Safe Speed Forums

Add in one second because you happened to be checking your mirror when it all went wrong and we're back at 2.1 seconds.

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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

I believe motorway mirror checks are highly over-rated (my discussion with vonhosen reinforced this); changing lanes is an obvious exception. I can see if the following vehicle is tailgating without taking my focus off the road ahead, the strobe of emergency vehicles and the lights of overtaking bikers are obvious anyway. Again, perhaps I’m judging by my own standards?

The only time I do such comprehensive mirror checks are when I’m doing a big speed (and you can bet I’ve left a big gap) and I’m looking out for plod

I'm sure this would apply in one set of circumstances but the number of variables is almost infinite, tyre condition, age/state of other drivers, etc etc, Also no "fat" at all, i.e. no safety/error margins. Ken

Yes, you are right, which is why I stated it as the minimum. It goes without saying that the following driver is not half asleep, know with confidence that their car is roadworthy and conditions are good. The age/state of other drivers don’t come into it. This exercise proves you can leave a gap of 1.1 seconds, accounting for the driver in front spotting something you don’t, but only if you’re alert and undistracted (excluding the pointless regular mirror check, but that’s for another thread).
Without doubt you would be leaving your destiny in the hands of others if you leave a gap of less than 1 second.

Steering back to the original point: Don’t assume that drivers leaving gaps of 1.1 seconds as territorial tailgaters (especially now that I’ve shown justification for it). It’s a shame that most leave less, some far less



With this I’m in agreement (except of course for drivers of german cars ).
Perhaps this is something else Paul could broadcast to the general public?

Curious. I'd like to read the discussion with VH if you can easily find a link.

If I'm driving 'in the traffic' I reckon about 10-15% of my total observation on a motorway is in the mirror. If I'm 'surfing the gaps' it's a bit less.

One of the big considerations is that I want to be well placed to escape any problems, and many of the problems come from behind (I slow, he doesn't for example).

I also find that higher concentration levels tend to be associated with more mirror use. If concentration drops a little, I notice it in the miror use first.

So your view is pretty strange to me...

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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

sorry smeggy, you really haven't shown justification for a 1.1 second gap (or 115 feet at 70mph.) There is no need to drive at this distance from the vehicle in front, it does nothing for you at all, and as said, it gives no margins at all. It does smack of "territoryism" though since at that distance there is more chance that the driver waiting to enter lane 3 will wait until you've passed. The trouble is the person who believes that the indicator stalk is a magic wand will enter it anyway and you'd be in a better position to take action if you had a proper gap. Thanks for your views though. Ken

Let’s be honest, 99.9999% of all motorway problems start in front of the drivers involved (and the side, but no amount of gap will save you from those), so it seems excessive that one should spend 10% or so concentration on the 0.0001% of the problem. (changing lanes being an exception).


If I find I have to slow then I would be more worried about what’s happening in front of me, then plan my potential escape routes given the real-time, dynamic situation unfolding in front. I am sure that almost all well meaning drivers placed under stresses will dismiss the situation behind them and simply act upon instinct based upon what they see head.

What could you do different if you look in the mirror to see you were going to be shunted? Would your resulting escape route (to escape the shunt) actually have been the following driver’s planned escape route? No amount of planning will save you from the actions of a following numpty.

Twice I have been involved in a shunt, one was on a motorway (I was passenger in the shunted vehicle for both). In bother instances there was no escape route to take.

Eh? Most motorway crashes are shunts. So that's a problem in front for one, but a problem behind for the other. 99.9999%? I don't see how it can be anything other than 50/50 on average.

But I don't rate myself as average in this. If there's something going on in front, I expect to deal with it in very good time. So for me, it's got to be ~90% of danger coming from behind.

Still curious...

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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

In which case I think you have misunderstood me.
I have already agreed there’s nothing to gain by leaving a 1 second gap (well actually there is, but nothing in terms of safety), so be inference I also agree there is no need for it. I have also agreed there is no margin for error, I stated it as a minimum. However, I have justified that it can be safe to do so - in a specific set of known circumstances.


That seems like a good thing (in terms of safety) given those circumstances.

Actually, you would be correct had I said I ‘keep to the minimum safe gap’ (whatever that may be), but I made no such inference; so I disagree that what I’ve said could be deemed to be “territoryism”

I have come to accept this: if someone really wants the gap in front of me they will take it, regardless of whether the gap is 1.1 seconds or 2 seconds; that’s far worse than any “territoryism”.

I was careful to word that post carefully; I said “start” as it is extremely rare for a person to emergency brake for what is, in actuality, no other reason (except a mechanical fault, but for that I would be more worried about why I feel the need to perform an emergency brake than the drivers behind). Hence the rest of my post becomes relevant; you can’t plan for the actions of a following numpty.

If anything, an event coming at me from another carriageway will be amplified most by myself being distracted for that one second of checking the rear mirror (just like that old seat-belt advert). This is likely in the run up to a shunt because drivers will take their instinctive escape routes. I would say drivers would be better off not doing anything unexpected.

On single lane roads I only use the mirrors enough to keep an eye out for cars behind that might be tempted to overtake me. At night time on single lane roads I hardly look directly into the mirrors at all most of the time - peripheral vision is enough to show whether there is anyone approaching from behind.

On multiple lane roads it's a very different kettle of fish - I use the mirrors extensively to keep track of relative speeds and distances of vehicles behind me so that I know what opportunities are coming up to pass slower traffic in my lane. Very little of my mirror use is to guard for people in danger of rear-ending me, that only gets attention during and after a sudden stop or when stationary at a juction with moving traffic behind me.

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Peter Humphries (and a green V8S)

Try applying these ideas on the two lane, dual carriageway A14, the east west trunk road that carries the heavy goods traffic between the east coast ports and the midlands past Cambridge. If you leave a gap much more than the length of an articulated lorry then the next thing you know is that one lorry will pull out to pass another. You are then looking at the back end of the two lorries for the next ten miles as one struggles to pass the other. On a wet, dark evening it can get particularly interesting.

It is little wonder that this road has such an appalling accident record.

If ever there was a case for restricting HGVs to the nearside lane, it should be on this stretch of road, then drivers might be willing to leave more space in front of them.

I love how Keni says "try it" like no-one who currently follows too close has ever tried leaving a bigger gap.

I am a field engineer. I do 40K-60K miles a year, mostly motorway, and it's given me plenty of time to experiment.

A lot of it is dependent on the time of day and the make up of the traffic, perhaps the most extreme example can illustrate the norm though.

I'm doing 70-ish in L3 which is currently at capacity (by which I mean that adding any more cars would cause it to slow down), L2 consists mainly of middle lane morons doing 60. L1 is mostly empty apart from the odd HGV every now and then.

Now I could just sit in L1 and undertake all the MLMs, it's not illegal, but you can get a pull for it and you can have the MLM drive straight into the side of you when their exit comes, so I dont. I'm in L3. I'm leaving a safe stopping distance...

...so the car behind me pulls into L1, undertakes me and the MLM, then pulls in front of me. I slow down to make the gap bigger...

...so the car behind me pulls into L1, undertakes me and the MLM, then pulls in front of me. I slow down to make the gap bigger...

...so the car behind me pulls into L1, undertakes me and the MLM, then pulls in front of me. I slow down to make the gap bigger...

(Repeat a few more times)

...so the car behind me pulls into L1, I get pissed off beecause I'm supposed to be overtaking this MLM and all these wankers keep pulling in. I close up the gap and finally overtake the MLM.

There is an incorrect assumption here, and that is that the vehicle in front will not stop instantly. Thats what I used to think too, until last year when a vehicle did just that. Luckily my 5 second gap saved me.

The reason for the sudden stop was hitting something solid, in this case the wreckage of a dozen or so HGVs. Because of bad spray I could not see beyond the lorry ahead. But perhaps a bridge pillar or a vehicle crossing the central reservation (or a head on with an overtaking vehicle on a single carriageway) would do just as well.

You can't assume that the vehicle ahead cannot stop in less than its normal braking distance, so it is much wiser to give yourself enough space to brake to a halt even if the vehicle ahead stops dead. At motorway speeds 5 seconds is probably about right.
You lose nothing by doing this, improve your safety and that of the vehicles following you, and can usually avoid having to brake when the vehicle ahead slows down.

No no no! See this earlier post in the thread: http://www.safespeed.org.uk/forum/viewt ... 6132#96132

The capacity of the road is highly dependent of vehicle spacing - think about it... at 2 seconds, that's 1,800 vehicles per hour per lane. At 5 seconds that's 720 vehicles per hour per lane.

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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

Purely out of interest, and not inferring that they may be more right than the rest of us, does anybody know what time interval is used for the likes of Mercedes and Jaguar adaptive cruise control?

Your logic is correct, but I don't believe there was any such assumption:

Not necessarily, it is just possible for a following driver to come to a complete stop (factoring braking and thinking time) in the distance created by the 2 second gap at 70mph, even if the following driver isn’t looking beyond the brake lights of the car in front (assuming good dry conditions, average roadworthy car – and no mirror checking at that instant).


If you are saying there is a trade-off between drivers leaving such large gaps and carriageway utilisation, then I would have to agree; however, mrtd did infer that the roads were wet (spray). A good compromise would be to get some drivers to use lane 1



EDIT:
This is my 1000th post
I was told I get my donations back when I reach 1000 posts, when can I expect payment Paul?

Driving at high speed on a wet road with visibility so poor that I couldn't see past the vehicle in front, I'd want a decent safe gap and five seconds doesn't strike me as excessive. But I think it would be bigger than necessary where there was a dry road and good visibility.

Somebody commented about being able to stop in a two second gap. By my estimate it's possible at 80 mph assuming the driver manages to maintain around 1G braking and has negligeable reaction time. If you have ABS then you shouldn't have any difficulty maintaining 1G in good conditions. Without ABS it's a pretty tall order. As others have said, you don't need to be able to stop completely in the distance that is currently clear, but I find it's interesting that the braking part of the stopping distance is potentially so small. Of course the reaction part of the stopping distance can be arbitrarily small (or even negative) too.

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Peter Humphries (and a green V8S)

I was wondering when someone was going to mention that!

Seems perfectly reasonable to me. 0.8g or even 1g braking could easily turn into 20g braking for the last few feet (depending on how long your bonnet is (was))!