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Orillion

116 posts

45 months

[news] 
Thursday 24th May 2012 quote quote all
The Black Flash said:
I also know that I can (and have) react and brake in well under three-quaters of a second, and have done multiple times.
Why have you found it necessary to react and brake quickly on multiple occasions?

davepoth

22,641 posts

79 months

[news] 
Thursday 24th May 2012 quote quote all
Nigel_O said:
Either way, if the car in front brakes hard and you're following at a 1 second gap, you're nearly always going to hit it. The only escape is if (as some advanced drivers do) you've spotted the brake lights coming on several cars ahead and you're on the brakes before the car in front. However, the OP admits to tailgating simply because other drivers do, so there's a very high chance that whilst he may not ram the car in front, he's going to be hit from behind by the other driver in the Lemming-train
That's it really; leaving a big gap means you can brake less hard, avoiding being shunted by someone behind. Whenever I'm in a line of traffic, regardless of the speed, I aim to manage the gap such that I don't need to use the brakes. Obviously it doesn't always work, but it makes my journey a tiny bit quicker, and knowing the theory behind traffic jams, makes a much bigger difference to the people behind.

Zeeky

2,286 posts

92 months

[news] 
Thursday 24th May 2012 quote quote all
When overtaking on a single carriageway, one lane either direction, do you offside from a 2 second gap or reduce it temporarily?

The Black Flash

5,157 posts

78 months

[news] 
Thursday 24th May 2012 quote quote all
Orillion said:
The Black Flash said:
I also know that I can (and have) react and brake in well under three-quaters of a second, and have done multiple times.
Why have you found it necessary to react and brake quickly on multiple occasions?
Oh I see where this is going rolleyes

Animals comming out of the hedge on country roads, normally. And once comming up to the brow of a hill to find a 4x4 + trailer on my side of the road, overtaking something. I'm satisfied that it wasn't my anticipation or planning that was at fault in each case, which is what you're insinuating. I always run through such incidents afterwards to consider if I could have done anything different beforehand.

Orillion

116 posts

45 months

[news] 
Thursday 24th May 2012 quote quote all
The Black Flash said:
...which is what you're insinuating. I always run through such incidents afterwards to consider if I could have done anything different beforehand.
No insinuation intended. Animals do pose a problem, and so do people driving on the wrong side of the road. Most people, I suspect, find themselves occasionally surprised. Reflection is a good thing.

Here is a link that you might find interesting:

http://www.advanced-driving.co.uk/forum/viewtopic....

Advertisement

Toltec

2,220 posts

103 months

[news] 
Thursday 24th May 2012 quote quote all
The Black Flash said:
Oh I see where this is going rolleyes
biggrin

Out of interest, what do you chaps with the glacial reactions do when someone pulls into your braking gap? Do you immediately brake hard to re-establish the gap? If having a gap less than two seconds is such a problem what is the correct response to loosing it?

Don't get me wrong, having a sub second gap is a risk, I just think it is manageable for brief periods until a more suitable gap can be re-established. I tend to like quite large gaps as it allows me to respond rather than react and it is much less tiring on a longer drive etc.



simoid

11,868 posts

38 months

[news] 
Thursday 24th May 2012 quote quote all
SVS said:
On rare occasions, vehicles can stop dead. It happened to me when the whole engine (!) fell out of my car yikes whilst I was driving in the outside lane of the M25. My car stopped dead.

It turned out that the bolts holding the engine in place had simultaneously sheared. It was obviously a freak accident. But then emergency stops are for emergencies, such as a freak occurrence.
Stopped dead, as in, instantly?

What speed were you going, and how are you still alive?

wink

MC Bodge

5,193 posts

55 months

[news] 
Friday 25th May 2012 quote quote all
The Black Flash said:
Orillion said:
The Black Flash said:
I also know that I can (and have) react and brake in well under three-quaters of a second, and have done multiple times.
Why have you found it necessary to react and brake quickly on multiple occasions?
Oh I see where this is going rolleyes

Animals comming out of the hedge on country roads, normally. And once comming up to the brow of a hill to find a 4x4 + trailer on my side of the road, overtaking something. I'm satisfied that it wasn't my anticipation or planning that was at fault in each case, which is what you're insinuating. I always run through such incidents afterwards to consider if I could have done anything different beforehand.
There is a suggestion that you should be able to stop (comfortably, without being on the absolute limit) within the distance you can see to be clear.

As you've discovered, large objects can be concealed by the brows of hills. Arguably, you should have anticipated and planned for this.

Nigel_O said:
The only escape is if (as some advanced drivers do) you've spotted the brake lights coming on several cars ahead and you're on the brakes before the car in front.
Surely this is what some drivers do?

Edited by MC Bodge on Friday 25th May 10:03

noumenon

1,152 posts

84 months

[news] 
Friday 25th May 2012 quote quote all
I read this thread with much fascination. My principle of safe motorway driving is based largely around never letting the gap to the car in front decrease under braking. Since a lot of people don't brake hard enough early enough, this is an easy way to help better judge how hard to brake. (I also look in the rear view mirror to try to assess if I brake harder will it cause the car behind to hit me.)

I hadn't consdiered the case where the car in front slams on the brakes. I am quite surprised at how quickly the gap disappears due to the unrecoverable speed differential. Yikes.

In terms of trying to get past slow drivers - better to be late in this life than early in the next. wink

Toltec

2,220 posts

103 months

[news] 
Friday 25th May 2012 quote quote all
The speed differential is not necessarily irrecoverable, providing you begin braking within the time gap and brake at least as hard as the vehicle in front you will not hit it. The problem is that there are circumstances when the vehicle or obstruction in front does slow faster than you can. It may hit a static or slower moving object, or a load loss or mechanical failure which digs into the road surface are examples. It may have better tyres and braking systems.




creampuff

Original Poster:

3,223 posts

23 months

[news] 
Friday 25th May 2012 quote quote all
[quote=MC BodgeAs you've discovered, large objects can be concealed by the brows of hills. Arguably, you should have anticipated and planned for this.

[/quote]

You can anticipate a stopped car on your side of the road over the crest of a hill, with another car oncoming and a hedge to your left, eliminating any escape options other than stopping. That's why you "drive at a speed where you can stop, on your own side of the road, in the distance you can see to be clear".

This person related cresting a hill and finding a 4WD towing a trailer rapidly approaching in his side of the road. You cannot plan for a car coming towards you, at high speed, on their wrong side of the road, on a blind overtake of another vehicle. For a start, there are some crests you would just never be able to approach at all if you had to consider this.

MC Bodge

5,193 posts

55 months

[news] 
Friday 25th May 2012 quote quote all
Yes, but you shouldn't be 'surprised' by something unseen over the blind brow, even if it is heading your way. It could always happen, even it is unlikely.

On a narrow road with poor visibility, you should drive to allow oncoming traffic to also stop. Not necessarily practical on an A-road, but it needs to be considered.

The plan could involve swerving into the hedge whilst stamping on the brake....


The alternative is to have your car blessed by god and drive like a Filipino Jeepney driver

An interesting experience wink

Edited by MC Bodge on Friday 25th May 14:09

Nigel_O

368 posts

99 months

[news] 
Friday 25th May 2012 quote quote all
Toltec said:
The speed differential is not necessarily irrecoverable, providing you begin braking within the time gap and brake at least as hard as the vehicle in front you will not hit it.
But this is part of the problem - the vehicle in front will have hit the brakes 0.5 - 1.0 seconds ahead of you, so even if you brake just as hard, the car in front will be travelling slower, simply because it started braking sooner. This is the basis of my contention that with a 1-second gap, you WILL hit the car in front. You might be already on the brakes when you hit it, but the chances of being able to brake hard enough to make up the speed differential are slim.

Its the same pricipal as the concertina effect in car racing - the gap opens and closes purely because one car is on the brakes (or throttle) before the other

Toltec

2,220 posts

103 months

[news] 
Friday 25th May 2012 quote quote all
Nigel_O said:
But this is part of the problem - the vehicle in front will have hit the brakes 0.5 - 1.0 seconds ahead of you, so even if you brake just as hard, the car in front will be travelling slower, simply because it started braking sooner. This is the basis of my contention that with a 1-second gap, you WILL hit the car in front. You might be already on the brakes when you hit it, but the chances of being able to brake hard enough to make up the speed differential are slim.

Its the same pricipal as the concertina effect in car racing - the gap opens and closes purely because one car is on the brakes (or throttle) before the other
You are wrong, but it is a common misunderstanding.

The gap is e.g. 1 second at 80mph, the car in front brakes and begins to lose speed.
In 1 second the car, assuming instant full braking and good brakes, the car loses 20mph.
The change in distance traveled is the average speed, 10mph, times time, 1 second.
This is 1/8 of the distance traveled at 80mph.
From the point of view of the following driver the gap is now therefore 0.875 seconds at 80mph.

The following driver after this second is also now instantly at full braking*
There is now a constant 20mph differential between the cars and so a narrowing gap.
This gap however is 0.875 seconds at 80mph and therefore 3.5 seconds at 20mph.
The front car will stop after a further 3 seconds so the remaining gap is 0.5 seconds at 20mph.
The rear car is still doing 20mph, however as it is still braking and will stop in 1 second.
Over that second its average speed is 10mph, so the 0.5 seconds at 20mph being 1 second at 10mph the car will stop just touching the bumper of the lead car.

In brief, providing the tail car begins braking before its passes the point** at which the front car began braking and can at least match the braking of the lead car there will not be a collision.

- * I know this is not possible, however all the tail car has to do is match the braking profile of the front car.

- ** This being where the front of the tail car passes the position of the rear of the lead car, i.e the gap.




Edited by Toltec on Friday 25th May 19:36

Mr Grayson

148 posts

55 months

[news] 
Friday 25th May 2012 quote quote all
Mr Toltec, when we drove together you had a very sensible approach to following distances which isn't necessarily represented accurately in your posts above. Try and post objectively to reflect your actual practice, rather than concentrating on numerical theories wink

Regards

noumenon

1,152 posts

84 months

[news] 
Saturday 26th May 2012 quote quote all
I think the calculation is rather simpler. Let's say both cars are doing 70. The car in front stands on the brakes and gets the speed down to 50 before the car behind starts to brake.

Stopping distance at 70 mph = 245 feet
stopping distance at 50 mph = 125 feet

So if there is more than 120 feet between the cars after the rear car starts braking, then all is good, otherwise it's a smash.

120 feet at 70mph is about 1.2 seconds and that is after the car in front has already eaten into the gap through braking.

Toltec

2,220 posts

103 months

[news] 
Saturday 26th May 2012 quote quote all
It is even simpler, as I said earlier you only need to start braking at or before the point the lead car did.

This why using a timed gap rather than a distance is so useful as a guide, it represents how long you have before you must brake to avoid a collision in a limit situation.

Ignore the distance and speed, the gap in seconds is how long it will be before you cross the point at which the lead car began braking. Providing you brake by that point and at least as hard you will not hit the car.

As Mr Grayson kindly reminds me, while the physics is simple people are not and we really should not drive at the limits of the physics.

A two second gap gives you time to notice the car in front braking and work out how hard they are braking so you can make an appropriate response. It not only gives you some leeway for the vehicle in front having better brakes it helps to prevent over-reactive braking. It helps you manage the gap to the car behind.

Personally I like to run with three or four seconds in hand if the traffic allows, it makes for a low stress and smoother drive.

On the other hand a half second gap does not mean you will always hit the car in front. It is much riskier and requires a high level of concentration and observation, it should also be quite transient and seldom if ever done by choice.

Unfortunately even if you can manage sub two second gaps many drivers cannot, I would say .7 to 1.5 reaction times are fairly optimistic, two plus is common given my observations. This means closing to a one second gap when you have another car running less than two behind is very risky as you have just ceded control of the situation and your safety to the car behind you.


kaf

323 posts

27 months

[news] 
Sunday 27th May 2012 quote quote all
I find it staggering that so many support reduced following distances in an advanced driving forum. No wonder there are so many rear end shunts.

There is no such thing as being 'too far' from the car in front.

Nobody has ever died as a result of being too far back from the car they are following.

martinalex

168 posts

51 months

[news] 
Sunday 27th May 2012 quote quote all
You're making your passenger feel uncomfortable and unsafe which is definitely inconsiderate and potentially crap driving. So why continue doing it?
Proving you're right with the PH opinion won't make her feel safer.
If you did it to your passenger as a chauffeur, I suspect you you wouldn't be chauffering for long

Dr Jekyll

6,881 posts

141 months

[news] 
Sunday 27th May 2012 quote quote all
kaf said:
There is no such thing as being 'too far' from the car in front.
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