"only a fool"…………

[martine]

Here's the thing I don't get, there are people talking about how the 2 second rule doesn't give you a big enough gap to stop at motorway speeds. It's only an issue if the car in front suddenly ploughs into a brick wall that you somehow didn't see until that point.

OK how about...
- an object in your lane - the vehicle ahead goes over it or swerves to avoid it (think tyre, bumper, load from lorry)
- the vehicle ahead has a head-on with one that's crossed the central reservation or simply going the wrong way
- and most commonly, you're following a high-sided van that swerves at the last minute to reveal a stationery vehicle. (this happened to me )

Even someone suddenly braking hard is an issue with a 2 second gap...remember the 0.7 'thinking time' - delay it a little bit and you're be in the back of them before you've even touched your brakes. It all happens so fast and that's what catches people out every day on the motorway. I bet 90% of the collisions on motorways just wouldn't happen if the everyone stuck to 2 secs or more.

[Mr Cholmondeley-Warner]

Here's the thing I don't get, there are people talking about how the 2 second rule doesn't give you a big enough gap to stop at motorway speeds. It's only an issue if the car in front suddenly ploughs into a brick wall that you somehow didn't see until that point.

It doesn't, because of what Martin said:

Yes but of course it's not a straight line...doubling your speed, doubles braking time but quadruples braking distance.

So the 2 second rule is too simplistic as well. In fact at 70mph you need about 5.5* seconds - the stopping distance (according to the HC) takes 4.8* of those, and if you allow 0.7 for thinking time that's just enough. In fact you'd need more unless your reactions and reflexes were super-sharp.

The real rule would be based on the stopping distance divided by the speed plus the thinking time. The stopping distance itself is derived from the speed based on an arbitrary deceleration so you can apply v**2=u**2 + 2as and adapt it so you get s=(v**2 - u**2) / 2a. I think the arbitrary deceleration is 0.4g, but someone like StressedDave would know better. *Edit - working it out from the HC figures seems to give 0.67g but I'm not very comfortable with all the imperial / metric conversions involved

If you're doing or just done GCSE maths or physics you'll be much better in tune with this than me

And Martin provided some examples of where an object like a brick wall can suddenly appear in which case most people will be stuffed.

*EFA

[MGF]

In Martin's example the stationary object would not suddenly appear. It would take time to develop and in that time you could be braking.

The 2 second rule is simply a rough rule of thumb for a safe following distance, that is, to give us enough time to react safely to traffic slowing ahead in most circumstances. It shouldn't be applied rigidly.

[martine]

In Martin's example the stationary object would not suddenly appear. It would take time to develop and in that time you could be braking.

I gave 3 examples. I can envisage all of them 'suddenly appearing'...think of a straight motorway with heavy traffic in all lanes - even with a 2 secs gap you have a very restricted view directly ahead.

[fungus]

Quote from Stephen Haleys Mind Driving, skill 7 Risk assessment and control. Page 140.

Conflicting advice.

Two measures are used in this rule.

The first is distances. "never get closer than the overall stopping distance".

And the other is time. "Allow at least a two second gap...".Although, the "at least" part is largely overlooked.

But which should you use - the distances or the 2 seconds? And are they the same, being together in the same rule?

In fact, they are very different, coinciding at only one speed - which calculates out to be 19mph. This is the speed from which takes 2 seconds to stop 9on Highway Code figures, including thinking time). Above this, the time increases far beyond that figure.

"From 70mph it takes an eternal 5.5 seconds to stop, and after two seconds you are still doing 51mph!"

You may think this is all right, being confident you can stop as quickly as the chap in front. But this assumes they do not hit something that stops them quicker. And this is exactly the mechanism that unleashes multiple pile-ups.

[fungus]

If you're doing or just done GCSE maths or physics you'll be much better in tune with this than me

Left school 47 years ago and not needed to use anything but basic maths since.
Gave up on maths when it got to algebreric equations.

[Ralge]

...You may also want to ask their 'grade' and look for a high one - qualified instructors are graded 4, 5 & 6, with 6 being highest. They are regraded at each check test.

Since April this has now been replaced with a simpler A or B grade...
https://www.gov.uk/government/news/standards-check-new-grading-structure-to-be-simpler-and-clearer

I'm not sure I buy into or understand the "simpler, clearer for Jo Public/Parent" message.
Firstly how much was known by Jo Public about the old ADI grading structure?
Secondly, how much does the same JP know about the new structure? The ADI press has been full-to-brim with CheckTest/Standards Check but what coverage has been given to the change in the ordinary press and media?
Thirdly, moving from fail grades (1,2,3) to a "4 (just scraped through), 5 OK, 6 pretty good" structure to one of B=OK, A = pretty good doesn't seem to me to be that much simpler or clearer in its move to fewer pass grades and from numbers to letters.

[Graham Wright]

Anyway - to get back to the original question.

I would like to discourage tail gaters and the suggestion of the "Only a fool ……" sticker was to maybe achieve that. When the bumper of a following vehicle cannot be seen in your mirror, the gap is considerably less than 2 seconds.

What we do, as experts (!) is up to us. It is the followers that concern me.

Yesterday, I had a heavy right up against my rear end at 50 m.p.h. I would guess the separation was about 6 feet. Slamming my brakes on would have stopped that but, probably also, my life.

Frequently, following BMWs and Audis in a traffic stream, I see their brake lights around every 4 to 5 seconds.

How about "Support Ferodo - be a tail gater"?

"Tail gating can win you points".

I am sure someone can be more imaginative.

Again yesterday, I had a small white van right up against the back window (M50 - you know who you are). I raised my hand in front of the interior mirror with palm facing backwards. It worked. Message received?

[Mr Cholmondeley-Warner]

In fact, they are very different, coinciding at only one speed - which calculates out to be 19mph. This is the speed from which takes 2 seconds to stop 9on Highway Code figures, including thinking time). Above this, the time increases far beyond that figure.

"From 70mph it takes an eternal 5.5 seconds to stop, and after two seconds you are still doing 51mph!"

You may think this is all right, being confident you can stop as quickly as the chap in front. But this assumes they do not hit something that stops them quicker. And this is exactly the mechanism that unleashes multiple pile-ups.

From 70mph it takes around 4.8 seconds to stop* if you use the HC stopping distance. I don't know where this 5.5 seconds came from (possibly by adding 0.7s reaction time but that's not enough) but it's been calculated using a different deceleration rate. However, if you watched the clip of the Car Limits coaching the other day you would have seen the journalist reducing his stopping distances (and therefore times) waaaaay below the HC figures with a little coaching and practice. The secret is to go full on the brakes (if possible without triggering the ABS) from the start. I'm not suggesting anyone should modify their 2 second rule downwards because of this, but it does show that people, cars and conditions vary enormously.

* my rusty workings

70mph=103fps
g=32 fps**2
v**2=103**2=10540
s=245ft
v**2=u**2 + 2as
therefore a=(v**2-u**2)/2s or (10540-0)/2*245 = 21.5fps**2 = 0.67g
v=u+at
therefore t=(v-u)/a = (103 - 0) / 21.5 = 4.8 seconds

EFA.

[martine]

From 70mph it takes around 2.3 seconds to stop if you use the HC stopping distance. I don't know where this 5.5 seconds came from...

Quick back of a fag packet calculation and I make it over 5 secs as well...thinking about it 2.3 secs does seem extremely quick from 70 so intuitively it seems wrong to me. I also suffer from Physics wayyyy back in my memory - so be gentle with me.

Serious thread diversion alert:
Amazing http://www.bloodhoundssc.com factoid: when Andy Green shuts off the power at the end of the measured mile (1000mph) he will get 3g deceleration just from aero drag alone (15 tons drag) That's losing 60mph/sec

[Mr Cholmondeley-Warner]

From 70mph it takes around 2.3 seconds to stop if you use the HC stopping distance. I don't know where this 5.5 seconds came from...

Quick back of a fag packet calculation and I make it over 5 secs as well...thinking about it 2.3 secs does seem extremely quick from 70 so intuitively it seems wrong to me. I also suffer from Physics wayyyy back in my memory - so be gentle with me.

Serious thread diversion alert:
Amazing http://www.bloodhoundssc.com factoid: when Andy Green shuts off the power at the end of the measured mile (1000mph) he will get 3g deceleration just from aero drag alone (15 tons drag) That's losing 60mph/sec

Now fixed. I don't know where I was getting that from yesterday

[GJD]

I would like to discourage tail gaters

So would I, ideally, but I don't see much prospect of being able to achieve it. Why would you think they're interested in your opinion? I suspect it would be more productive and constructive to focus on how you will manage being tailgated when it happens.

[GJD]

In fact, they are very different, coinciding at only one speed - which calculates out to be 19mph. This is the speed from which takes 2 seconds to stop 9on Highway Code figures, including thinking time). Above this, the time increases far beyond that figure.

Not sure if that's your words or a quote from Mind Driving? Seems a strange way to define the point at which the two rules coincide. If you're worried about whether the 2 second rule is closer than HC stopping distance, how many seconds it takes you to stop is irrelevant. What matters is is how much distance it takes.

At 19mph, your HC stopping distance would be 37 feet and your two second following distance would be 55 feet, so if the car in front came to an instant stop somehow, you'd have plenty of space.

HC stopping distance equals 2 second following distance at about 39mph.

[Mr Cholmondeley-Warner]

If you're worried about whether the 2 second rule is closer than HC stopping distance, how many seconds it takes you to stop is irrelevant. What matters is is how much distance it takes.

That may be true, but have you tried measuring the distance between you and the car in front on the move?

[GJD]

If you're worried about whether the 2 second rule is closer than HC stopping distance, how many seconds it takes you to stop is irrelevant. What matters is is how much distance it takes.

That may be true, but have you tried measuring the distance between you and the car in front on the move?

No. The point I was making was academic really. The way fungus described the coincidence of the two rules just seemed odd. For the record, I don't find the fact that the rules coincide at 39mph to be particularly relevant to anything, it's just how the maths happens to work out.

Outside of academic internet discussions, I haven't used HC stopping distance numbers since I passed my driving test. I never think of following distances or distances to hazards in numbers of metres or feet. Once in a blue moon I might count out the two second rule as something to do to pass the time, but generally I judge it by, well... judging it.