and 90 seconds at 40mph.
So, if a tractor pulls out of a side road one mile from your house, 103 seconds after you leave, and you do an average 35mph, you'll hit it, having absolutely no time to react. If you do 30mph average, you'll have a whole 17 seconds to react - no problem at all, you won't even have to slow down. But if you do 40mph average, you'll pass the side road 13 seconds before the tractor pulls out - you'll see it in your rear-view mirror.

So, tell us again: how exactly does going slower give you more time to react?

Or, if there's a significant error in either my logic or my calculations, kindly point out exactly what it is.

On the first, first frost in winter, find a LARGE EMPTY CAR PARK WITH NO
BARRIERS ETC TO DENT CAR. Have your first gentle skid of the winter here.You can always have more . That way YOU'VE had your first skid of the winter where YOU want to have it and it gives you extra confidence - I still do it even after 38+ years on the road.
Ont the locked wheels issue - maybe i am getting old, but in years gone by we relied on ANTICIPATION to prevent locked wheel situations and an old fashioned idea called CADENCE BRAKING IF WHEELS DID LOCK.
i.e. BRAKE, WHEELS LOCK , RELEASE PEDAL FOR A FRACTION, THEN BREAK AGAIN. REPEAT UNTIL STOPPED. STOPS YOU FAR FASTER, AND IN CONTROL !!

All that's correct as far as I can see, but the friction equation is nowhere near adequate to describe the extremely complex tyre / tarmac interface.

Drag racing cars achieve at least 4g acceleration from a standing start without aerodynamic benefit. I think there are at least two reasons why mu substantially exceeds 1 in this example.

One is the glue effect - clearly a one gram block well glued to a surface is going to take far more than 1 gram of sideforce to move it. Hot and sticky tyres clearly exhibit a glue effect.

The other is the rack railway effect - the tarmac isn't smooth and some portion of the total mass has to be LIFTED to pass over taller elements. In the case of a rack railway, huge forces can be developed without depending on friction at all - the pinion bears on vertical rack surfaces. If you want to "beat the rack" first you get the force against the vertical rack surfaces, and then you have to lift the mass past the height of the rack.

However neither of these effects appears to be large for road cars and typical emergency braking - few cars achieve more than 1g of deceleration under emergency braking. Neither have things changed very much - decent cars were achieving around 1g braking in 1938 (to my certain knowledge) and probably much earlier.

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Paul Smith
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Getting practice is skids is excellent advice, but there are a few notes of caution:

* You don't want to try to practice skid control on ice surfaces - there isn't anywhere near enough grip with normal tyres. It's occasionally possible to find a sloping ice surface where there's nothing you can do to prevent gravity accelerating the vehicle down the slope - and all slopes come to an end - usually with a curb or something inconvenient. So what's needed is a snow surface.

* The surface has to be fairly smooth too. Hitting the edge of a 4 inch pothole (or other ridge) at just 25mph sideways could sometimes provide enough sudden grip to initiate a roll. Never practice skidding where there are curbs, potholes or other ridges or edges!

* If you've never previously experienced skidding, professional instruction is brilliant - consider getting tuition on a "proper" skid pan. It's great fun and it "upgrades" every car you'll ever drive.

* But large smooth flat empty snow covered car parks are great places to practice and get a quick refresher course. Industrial estates usually have something to offer when it snows at the weekend.



Cadence braking is a very valuable technique, but it takes quite a bit of practice before you actually DO use it if there's an emergency. The "natural tendency" to freeze on the brakes is quite hard to overcome. The "secret" trick is to (always) look for your escape route. The "freezing on the brake" effect is usually associated with total focus on the object you're about to hit. Look away. Release brake. Steer away.

Cadence braking is very unlikely to stop you in a shorter distance.

Cadence braking does not apply to vehicles with ABS. Looking for an escape route applies to all vehicles.

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Paul Smith
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I’m sure you already know this, but you have asked me to explain, so I will. Going slower gives you more time before things turn critical when things do or nearly do happen. The times come from the fact that you have less speed to shed before you can come to an emergency stop, and shedding speed takes time. Furthermore, if that extra time does not enable you to avoid crashing, at least you crash at lower speed, which is in itself a good thing due to F=MA. Of course, the same applies for a dog running out, child running out, brakes fail near junction, large pothole comes into view, etc. etc.

The error is that I only include reactions to real-world things that do happen, not to the zillions of unconnected hypothetical scenarios that do not play out because of changed timelines in a parallel world of optional events

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It's true that if you are travelling more slowly at that critical instant when things go wrong you have more time.

But real world data indicates very strongly that average impacts are way way below free travelling speeds and speed limits. This means that drivers' adjusted speed is a far larger contributor to accident severity than free travelling speed.

We also know that adjusting speed depends on proper attention and that the "means of speed reduction" (i.e. camera enforcement) affects attention.

That's why the policy doesn't work and won't ever work. The side effects on attention are larger than the potential benefits of reduced free travelling speed. To much speed enforcement is making us less effective at avoiding crashes and less effective at reducing crash severity.

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Paul Smith
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Leaving aside the "parallel world" theory, there is still another angle on this which you haven't considered, which is that driving slower actually brings a whole lot more people into a zone of potential risk of collision.

Take a simple example of a pedestrian stood in the side of the road, who may or may not decide to suddenly run in front of your car. Lets say he runs at 5mph average, and that he can accelerate to this speed instantaneously. A crude approximation but it will serve to illustrate the point - we can fill in the accurate numbers later if so desired.

In 1 second he can cover just over 7 feet. So if he is stood 7 feet away from the path of your car you can disregard him as a potential hazard once you are within 1 second of passing him, as he no longer has time to get in front of your wheels, so to speak. At 30mph this means that once you are within about 40 feet he is no longer at risk from running in front of your car, and you can concentrate on the next hazard.

But if you slow down to 20mph he remains in the danger zone until you are about 30 feet away from him.

This illustrates beautifully the S of the COAST acronym - that of "space". If we are driving at 30mph and we worry that someone (say) 50 feet away is likely to run into our path we can either slow down, or we can consider steering away from the potential hazard, so he is now (say) 12 feet from our intended path. By applying the sort of maths above, it can be seen that where possible the second approach is infinitely preferable, as it delivers safety instantly - once we've adjusted our road position he moves immediately out of the zone of potential danger - whereas slowing down actually moves the potential hazard deeper into this danger zone and increases the time span in which we have to continue monitoring him. As a means of collision avoidance, slowing down should really be a second choice to be used when space isn't available, as it radically increases the time exposed to danger.

This explains why we need to consider doing all those other things we do, like constantly reviewing and adjusting our road positioning in relation to hazards, and indeed using acceleration to move hazards out of the danger zone. It also explains that strange phenomena that I'm sure we've all experienced, where we start to brake in response to a hazard (typically an urban pedestrian) and the hazard seems to encroach even more, such that we end virtually down to walking pace and yet still feeling very uncomfortable about the proximity of them. In a lot of these cases early observation and anticipation coupled with appropriate speed selection and positioning can remove the hazard potential in an instant.

Sorry for the long post, but I think this illustrates the point that speeding up can sometimes be an equally valid means of collision avoidance. The other classic example is the vehicle you see approaching froma side road at 90 degrees to your path and looking like he isn't going to make the give-way. If you can accelerate to ensure you pass the junction before he gets there then he ceases to pose a hazard to you. But the instant you start to brake then you often move right into potential conflict and in effect you have to commit to being able to come to a complete standstill to avoid him.

Slow doesn't always mean safe!

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While it is true that average impacts are below free travelling speeds, this does not present a complete picture. Very few accidents are totally unforeseen, and therefore a relatively random amount of noise is introduced into the figures depending on the amount of speed that the driver(s) managed to shed before impact. This in no way negates the implication that it is safer to have less speed to shed in the fist place, i.e. by driving more slowly.



But we have already determined that many accidents are caused by inattention, rather than attention to certain things. Cameras compel drivers to be attentive, and this at least cancels out your conclusions. Of course, in my opinion, it far out weighs them.

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... but fast is less safe on average. Even then, I would mind my own business if it was only the speeders who got hurt, but it also endangers others (e.g. me) so I work the averages and drive more patiently, for everybody's sake. Beside which, I like listening to radio 4 in my car.

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[Eric_Morecambe_Mode]
There's no answer to that...
[/Eric_Morecambe_Mode]

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That's not noise - that's real road safety. In far more cases still the risk is mitigated to a near miss, and in more cases than that the risk is mitigated to nothing at all.

There's plenty of evidence to support the idea that driver responses are getting duller in the speed camera era. Have you seen this page:

http://www.safespeed.org.uk/factors.html ? And this:

http://www.safespeed.org.uk/effects.html ?




Cameras compel drivers to be attentive to what? Real road risks? Or cameras and speedos? If they are more attentive to cameras and speedos then they are certain to be less attentive to the road ahead. Attention is finite after all.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

It compels them to be awake. You'd be surpised how awake drivers become after they have a ticket! Of course, it's too late then, for that instance, but they'll think next time.

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Can you show any evidence at all to support your suggestion? I've never seen any. (Neither do I believe it, but I'm open to any better evidence.)

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety