Safe Speed Forums

It is common ground among most contributors here that a two seconds minimum separation from the vehicle in front provides a safety margin which allows time to react (the "S" and "T" in COAST).

I believe most will also have realised that the two seconds rule does not and is not intended to equate to the "stopping distance" that is used to guide the choice of speed as per the Safe Speed rule.

If the two seconds rule does not equate to stopping distance, why is it advisable to leave a longer separation in less than "good" conditions (e.g.in the wet)? I suggest it is NOT on account of longer stopping distance (because that will apply equally to the car in front) but because of the likelihood that a wet road is accompanied by reduced visibility and an increased risk of the vehicle ahead skidding or taking some unexpected evasive action or generally behaving in a less predictable way.

Comments?

I believe the most basic reason is that braking differentials increase markedly in the wet. This especially applies with differences in tyre tread depth in very wet conditions. e.g. If the guy in front has new tyres, while yours are nearly worn out he'll have quite a big advantage.

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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

Good point and I agree that there ccould be substantial differences in braking perfomance between cars on wet roads that would not appear on dry roads. Still, even if I'm driving on new tyres with good wet road performance I might (probably would) still increase separation. So are the other 'reasons' applicable as well?

It seems to me that this is part of the "..generally behaving in a less predictable way." that observer makes a point about.

Bad driving conditions increases the "noise" the driver has to deal with in order to drive safely and as such the driver needs to allow more reaction time to deal with the unexpected.

Bad conditions frequently reduce visibility, which in effect reduces reaction times. As the separation gap is based on reaction time it's a logical way of compensating for a loss of visibility.

Furthermore, in conditions of reduced adhesion it is important to do everything more smoothly and progressively. If you need to make the transition from accelerating to braking behind another car an increased gap gives you the extra time required to make that transition more gradually and smoothly, compared to dry conditions, where it can be safely done more quickly.

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As the conditions get worse visibility decreases and a degree of tiredness/strain increases the ability to react correctly.
In this case with normal 2 second clearance , you could stop without problems BUT strain/tiredness makes your reactions ragged.
There is also the possibility that you have not used the brakes for a long period - with a high volume of water the pads become wet . Applying the brakes dries out the pads, sometimes evenly, sometimes not and the effect is not always the same - one minute braking is gentle - next the pads are dry and brakes lock ( for those of us who aint got SBS)
Add this lot together and i always want at least four seconds minimum -and some companies put a software link from brakes to accelerator so that you can't keep dabbing the brakes to dry them out( like us old timers used to do in the old days)-- VW Caddies of S vintage spring to mind.
And for those of you with a Vauxhall, certain drum handbrake shoes glue the shoes to the drum when wet when left parked overnight.

I like to quantify things to get a better understanding, so here's my (somewhat tedious) take on it:

At a speed of 40mph, with a two-second gap, you'll be approx. 36 metres behind the car ahead. He does an emergency stop, it takes him approx. 2 seconds to stop, and his braking distance is approx. 18 metres.
The total distance you have to travel to reach his back bumper is therefore 36 + 18 = 54 metres. If your stopping distance is also 18 metres then you can travel on for up to 36 metres before hitting the brake, in order to end up just behind him. 36 metres at 40mph is two seconds. So you can allow up to two seconds to react.
Let's say that you react after one second. After one second you have travelled 18 metres, so you have 36 metres in which to brake. If you stop in 18 metres, you will stop 18 metres from his bumper. If, however, your brakes are only working at 50% efficiency, you'll take 36 metres to stop - and you'll end up right behind him. If your reaction time is half a second and your brakes are 50% you'll stop 9 metres behind him, but if your reaction time is one and a half seconds with 50% brakes, you'll need about 9 metres more than you have, so you'll plough into the back of him at about 28mph. So if your reaction time is that slow, your brakes have to be at least 75% as efficient as his to avoid running into him.

Let's change the speed to 20mph now. At 20mph you cover approx 18 metres in two seconds, the braking time is approx. one second and the braking distance is approx 4.5 metres.
So, in the same scenario as above, you'll have 22.5 metres in which to stop. If your reaction time is one second, you'll have 13.5 metres in which to stop, so your brakes only need to be 33% efficient. With 50% efficient brakes you can now afford to take one and a half seconds to react.

So it seems that lower speeds are more forgiving of bad brakes/bad road surfaces than higher speeds - but you can compensate for this, to a point, by being quick on the draw. Similarly, good brakes/low speeds can, to a point, compensate for (not too) long reaction times.
On the other side of the coin, with good brakes and good reaction times, a one-second gap can be sufficient at times - not that it should be recommended.

But it must be stressed that if you're inattentive then no gap is big enough. A five-second gap isn't going to help you if you take your eyes off the road for six seconds at the wrong time - you'll hit the car in front at full speed, whatever speed that happens to be.

Cheers
Peter

Totally with you there,Pete. Wish someone could get that into head of the driver who changes lanes on a motorway and reduces your gap - that way you don't have to be innattentative - and if it's done as you're checking mirror/speed - bang goes at least one second
The same thing that makes me give car in front extra gap leading up to a camera.Will he or won't he do panic brake at the camera.

When approaching a known hazard (such as a camera ) is it enough to be prepared for the expected hazard (panic braking) or should a greater following distance be introduced as well?

The overall time taken to react to an expected event is significantly shorter that the overall time taken to react to an unexpected event (thinking time starts to have an effect here)

I'm not sure I understand your main question. Can you re-word that please?



Actually, a number of studies show that reactions in an emergency are rather faster than in non-emergencies. I'm not saying that the same as your point, but it's interesting, nevertheless.

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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

Probably didn't word it very well

Is it sufficient to be aware of a hazard as you approach it, or should your following distance be increased as an extra precaution? (shorter reaction time will already mitigate the risk to some extent)

I don't think the two are related. Suppose you have a vehicle 2 seconds ahead. You might also be adjusting speed or position ready for a hazard 10 seconds ahead.

If the vehicle ahead is the hazard, you'll have to decide what to do about it. A nice early lane change is a good solution where practical.

If the vehicle ahead becomes erratic, then a large gap increase might well be wise. 2 seconds gap won't be a full stopping distance at higher speeds.

(I hope that properly covers your point...)

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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

Extending Peter's take, I tend to leave longer gaps than 2 seconds at very hidgh speed and on wet roads, principally because brake efficiency varies from car to car much greater under those conditions. Also, as Peter points out, a lower braking efficiency makes more difference at high speed. Take it (hypothetically of course) up to 120 - brakes 90% as good as the bloke in front who plants the pedal... Quite sobering.

It's also sensible to have a background awareness of differences in vehicle types. If you're driving a fully laden Transit and the vehicle in front is a brand new Impreza then some extra space would be well advised.

However in clear conditions we can (in a very general sense) tend to know if the vehicle in front will brake suddenly. We should be observing the road ahead of him and in general if there are no hazards we can tend to assume that he won't be braking suddenly. This is how we justify a transient close following distance (1 second ish) in preparation for overtaking.

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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

A thought struck me reading the last bit by SS - RE the transit.( Now someone will say it so i'm going to get in first - sometimes thought do )

Not just the difference between a fully laden Transit ( actually the brakes on these are quite good - rear axle got pressure limiter and front got big discs) and a brand new Impreza ( but in two different worlds)- try the difference between two of same types - e.g. transits - we use 3/4 all of similar mileage and year , yet one will stop on a sixpence ( ancient coin of yesterday for the young set) and the worst needs space - all passed as acceptable by main dealers.
Now try jumpng in and out of this lot on a daily basis and for the first mile or so you wonder "does this stop"

I suggest you're more likely to be observing differences in pedal sensitivity rather than ultimate braking performance.

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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

There you are, driving along that stretch of 30MPH dual carriageway with the camera off to the left so you check your speed as you approach it, and check again, and check again as you pass it and just to make sure, you glance at your mirrors to see whether it flashed while you were in the zone.

Thud

That was the 11 year old kid you just hit who followed the ball out into the road you didn't see because you were looking at your speedo (because you want to keep your licence) rather than observing the road ahead.

Still at least you can categorically assure the investigating police officers you weren't speeding and that it had no factor in the accident.

And at the kids funeral, you can say how sad it is that little johny wasn't one on the 90% we're told aren't killed when struck at less than 30 mph.

Absolute lunacy that we have these things at the roadside

[quote="Observer"]It is common ground among most contributors here that a two seconds minimum separation from the vehicle in front provides a safety margin which allows time to react (the "S" and "T" in COAST).

In my opinion all such "rules" are an indication that the government/authorities believes drivers need a code to enable them to drive safely. Maybe there is some value in this to inexperienced/poor drivers but, surely experienced drivers can judge what gap is needed in the prevailing circumstances. Factors to be considered are: speed, road surface, conditions, known stopping characteristics of own vehicle, known reaction time of self, estimated stopping characteristics of vehicle ahead, manner of driving of vehicle ahead etc. All of these are second nature to a good driver. Of course some drivers seem to be incapable of performing the simplest time and space calculations but these poor souls are also unlikely to be able to count accurately to two seconds!

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I'm not always right, but my score is higher than the government's

If only they did. The goverment's idea of a code is Speed Kills and not much else. The two second rule is a good example of something that used to be tlaked about and promoted with ads on the TV, but has been pretty much forgotten about since they decided that the inflexible enforcement of speed limits ought to cure everything.

Fifteen years on the road and I make more use now of the two second rule than I did back then. You're right to say that other factors need to be considered but when driving the two second rule is a good place to start. Fantastically grippy road surface? I can drive a bit closer than two seconds. Smashing down with rain? Need more space. And so on.

There's nothing wrong with using this sort of thing as a code for safe driving, whether a driver is experienced or not. Thinking about it I don't think I knew what COAST was until I started visiting these forums, but now I try to use it all the time i'm behind the wheel. It's a code for safe driving isn't it? Same applies to the "tyres and tarmac" one someone came up with recently, and even mirror-signal-manoeuvre that we all learnt when we still had L plates. Add 'em all together and we all ought to be able to, by and large, be left to get on with the task of not hitting anything.

If only the government could see that.

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Make everything as simple as possible, but not simpler - Einstein

Not if my daily perambulations along the M54 are anything to go by they can't.
Tailgating is endemic.
Regular cleanups following rear-enders are witnessed.
Even a quick fright when the vehicle ahead brakes causing the dozy tailgater behind to anchor up doesn't seem to convey the simple message - 'hey jerk..you were driving too close, back off', becuase they can be seen doing it all over again once the traffic gets up to speed