Safe Speed Forums

Spoke to him once he'd calmed down.

He's got virtually the same car as me, a B7-platform Audi A4 2.0 TDI Quattro, although his hasn't had the remap that mine has so it's "only" 170bhp rather than 210. His is a bit newer than mine (07 rather than 56 plate) and he's on Bridgestone Potenzas rather than Pirelli P-Zero Rossos. Still a 235/45R17 profile though with ZR rating.

The car has a 60/40 split to the rear under normal conditions but can vary the torque split (I've felt this happen in mine when I've been having a lark about on loose gravel).

He was doing 60mph and had put the cruise control on so he wasn't tempted to welly it in adverse conditions. Him and I both agreed that this was his fundamental mistake as when the car started to twitch it takes far longer to disengage cruise control than it does to lift your right foot.

ESP was turned on of course.

He knows pretty much what he did wrong even though it was for all the right reasons.

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Magistrates rule #1: "Never let justice get in the way of a conviction."

Can cruise control be an important factor here? I've found it very unsettling to drive with CC on in conditions of dubious traction.

There were some scare stories going around that cruise control could send a car out of control under aquaplaning conditions, but these proved to be false.

It is possible that the CC cut power too harshly (if the wheels started to spin), it's possible an advanced car like the Audi can apply the brakes as part of the cruise control system (it certainly can as part of the ESP system). Either could unbalance the car.

But I suspect it's simply that handing control back from the computer to the driver is not seamless, or quick enough. I use CC and my automatic reaction when I need it off is to jab the brake pedal rather than the suspend button. Even using the suspend button it is impossible to match the throttle setting and get a seamless switch, unless I have already overridden the CC using the throttle. Whichever way it's not ideal in such a situation.

My advice would be in wet/slippery weather to not use cruise control but instead to learn throttle control. What does Roadcraft call it? Accelerator sense?

I don't use CC when it's windy because I don't have the level of control I can get using the throttle.

What puzzles me is why would anyone want Cruise Control switched on in the first place? If you are alert, surely its no effort to control the throttle by sub-concious foot control?

Absolutely. A cruise control is only useful if you're all-but a ban and paranoid about speed limits - it has no place for adverse conditions, speed of progress being SOLELY liked to conditions not speed limit or, even worse, a fixed numerical speed.

I really like having CC on (in good conditions) in my car.

1. It enables me to cover the brake with my right foot which reduces my braking reaction time.

2. It stops my psychotic little car going too fast!

The average Transfer Time (that is time taken to move foot from throttle to brake) is 0.2 sec as it is a spontaneous reaction. So at 70mph, thats 6.25m!!
Considering your overall stopping distance is going to be around 96m its not alot and an alternative could be to drive with an gap of a further 6.25m or 3mph slower!

However, here's the bad news, it is extremely unlikely that braking from a hovering your foot on the brake pedal is a spontaneaous reaction and so your reaction time to make Choice Reaction will probably be greater than the saving of pedal transfer time!!!!!

So being trained to control a skid is a Bad Thing and being able to brake quicker is a Bad Thing. Right...

Actually I think Lucy is right.

Hovering over the brake reduces the stamp factor. HOWEVER, taking up the slack - already braking gently approaching a higher hazard density is VEY good - one is already feeling the brakes and pressure increase will be instant but controlled.

The figure I have always known was that the 'average' driver takes half a second to start braking but it is mostly about thinking time not to do with physical ability - although this is a smaller part of it.

So then there is a problem that if you start to drive with your foot hoovering over the brake pedal this has disadvantages and advantages, but is overall not best practice.

Your vision should tell you about the road ahead, so the throttle is priority not braking.
If you are driving so that you can stop in the distance that you can see to be clear in, then why are you going to have a surprise when 'stomping on the brake is going to become necessary.
Good spacial awareness should also elevate any sudden change in environment.
Concentration, good observation, attention, anticipation should all also be strong attributes that also prevent all surprises.

Am old AA survey showed that if you braked hard then the 6th was usually an accident, so using any hard braking that 'one' does as a warning that there is a failure going on in other areas of our driving is a real tale tell sign.

I see use of the Cruise control in areas of safe conditions and as soon as conditions are anything but ideal it is taken off. Certainly for wet weather it absolutely would be off, you have to feel and see for the water and signs will usually be about to tell you the quantities of water are such that aquaplaning is possible.
With a 60/40 split any immediate lift off with so steering input should have helped him get the front wheels crucially increasing grip (from the weight transfer to the front).

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Safe Speed for Intelligent Road Safety through proper research, experience & guidance.

The science of braking is a complicated one with various lay-term used. The “1/2 second reaction time” is also a general term which is used for convenience by those who wish to make braking time/distance calculations to make their point.

However the industry, and so courts requiring expert evidence, would describe “reaction time” as follows:

Prebraking Time
This is the period of time that elapses between the moment the hazard is recognised and the start of braking. It is generally around 1 second. Prebraking time is a combination of the four times below:

1 Reaction time: Time that elapses between the moment a defined incitement to action occurs and the start of the first specifically targeted action. It is at least 0.3 seconds for a choice reaction if a driver needs to make a decision to perform a preventative or evasive action in response to conscious hazard recognition. This is commonly quoted as the “1/2 second reaction time”, but may well be several seconds.

2 Transfer time: Time taken to move foot from throttle to brake – about 0.2 second.

3 Response time: Time it takes to transmit the pressure applied at the brake pedal to the point where braking becomes effective.

4 Pressure build up time: Time taken between the brake action taking effect and reaching fully effective braking.

These 4 times combine to about 1 second, hence using ½ second to make calculations in relation to braking and safety is misleading and may lead to a safe speed being faster than it is.

However the point make about cruise control and “hovering your foot over the brake” is this. Whilst the larger transfer time of 0.2 second may be exchanged for a shorter one, he fact is it will probably be at be a saving of 0.1 second, but this is out weighed by an addition choice reaction time of on average 0.5 second. Do he maths and you will see that this hovering trebles Transfer Time – it does not reduce it.

The conclusion is that if some one “hovers the brake” in this fashion, they are misperceiving the risk and will therefore be driving too fast based on their false perception of an acceptable risk.

It is true that speed kills – accidents are often caused by driving too fast. However people will drive at a speed that they believe is acceptable. The problem arises when they place over reliance on driving techniques and technology and mis-judge an acceptable level of risk and naturally drive too fast.

I hope my views are compatible with Safe Speeds aims.

I picked up on the key word for safe driving in Safespeedv2, and that is anticipation. Anticipation leads to preparation which leads to survival – a maxim of mountaineers which could be equally adopted by drivers.

A note on weight transfer – it cuts both ways, you loose traction on the rear. On cornering it can induce loss of steering (some ESPs actually don’t back of the throttle if you aren’t braking to compensate this when required) due to increase of lateral momentum. When cornering, you should ideally remain at a constant speed or accelerate slightly in a front wheel drive car due to the vehicle dynamics.

Also Electronic Brakeforce Distribution (EBD) and other mechanical devices will itself apply the most effective braking distribution regardless of weight transfer (it compensates for weight transfer).

You’ve quantified points 1 and 2 and left 3 and 4 open, yet you’ve given a total for all of them. Does it really take ~500ms for 3 and 4?



I think I disagree. The choice is done in advance – in anticipation (when not actually needing to brake).
Also, the 'choice reaction' can be overridden at any time by a needed 'panic reaction'. The reaction time to an event that requires action shouldn’t be any longer, should it?

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Steve: on average in passenger cars, 3 & 4 combine to 0.3 seconds. In accordance with the EU Council of Ministers Directive EEC 71/320 Addendum 3/2.4 the sum of the Response Time and Pressure Build Up Time (3 + 4) may notexceed 0.6 s.

Any questions on that?

Sadly there is no EU Directive for the manufacture of humans. However you are welcome to your opinion but the generally accepted view is this brake hovering increases braking as previously explained. The choice to make an emergency brake is NOT made in advance. The reaction to press the brake directly (not from throttle) is not spontaneous. I don't think you understand the technical definition of choice reaction.

As I originally said the science of braking is a complicated one.I would elaborate but can't justify the time because my posts all pass through the moderators and may not be posted as some people have been upset that I have more technical knowledge than them and take the huff when I correct them in the interests of road safety.

If you don't think I know anything about braking then so be it. But if you kill someone and claim in court you were a better driver for brake-hovering, than don't say you haven't been warned!

Another point to advocate CC - when you've got something with a fair bit of poke (and remember my A4's had a remap to 210bhp) it's easy to drift into 3-figure speeds when you're purely watching the road ahead and not keeping an eye on the speedo. The A4 is a very quiet and smooth ride, which makes this even more of an issue.

For example I managed to cook the brakes on a downhill sliproad when, without realising it, I'd hit over 140mph along the carriageway (almost deserted, late at night, was coming off the M5 at J17 for diesel on the way back from Devon). Applied the amount of braking I'd usually apply for my normal motorway cruising speed of 85mph. And realised I wasn't going to stop for the red traffic light. So braked a lot harder - just before the point where the ABS kicked in. A very cursory glance at the speedo showed I was still doing nearly 100mph at the point where I'd expect to be down to around 45mph.

Let's just say that in spite of 13" vented discs the brake pedal felt a little spongy for a good few minutes after that!

Disclaimer - I was tired and driving almost on "autopilot" at the time.

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Magistrates rule #1: "Never let justice get in the way of a conviction."

And that's a disclaimer? hmmm.

Yes, where did you get the 0.3s? (is it in EEC 71/320, if so can you point me to it, I can't find it)
Also, aren't modern car brakes equipped with advanced filtering to apply increased braking when the pedal is pressed quickly?
(granted that doesn't apply to all cars)


By whom? Can you substantiate that?


I don't understand how panic mechanism cannot override the choice mechanism (if that’s what you’re saying). I am most interested to hear the explanation, so please do explain. 'I can do science me'.


Emotive nonsense. I could just as easily have said: "But if you kill someone and claim in court you were a better driver for not brake-hovering, than don't say you haven't been warned!", no?
What I’m after is logic and reasoning, not simple claims. I think it fair to say that most of us at SafeSpeed are here because we don’t take claims at face value.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Lucy,

you have been placed on moderated status purely because of the overly confrontational nature of your posting style; all non-antagonistic posts will be approved.
You can score your goals but only if you play the ball, not the people.

Well Steve, when I am off moderated status I will quite happily put a little more effort into it, but I'm not wasting 1/2 hour carefully crafting a post that may not get posted if its interpreted as antagonistic.

If you think my posts are antagonistic, you should see what goes on, on my local paper posting board!!! (and that is 100% moderated). I really do think that this site is a bit sensitive to criticism as I note that very few of my posts are being accepted, hence I have better things to do.

At the end of the day, if you don't think I know what I am talking about, thats fine with me - plenty of other people outside Safe Speed do.

Lucy,

I've just checked the logs, you are mistaken: thus far all your posts have already been approved, all in a timely manner - none have been disapproved. Due to the nature of the setup, instant approval cannot be given.

However, any further posts on the issue might be rejected.

It's most useful to me when there is no throttle control required. Instead of having my foot planted in a fixed position for extended periods of time I can take it off the pedal and give it a rest from doing nothing.