"only a fool"…………

[Ralge]

...I would just guess that a decent following distance would be:
Minimum number of car lengths = (speed limit / 10) + 1
For example, the speed limit is 70, so the minimum following distance should be 8 car lengths?

mmmm...not sure about your maths...8 car lengths would be around a 1 second gap @ 70mph...eeeek

Bearing in mind the 'official' thinking time is 0.7 seconds, you'd be awfully close to the vehicle in front before even starting to brake...

"Official" is out of date if recent research (Direct Line) is anything to go by which puts this element at 0.98 i.e. much nearer 1 whole second.

[michael769]

"Official" is out of date if recent research (Direct Line) is anything to go by which puts this element at 0.98 i.e. much nearer 1 whole second.

0.7s is close to the best case reaction time for an alert and trained driver. Recall that the HC figures are supposed to be the shortest stopping distances not the average.

1s is about average, but inexperienced and young drivers are closer to 1.5s.

[martine]

"Official" is out of date if recent research (Direct Line) is anything to go by which puts this element at 0.98 i.e. much nearer 1 whole second.

0.7s is close to the best case reaction time for an alert and trained driver. Recall that the HC figures are supposed to be the shortest stopping distances not the average.

1s is about average, but inexperienced and young drivers are closer to 1.5s.

I reckon even experienced drivers are much higher than 0.7 (or even 1s) in practice. I'm conscious I don't react with max. braking if I see something bad ahead...I may start braking but not optimally until I'm convinced it's a serious problem. I think my 'experience' works against me sometimes.

[michael769]

I reckon even experienced drivers are much higher than 0.7 (or even 1s) in practice. I'm conscious I don't react with max. braking if I see something bad ahead...I may start braking but not optimally until I'm convinced it's a serious problem. I think my 'experience' works against me sometimes.

IIRC the tests do control for expectation bias, and the 1s figure is achieved by drivers who were unprepared. I think we tend to underestimate just how quickly we can react, but we also underestimate just how long 1s is and how far we travel in that time.

I seem to recall that a lot of drivers to fail to hit the brake pedal hard enough in an emergency until the last instance when the "oh I'm gonna hit it" realization strikes.

Of course reaction times are somewhat different for the folks whose heads are constantly swiveling round to chat to other occupants - I suspect that for many of them even 2s is an impossible dream.

[TheInsanity1234]

...I would just guess that a decent following distance would be:
Minimum number of car lengths = (speed limit / 10) + 1
For example, the speed limit is 70, so the minimum following distance should be 8 car lengths?

mmmm...not sure about your maths...8 car lengths would be around a 1 second gap @ 70mph...eeeek

Bearing in mind the 'official' thinking time is 0.7 seconds, you'd be awfully close to the vehicle in front before even starting to brake...

My maths has fallen foul again.
I'm picturing the distance in my head, and it seems big enough, but I'm evidently not counting the spaces right.
I felt like I should put a x2 into the end, but then the gap came out at 16 car lengths, which seems like an awfully big gap.
I think my real problem is just trying to put an every-day sub-conscious thing into a calculation, it's difficult to be reasonably accurate with numbers (Especially when I've not had a chance to put any thinking into practice)

In fact, have any of you noticed that in some places on the motorway, there are chevrons painted on the road and you have to follow someone with at least 2 of those in between you and the car in front.
The gap that they make you follow at varies slightly, depending on your position in the car, but the average I've counted is between 1 and 1.5 seconds. Those distances are closer than the 2 second rule that the government keep talking about?

With a good view of the first chevron in front and with the car ahead already beyond the second (how the roadside board portrays it) I.e. fully two chevrons' space ahead, I measure it as a 2-second gap so am at odds with your assessment of it somehow.
(Incidentally, when have the Govt mentioned a two-second gap recently?)

The true benefit of such a gap as a MINIMUM, for me, can be understood if the answers are known to the questions:
- if it is a good idea to remind drivers what a 2-second gap looks like at 70mph, why aren't there chevrons on every stretch of m/w?
And
- whatever the reason for this, why is it a feature of this particular chevron-ed stretch? What is the root cause, the common denominator of every chevron-ed m/w bar one that I have seen that generates the need for the chevrons?

That's how my parents follow cars, with 2 chevrons in between us.
I measure it by counting the time it takes for us to reach the chevron that the car in front goes past.
It almost always comes out as a bit too short.
Whenever I've seen them, the motorway is usually very straight and level, so I suspect that may have something to do with it.

[Horse]

Note to DfT: increase m/w speed limits by all means but enforce space à l'allemande, the German way.

IIRC, the proposed increase to 80mph would only have been on motorways where enforcement was possible (ie managed motorways).

Bearing in mind the 'official' thinking time is 0.7 seconds, you'd be awfully close to the vehicle in front before even starting to brake...

Although, presumably, the car in front is still travelling forwards?

[martine]

...but then the gap came out at 16 car lengths, which seems like an awfully big gap.[/quote
No sounds about right to me.

I think my real problem is just trying to put an every-day sub-conscious thing into a calculation, it's difficult to be reasonably accurate with numbers (Especially when I've not had a chance to put any thinking into practice)

Exactly - so why not stick to the 2 seconds gap guide...it's easy to check and works? Just make sure you count 'one thousand, two thousand' and not '1,2'.

[michael769]

My maths has fallen foul again.
I'm picturing the distance in my head, and it seems big enough, but I'm evidently not counting the spaces right.
I felt like I should put a x2 into the end, but then the gap came out at 16 car lengths, which seems like an awfully big gap.

Distance is non-linear with respect to acceleration - mathematically it is a first order integral.

It's almost impossible to picture, which is why the guideline of 1m per mph of speed (which gives rise to the 2 second rule) - which is linear is preferred.

[125isfine.]

This always struck me as a good maxim.

It would be good for a sticker in the back window.

Does one exist?

I don't know if one exists, but I made my own version up a few years ago.
I couldn't possibly put it in my back window because of section 5 of PACE.
It ends in " vehicle in front " I'll leave it for you to fill in the gaps

[waremark]

I consider the 2 second gap to be mainly for reaction time - and not sufficient to deal with a vehicle in front coming to a sudden stop. I know this is not the official view. Since braking distance changes with the square of speed, a gap fixed by length of time could not possibly work for a wide range of speeds. Over about 40, in 2 seconds you travel further than the Highway Code overall stopping distance.

The motorway chevrons are positioned 40 m apart, so two lengths (which is what is meant, not two chevrons) is 80 m, which is more than 2 seconds but less than the HWC overall stopping distance at 70 of 96 m. An official publication calls this a safe driving distance at 70 mph.

I cannot imagine what 16 car lengths would look like when there aren't 16 cars lined up end to end. If you want to estimate a following distance on the motorway, you can calibrate your judgement by looking at the white with red and blue marker posts on the left - they are 100 m apart, so if you want to estimate a reasonable following distance at 70 aim for something a bit less than the gap between marker posts.

Incidentally, people often talk about following distances; they don't often talk about the space beside you. On a multilane road I want to have empty space beside me for as much of the time as possible. If something goes very wrong 2 seconds in front of me, I may not be able to stop but I will be able to change lane providing the space beside me is empty.

[waremark]

Mr Insanity, have you worked out how to choose your driving instructor? With most driving instructors, you would drive them mad by thinking you knew better, and they would drive you mad because you would know better! I recommend you to look for someone who advertises that they also coach advanced driving. 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.

Unfortunately, an excellent instructor may cost a little more per hour - but could be better value in the long run.

[Ralge]

...I would just guess that a decent following distance would be:
Minimum number of car lengths = (speed limit / 10) + 1
For example, the speed limit is 70, so the minimum following distance should be 8 car lengths?

mmmm...not sure about your maths...8 car lengths would be around a 1 second gap @ 70mph...eeeek

Bearing in mind the 'official' thinking time is 0.7 seconds, you'd be awfully close to the vehicle in front before even starting to brake...

My maths has fallen foul again.
I'm picturing the distance in my head, and it seems big enough, but I'm evidently not counting the spaces right.
I felt like I should put a x2 into the end, but then the gap came out at 16 car lengths, which seems like an awfully big gap.
I think my real problem is just trying to put an every-day sub-conscious thing into a calculation, it's difficult to be reasonably accurate with numbers (Especially when I've not had a chance to put any thinking into practice)

In fact, have any of you noticed that in some places on the motorway, there are chevrons painted on the road and you have to follow someone with at least 2 of those in between you and the car in front.
The gap that they make you follow at varies slightly, depending on your position in the car, but the average I've counted is between 1 and 1.5 seconds. Those distances are closer than the 2 second rule that the government keep talking about?

With a good view of the first chevron in front and with the car ahead already beyond the second (how the roadside board portrays it) I.e. fully two chevrons' space ahead, I measure it as a 2-second gap so am at odds with your assessment of it somehow.
(Incidentally, when have the Govt mentioned a two-second gap recently?)

The true benefit of such a gap as a MINIMUM, for me, can be understood if the answers are known to the questions:
- if it is a good idea to remind drivers what a 2-second gap looks like at 70mph, why aren't there chevrons on every stretch of m/w?
And
- whatever the reason for this, why is it a feature of this particular chevron-ed stretch? What is the root cause, the common denominator of every chevron-ed m/w bar one that I have seen that generates the need for the chevrons?

That's how my parents follow cars, with 2 chevrons in between us.
I measure it by counting the time it takes for us to reach the chevron that the car in front goes past.
It almost always comes out as a bit too short.
Whenever I've seen them, the motorway is usually very straight and level, so I suspect that may have something to do with it.

Very straight, level, long straight stretch - is precisely the significant bit.
It costs too much to have these chevrons everywhere.
There's a higher incidence of rear-ends on these chevron-ed stretches, hence the special treatment.
Can you think why, on a full, busy stretch of motorway like these, space is vitally important?
What problems accrue from the m/w's straightness when combined with lack of space here?

[michael769]

Mathematically, distance is a second order integral of acceleration, its velocity which is the first order integral

Indeed. I was thinking of speed not distance...

The 2 second rule is not.intended to give sufficient distance to stop, it is to.give you time to react.to a braking vehicle in front with a bit of a margin for the fact that most people don't brake hard enough initially. It's a larger gap than 1m per mph.

True it is based on the premise that the vehicle in front cannot stop dead. The idea is to get enough space to react with an allowance to accommodate differences in braking performance generally it works - but clearly if a vehicle strikes an immobile object it could come close to stopping dead and then a 2 sec gap may turn out to be rather inadequate.

There is a video on you tube (I cannot find it just now) where a car is following truck which suddenly swerves into lane 2 revealing an already stationary vehicle which the following car piles straight into.

But generally given how few drivers observe the 2 sec rule on motorways, we have more to worry about than brick walls appearing in running lanes.....

[TR4ffic]

Agreed, the 2 second rule has nothing to do with stopping distance - particularly at motorway speeds; it is about reaction time but, more importantly, IMO it's about giving you the time and space to observe (and react to) what's happening ahead of the traffic in front. On a motorway, I will spend time scanning ahead (eyes on 'main beam') as far as traffic and the road will allow - this could be a mile plus in certain circumstances - eyes on 'dip', check mirrors (repeat as necessary), assess what's going on around you and plan your lane changes and speed to maintain best progress or slow early if necessary.

If I happen to be behind a vehicle (lorry/van) that is restricting my forward view of what's in front of them and what's happening, I'll stretch the 2 seconds. Like Waremark, I'll try to travel next to gaps where practicable and if in L2 or L3, and passing traffic to my left with a low relative overtaking speed, I will stretch the 2 seconds so that I hang back a bit and then increase speed to minimise the time I'm next to them - I do this particularly when passing large foreign/left-hand drive lorries (look out for the foreign plates and the downward looking mirror on the off-side cab door).

Do you ever feel disadvantaged by maintaining a 2 sec gap on busy motorways and the like? Planning a move out into L3 to pass slower traffic in L1 & L2 - a lorry with 2 or 3 or more cars behind it - you maintain a 2 sec gap and the cars behind the lorry take the invite to pull out into L3 in the gap you've left - one after the other - so you almost end up going slower and slower to accommodate. Meanwhile, vehicles behind you are bunching and in the now emptying L2 I've even had cars pass me on the left and pull into my 2 sec gap. Have you ever experienced this? I'm not saying it happens every day but I do a lot of motorway commuting... How do you deal with or avoid this situation?

[Ancient]

Do you ever feel disadvantaged by maintaining a 2 sec gap on busy motorways and the like? Planning a move out into L3 to pass slower traffic in L1 & L2 - a lorry with 2 or 3 or more cars behind it - you maintain a 2 sec gap and the cars behind the lorry take the invite to pull out into L3 in the gap you've left - one after the other - so you almost end up going slower and slower to accommodate. Meanwhile, vehicles behind you are bunching and in the now emptying L2 I've even had cars pass me on the left and pull into my 2 sec gap. Have you ever experienced this? I'm not saying it happens every day but I do a lot of motorway commuting... How do you deal with or avoid this situation?

Yes. <shrugs> I let them do it, stop worrying (never in a hurry on four wheels ), move up towards the rear of the lorry when they've stopped dodging in front of me (to discourage a car behind deciding to move left and follow them), wait for the 'tunnel' aheaad to clear, accelerate through past the lorry. Usually enough have moved back to L2 to proceed at cruising speed.