Safe Speed Forums

Hi Keni, I'm a mechanical engineer too but I'm afraid I can't understand your "uphill" bit! Not sure what calculations you want me to do or, indeed, where you get the idea that I think 1 second is safe!

Got the chance to try your suggestion this morning on the way to Carlisle up the A595 - NSL single carriageway. It all went pretty much exactly as I predicted (usually does!) I wasn't in any great hurry and the weather wasn't that good. After a reasonable spell of uninterrupted driving, I came up behind an artic. I left your suggested gap (and then some!) and after a short while, there was a car between me and the lorry. A bit later, there were 2 cars between me and the lorry. Shortly after that, there were three...

...then 4...

I'm not entirely sure I'd agree that some of the motorists in this gap were "more competent than me" but one of them was certainly a lot (ahem!) "braver"! Eventually, we got to a nice straight bit that offers safe overtaking (that's one of the places they usually hide the scameras!) and about three of them got past the truck - one of whom would probably have been me, had I decided to drive closer to the truck in the first place.

Now I completely agree with you that I had a reasonably pleasant, safe, stress-free journey but I can't agree that I got there just as quick. It might work on multiple-lane roads but not on single carriageways. It's certianly true, however, to say that over the next 15 miles, I probably only lost 2-5 minutes compared to some of the more "enthusiastic" overtakers (depending on how fast they went after they passed the truck).

People often say this but the more I think about it, the less I agree with it.

Not 100 yards from my front door, there are a couple of "S" bends on a single track NSL road. Only wide enough for 1 car and high hedges on each side. If I'm driving round those bends in the dry and a stationary object (usually a sheep) comes into my field of view, I can safely stop if I'm doing less than 40 before I hit it. If, however, what comes into my field of view is another car, also driving such that he can stop in the distance he can see to be clear, the result will be carnage!

It started me thinking that the saying should be changed to "...be able to stop in HALF the distance you can see to be clear" - but that only works if the guy coming the other way abides by it too!

Last year, on just such a road, I got hit quite hard by some muppet doing rather more than 40. I had pulled right into the hedge and come to a complete stop before he hit me but there was no way I could have reversed!

I know this is drifting off-topic but I'm firmly convinced that if the proposals to limit rural roads to 40 go through, we will see MORE of this sort of accident as people stop thinking for themselves and start driving at 40 because "the government" has decided that it's safer.

I think the most general form is that you should be able to stop using road that will be clear when you need it. Or in other words, stop without hitting anything. But somehow that doesn't have the same impact* as the original.


* sorry

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Peter Humphries (and a green V8S)

This is properly discussed and explained on the following page: http://www.safespeed.org.uk/background.html

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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

So which is the most important, safety or maximising the capacity of the road? I would go for safety every time!

The problem with road capacity is entirely due to the failure of our roadbuilding programme to keep pace with the increase in traffic over many years. Are we now being asked to compromise our safety to let the politicians off the hook?

If that long gap is necesssry to ensure safety, then fair enough. Personally I don't think it actually is necessary in typical conditions, and I think we would be incurring a considerable loss of road capacity (and hence convenience) for a negligeable gain in safety. The goal is not safety regardless of cost, it is the best compromise between safety and convenience. If road safety was all that mattered, we'd just push every vehicle in the country off a cliff and reduce vehicle accident rates to zero at a stroke.

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Peter Humphries (and a green V8S)

Of course safety comes first. But increasing and increasing the inter-vehicle gap doesn't increase and increase safety. A 2 second gap in good conditions is perfectly safe and contributes ZERO to system risk. To get a risk you have to have an observation or planning failure.

It's the same will all 'safe speed for the conditions' decisions The aim is to get the speed to the point where you can ALWAYS GUARANTEE to stop comfortably within the distance that you know to be clear. While you're observing this, the system risk contribution of speed is ZERO. It simply isn't true in any general sense that slower is safer.

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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

Until something unexpected occurs, then the distance you know to be clear reduces at an exponential rate, more than likely at a rate > normal reaction time.

[quote="SafeSpeed
Of course safety comes first. But increasing and increasing the inter-vehicle gap doesn't increase and increase safety. A 2 second gap in good conditions is perfectly safe and contributes ZERO to system risk. To get a risk you have to have an observation or planning failure.

It's the same will all 'safe speed for the conditions' decisions The aim is to get the speed to the point where you can ALWAYS GUARANTEE to stop comfortably within the distance that you know to be clear. While you're observing this, the system risk contribution of speed is ZERO. It simply isn't true in any general sense that slower is safer.[/quote]

I think that up to a point increasing the gap DOES improve safety, but a point comes where there is little or any gain I agree. I'm sure we agree that tailgating is dangerous, and that a 2 second gap is a lot safer.

But I also find that a larger gap gives me the time to observe more widely, instead of having to concentrate on the brake lights ahead. This means I can anticipate further ahead, and I believe this also adds to my safety factor and the smoothness of my driving.

Where speed comes into the equation is that your time to stop is a function of speed (I agree that for a competent driver, safety is not speed dependent within reason). Shedding 20mph per second is a good rule of thumb for maximum braking in a well maintained modern vehicle on a dry road. This means with a 2 second gap (ignoring thinking time) you will have around 4 seconds to brake to a halt. This is just about possible at 80mph, but at higher speeds 2 seconds is insufficient, and you will not be able to stop in the distance you know to be clear. In most conditions it is true that you WILL be able to stop in the distance that you EXPECT to be clear when you get there, but that isn't quite the same thing, and must be slightly more risky than KNOWING that your stopping space is clear.


Perhaps the most worrying thing though is that the 2 second gap is based on everything being optimum. Maximum braking effort, dry road, alert driver, well maintained brakes and tyres. If any one of these factors is less than optimum, you could be in trouble, and it might be sensible to leave a bit more margin. For example, if I have to stop I would prefer to be able to do it under moderate rather than maximum braking effort.

2 seconds should therefore be regarded as an absolute minimum, rather than an ideal IMHO.

If 'something unexpected' happens we have had an entirely separate safety failure. Just the sort of failure that we should be addressing with policy, as it happens.

And of course, the 'something unexpected' may render the speed inappropriate, but we're still in the original position of appropriate speed contributing zero to system risk.

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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

That would be nice. However, I’m willing to bet money that the traffic flow rate (at 70mph, although the speed is irrelevant) on the M3 during peak times, will inherently result with the average gap between vehicles is less than 2 seconds. Try to imagine peak time traffic with 63m (2 seconds) between vehicles in all lanes; we’re so far from that it’s unreal. So what to do? Anything we try to do is already a compromise.

Before anyone says it: the increased gap required for unfavourable conditions can be achieved with slower traffic – that’s not unreasonable.

I agree with the bulk of your argument, however, I DO drive at about 2 seconds and I have NEVER had a problem in hundreds of thousands of miles of motorway driving. 2 seconds (in good conditions) seems to me to be an excellent buffer zone.

In bad conditions where braking performance differentials tend to be greater, then 4 seconds is much better.

With hazards ahead - maybe a sea of brake lights - I'll slow very early and might end up with eight or ten seconds ready to pass it back to those behind who might be unaware of the problems ahead.

So I don't really know where that leaves us. In practice and in theory I find 2 seconds to be just fine. Are there other folk around here who have had problems with a 2 second gap? What problems?

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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

[quote="SafeSpeed"

So I don't really know where that leaves us. In practice and in theory I find 2 seconds to be just fine. Are there other folk around here who have had problems with a 2 second gap? What problems?[/quote]

To be honest I have never had a problem with a 2 second gap myself. Perhaps my nervousness about this type of situation springs from the fact that the only two accidents I was ever had involved being hit from behind by somone who failed to stop when I did. So if I have to brake to avoid someone ahead I prefer to use less than maximum effort to give the driver behind more time to react.

The two second gap is a problem when a tailgater is following you and someone overtakes you and fills the gap, which usually occurs in heavy traffic. You're then in an immediate danger situation being boxed in with poor visibility forward to spot any hazards and someone up your chuff who can't be trusted to react.

Yes, that's absolutely true. If you can't dump a tailgater immediately, the best thing you can do is grab an extra 2 seconds ahead to pass back if required.

That advice has long been displayed on the Safe Speed tailgating page: http://www.safespeed.org.uk/tailgate.html

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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

It doesn't work on multiple lane roads either.

So you lost 5 minutes in 15 miles. Now try that as a field engineer with 150 miles to get to your customer, suddenly that has become 50 minutes, assuming an even spread of missed opportunities and random small delays caused by people cutting in.

Those 50 minutes mean you will be leaving your job 50 minutes later than you would have done, which generally puts you into worse traffic than if you'd left earlier, so lets assume a 20 minute delay due to having to use the M6 near Birmingham at 3:30 instead of 2:40. That brings us up to 70 minutes.

Lets drive home to the same standard that we drove there and lets be generous and ignore the fact that increased traffic densitity means that more people will be cutting in front of you, and just tack on another 50 minutes to the journey.

That's an extra 2 hours, unpaid, on your working day, every day. No wonder the reps and van drivers get so pissed off by it and will take risks for what are apparently minor gains, such as leaving a smaller gap to maintain their place in the queue.

I think anyone who uses the "only costs you a couple of minutes" argument should be made to do a job that involves driving from, say, Milton Keynes to Bradford and back every day.

This is precisely the reason why I no longer support the SafeSpeed rhetoric.

Can you clarify? Are you sayiing you no longer support SafeSpeed because you disagree with Lum?

So do I. That post is a load of twaddle - and I'm sure you'll find Paul thinks so too.

'Fraid so. Sorry Lum.

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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

Not Lum in particular, but that attitude in general is what I see of most SS advocates. "I'm inconvenienced and I can't see any reason for the speed limit, or these lights, or whatever.." so I'll do what I like. Yes, there's some genuine people, don't get me wrong, but I bet the majority of 'external' SS supporters are petrol heads who are revolting against the restrictions that DfT is rolling out. They probably couldn't care less about people being killed, as long as it doesn't result in a lower speed limit / enforcement.