Motorway 'safe' distance chevrons

[0000]

25NAD90TUL said:

M5 Taunton 2011, seven people dead and fifty one injured. Some people avoided the carnage completely, some sadly were in the thick of it, yes if you hit a lorry that has stopped in a pile up it is stopped, you aren't going to move it. It is stopped dead.

Was that the one where bonfire smoke blew over the carriageway in the dark? Not sure stopping distances were the problem there.

[0000]

25NAD90TUL said:

I'm maintaining that 2 seconds at 70 is inadequate, 7 is erring on the safe side, 4 has been suggested which is somewhere near the middle of these two extremes, and has to be well better than 2.

2 seconds isn't an extreme, it's a widely observed general rule for the masses. 7 seconds is an extreme.

[SK425]

25NAD90TUL said:

SK425 said:

I don't know about anyone else, but if I'm following someone at about 2 seconds at 70mph, it's on the basis that I can see what's going on up ahead and I'm going to get a hell of a lot more than 2 seconds notice - a hell of a lot more than 7 seconds notice for that matter - of something like a pile up ahead. If your main or only source of information is the vehicle immediately in front of you then following as close as 2 seconds is rather bold, but there's usually loads more information available than that.

If I can't see far enough ahead for some reason - e.g. following a large vehicle, or driving through fog or smoke - I'd increase my following distance, but I don't generally feel the need to use a following distance suitable for poor visibility when visibility isn't poor. In normal conditions I've never found a two second gap - or maybe even a bit less - has made it difficult to deal with encountering stationary traffic ahead.

So we're assuming now that I leave a 7 second gap but meanwhile I'm looking no further ahead than the car in front's bootlid? C'mon.

I wasn't assuming that. You mentioned the M5 bonfire incident and I thought your point was that visibility is sometimes (very) poor. I was just discussing how I choose to vary my following distance depending on how far I can see ahead. Normally I can see far enough ahead that 2 seconds is more than adequate. Occasionally I can't.

It's not just motorway following distances I approach in this way. Blatting along a clear B road on a warm, dry summer's day, I don't drive the same as I would if the road were cold, wet and icy.

[waremark]

0000 said:

25NAD90TUL said:

I'm maintaining that 2 seconds at 70 is inadequate, 7 is erring on the safe side, 4 has been suggested which is somewhere near the middle of these two extremes, and has to be well better than 2.

2 seconds isn't an extreme, it's a widely observed general rule for the masses. 7 seconds is an extreme.

it's a widely observed disregarded general rule for the masses - and I agree that a 2 second gap on a multi-lane road is far from extreme.

[Toltec]

25NAD90TUL said:

Ah, superb, 'if you can't deploy the brakes within 2 seconds' but you have only allowed 2 seconds between the car in front remember, the 2 seconds in which you deploy is the thinking distance, this is added to the stopping distance, so at 7 seconds you have 2 seconds to think and 5 seconds to actually bring the car to a halt. Looking in those terms it isn't that much time actually.

Five seconds at seventy will give you at least ten seconds to come to a stop, this is with a constant braking force just sufficient to stop you before passing the point which was originally seven seconds away. You would be braking approximately half as hard as the test/reference car used for HC distances.

At two seconds if you are on the ball and driving a modern high performance car you might just be able to stop before the end of your gap. At three seconds there is enough time to make sure the driver behind gets to see you are braking before really going on the brakes. At four seconds there is loads of time to decide how hard the car in front is stopping, warn the driver behind and decide if changing lanes would be a good idea or not.

WinstonWolf said:

You do realise you're leaving ten times the recommended thinking distance and more than double the complete stopping distance?

Thinking time is about 0.7 seconds so three times not ten, but definitely more than double the time/distance for a modern car to stop in the dry.

[Toltec]

waremark said:

it's a widely observed disregarded general rule for the masses - and I agree that a 2 second gap on a multi-lane road is far from extreme.

It is more that you want at least four seconds from the back of the car in front to the front of the car behind...

[25NAD90TUL]

waremark said:

it's a widely observed disregarded general rule for the masses - and I agree that a 2 second gap on a multi-lane road is far from extreme.

So you consider a 2 second gap at 70 is adequate?

I'm signing out in a minute, I'm going to go up the A30 at 70 using a 7 second gap from the car in front and have another look at the distance between us, I'm sure it isn't as much as some suggest, someone said it's over 50 car lengths and I'm sure it's not that far, in reality when I use the 7 second gap it doesn't actually look excessive to me.

Is it being suggested that even if the car ahead is half a mile away that I should rush up behind it to get that 2 second gap?

[25NAD90TUL]

0000 said:

Was that the one where bonfire smoke blew over the carriageway in the dark? Not sure stopping distances were the problem there.

Actually no it wasn't.

The result of the inquest and the manslaughter trial was that dense fog was the cause. The smoke only added to the problem. No charges were brought against the guy who had the bonfire.

[25NAD90TUL]

Toltec said:

At two seconds if you are on the ball and driving a modern high performance car you might just be able to stop before the end of your gap. At three seconds there is enough time to make sure the driver behind gets to see you are braking before really going on the brakes. At four seconds there is loads of time to decide how hard the car in front is stopping, warn the driver behind and decide if changing lanes would be a good idea or not.

All well and good, but you are assuming the driver in front is as skilled and aware as you are. I'm not prepared to take the risk that he might not be.

[WinstonWolf]

25NAD90TUL said:

Toltec said:

At two seconds if you are on the ball and driving a modern high performance car you might just be able to stop before the end of your gap. At three seconds there is enough time to make sure the driver behind gets to see you are braking before really going on the brakes. At four seconds there is loads of time to decide how hard the car in front is stopping, warn the driver behind and decide if changing lanes would be a good idea or not.

All well and good, but you are assuming the driver in front is as skilled and aware as you are. I'm not prepared to take the risk that he might not be.

Ideally you're looking well ahead of him too and anticipating any reason why others might brake.

[25NAD90TUL]

Accidents, collisions, emergencies are what they are, they happen suddenly, they don't slowly unfold for you to see and react to. It therefore follows that the farther away from the emergency you are, the more time you have to react. It's not a bad thing having fast reactions, but a very foolish thing to be relying on them all the time.

My reactions aren't that fast, I'm nearly 50. When they were faster I didn't make the mistake of relying solely on them, the graveyard is full of drivers who thought their reaction times were quick enough.

Now ok I am listening, I want to know what the benefit of closing that gap up to 2 seconds is? If we're all travelling at 70 you aren't getting to your destination any faster, well I concede maybe 5 seconds faster.

[StressedDave]

Ok, I've got a few minutes free to slay some demons...

1. The two second rule is not there to provide sufficient distance to brake to a stop, but to provide more than sufficient distance to observe and react to the vehicle in front slamming its brakes on.
2. A mechanical failure cannot cause a vehicle to decelerate at a rate greater than the coefficient of friction between tyre contact patch and road surface. A locked tyre has a coefficient of friction around 0.7, modern ABS systems can achieve 1g plus, and metal on tarmac can reach about 0.5g. Nothing stops instantly.
3. The motorway accidents you use as justification for an enormous gap were caused by drivers failing to drive at a speed whereby they could stop in the distance they could see to be clear. If you really need a complete stopping distance between you and the vehicle in front, might I suggest you raise your vision a bit...

[WinstonWolf]

5 seconds per vehicle

Two seconds is an appropriate balance between leaving a safe distance to react in and achieving reasonable throughput on a given road.

[25NAD90TUL]

WinstonWolf said:

Ideally you're looking well ahead of him too and anticipating any reason why others might brake.

Yes I'm with you on that point. I'm not just arguing about this out of boredom or being ttish. I fully believe in what I am suggesting otherwise I wouldn't be suggesting it or using it.

Now I'm asking what is the benefit of using a 2 second gap over a 4 or 7. I mean I've said why I think it's no good, now tell me why it is good, and in fact why it's better?

[25NAD90TUL]

StressedDave said:

Ok, I've got a few minutes free to slay some demons...

1. The two second rule is not there to provide sufficient distance to brake to a stop, but to provide more than sufficient distance to observe and react to the vehicle in front slamming its brakes on.
2. A mechanical failure cannot cause a vehicle to decelerate at a rate greater than the coefficient of friction between tyre contact patch and road surface. A locked tyre has a coefficient of friction around 0.7, modern ABS systems can achieve 1g plus, and metal on tarmac can reach about 0.5g. Nothing stops instantly.
3. The motorway accidents you use as justification for an enormous gap were caused by drivers failing to drive at a speed whereby they could stop in the distance they could see to be clear. If you really need a complete stopping distance between you and the vehicle in front, might I suggest you raise your vision a bit...

On point 2, in the pile up example I used the vehicles being hit were already stopped, friction coefficient or no.

Point 3, I didn't say I 'needed' the gap, just that I preferred it, need was never mentioned. I could say I feel I need more than 2 though.

Why is it wrong to allow a more than adequate safe gap anyway? What benefit does closing the gap have?

I can assure you that my forward obs isn't lacking at all thank you.

And your maths may work perfectly in the office on paper, I'm talking about the real world not one of the scenarios on your training courses. With or without the maths are you suggesting that the locked diff in my fictitious example would result in a nice steady controlled stop? Have you experience of a locked diff? From your experience stated earlier clearly not, the two examples you stated were not a locked diff at all. Sure it won't stop just dead, momentum will push it on somewhat, there will be no ABS, ABS can't free a wheel locked by the driveshaft, as you know. So yes while it will not stop 'dead' instantly, it's hardly going to be a controlled stop.

[StressedDave]

25NAD90TUL said:

Why is it wrong to allow a more than adequate safe gap anyway? What benefit does closing the gap have?

I can assure you that my forward obs isn't lacking at all thank you.

And your maths may work perfectly in the office on paper, I'm talking about the real world not one of the scenarios on your training courses. With or without the maths are you suggesting that the locked diff in my fictitious example would result in a nice steady controlled stop? Have you experience of a locked diff? From your experience stated earlier clearly not, the two examples you stated were not a locked diff at all. Sure it won't stop just dead, momentum will push it on somewhat, there will be no ABS, ABS can't free a wheel locked by the driveshaft, as you know. So yes while it will not stop 'dead' instantly, it's hardly going to be a controlled stop.

There may be no benefit to you, but if everyone did it, you'd drastically reduce the capacity of the motorway. For ease of maths, let's pick a six second gap. Each lane therefore has a capacity of ten vehicles per lane per minute, at a two second gap it's thirty cars per lane per minute.

As for your 'locked diff' that you keep banging on about, a seized drivetrain is entirely the same as having all four wheels locked. Now, it may surprise you to know that I wasn't chained to a desk waffling about theoretical situations. The vast majority of the theorems that were used to back calculate speeds and reconstruct accidents had to be proved experimentally. I've been the unlucky sod at the wheel proving goodly chunks of theory involving front wheel, lockup, rear wheel lockup and four wheel lockup at speeds up to 100mph in everything from a Caterham 7 up to 4wd behemoths, not yo mention lgv and psv. Factor in a few years worth of working in R&D in differential research and I might suggest my credentials go a liitle further than owning a '90 and having worked as a recovery driver.

[25NAD90TUL]

StressedDave said:

There may be no benefit to you, but if everyone did it, you'd drastically reduce the capacity of the motorway. For ease of maths, let's pick a six second gap. Each lane therefore has a capacity of ten vehicles per lane per minute, at a two second gap it's thirty cars per lane per minute.

As for your 'locked diff' that you keep banging on about, a seized drivetrain is entirely the same as having all four wheels locked. Now, it may surprise you to know that I wasn't chained to a desk waffling about theoretical situations. The vast majority of the theorems that were used to back calculate speeds and reconstruct accidents had to be proved experimentally. I've been the unlucky sod at the wheel proving goodly chunks of theory involving front wheel, lockup, rear wheel lockup and four wheel lockup at speeds up to 100mph in everything from a Caterham 7 up to 4wd behemoths, not yo mention lgv and psv. Factor in a few years worth of working in R&D in differential research and I might suggest my credentials go a liitle further than owning a '90 and having worked as a recovery driver.

Thank you for that post. I know what you're credentials are, I'm not new to this AD forum game. I've been in road test with RR, time served in the motor trade, done a few AD tests along the way and driven a lot of performance and prestige cars professionally. That's without the off-roading F1 Stocks and Banger racing. My credentials also go a lot further than owning a 90 and being a recovery driver. Why are we bickering about this? A large gap considerably reduces the chances of you hitting something does it not? I care not what that something is or how quickly it stopped or how inert it is when you hit it, my point is that the Mways ARE too crowded, people DO drive too close to one another and people DO die, far too often for my liking, if I wish to increase the chances of me NOT being one of them, where is the problem?

Please understand that this post isn't delivered with any dis-respect, I know who you are and what you do and I'm pretty sure you are a way better driver than I am, that doesn't alter my thinking though on this subject, in fact I'd go further and say that I like as big a gap as I can get, sometimes I travel at night when going to pick up cars and in that respect I DO have the M5 all to myself quite often.

I know you're not going to like this but why earlier did you only mention those two examples of drivetrain failure when you had all that experience to draw from?

[Toltec]

25NAD90TUL said:

Toltec said:

At two seconds if you are on the ball and driving a modern high performance car you might just be able to stop before the end of your gap. At three seconds there is enough time to make sure the driver behind gets to see you are braking before really going on the brakes. At four seconds there is loads of time to decide how hard the car in front is stopping, warn the driver behind and decide if changing lanes would be a good idea or not.

All well and good, but you are assuming the driver in front is as skilled and aware as you are. I'm not prepared to take the risk that he might not be.

The skill of the driver in front is not relevant, if you have a three second gap and get on to full braking within a second then, in a modern car, you will stop before hitting the car in front, even if it stops dead.

A modern car can brake from 70mph in 4 seconds without being on the limit, driver permitting.

Over that four seconds your average speed will be 35mph so you will travel the same distance as you would in two seconds at 70mph. That means two seconds of gap at 70 mph is enough braking distance to stop, then just add a second to decide you need to do an emergency stop and get on the brakes. Therefore if the traffic is heavy it is difficult to justify having a larger gap than three seconds, unless you are in the lhd lane and therefore not using an overtaking lane.

In light traffic where the road is not capacity limited then I do not see an issue with using a larger gap, it makes for a more relaxed drive and it allows other cars to move into your lane either to overtake a vehicle on the left or after overtaking you. In heavy traffic, you mentioned the M6 earlier so you know what WinstonWolf means about that, you just cannot expect to have a seven second chunk of road.

On the other hand, what I am behind matters, I once had to avoid a propshaft that came flying out from under a knackered old van and on another occasion only avoided an 8x4 sheet of plywood by swerving to the other side of the road and ducking.

In the first case the prop bounced into the air converting its forward momentum into vertical movement so effectively decelerating very quickly, in the latter, well an 8x4 sheet makes an effective air brake.

Edited by Toltec on Monday 13th January 21:10

[Tonsko]

What about doubling the 2 second gap when it's wet?

/cat amongst the pigeons.

[0000]

25NAD90TUL said:

0000 said:

Was that the one where bonfire smoke blew over the carriageway in the dark? Not sure stopping distances were the problem there.

Actually no it wasn't.

The result of the inquest and the manslaughter trial was that dense fog was the cause. The smoke only added to the problem. No charges were brought against the guy who had the bonfire.

Smoke, fog, still not stopping distances then.