Safe Speed Forums

Some time ago I remember reading a report about a survey carried out in America (I think) which was all about speed and accidents.

I seem to remember that they concluded that the safest speed to travel at was the same speed as everybody else. i.e. if everybody was doing 35mph then that was the safest speed.

Presumably because traffic was evenly spaced out and no bunching and braking to adjust speed and it cut down on tailgating.

Does this ring any bells with anybody else?

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Shooting is good for you and too good for some people.

I don't disagree.

Swot up on "The 85th percentile":

http://www.safespeed.org.uk/speedlimits.html

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

That's not really relevant is it. The 85th percentile speed is the speed at or below which 85% of drivers choose to drive. It says nothing about the modal speed or any other statistical measure. Further, it would be wrong and dangerous to suggest that the 85th percentile speed is the safest speed at which to travel because that could result in drivers who are not competent to drive at that speed choosing to do so because they (wrongly) think it is safer.

On a single lane road, it is impossible (without overtaking, which produces an additional risk) to drive faster than the car in front without crashing. It is possible to drive more slowly but that carries an additional risk of being tailgated. Both risks can be avoided by driving at the same speed as the car in front so (excluding other factors) that could be said to be the safest speed.

On a multi-lane carriageway, I have found that there is more bunching and tailgating if all traffic is forced to maintain the same speed. It may be safer to have all traffic moving at the same speed if no drivers have the desire to travel at a different speed. As that will never be the case, I would suggest that a reasonable (not excessive) speed differential between lanes is safer than no differential because: (a) it tends to stretch out the gaps between vehicles; and (b) it provides mental stimulation.

You’re right, that wasn’t really that relevant was it; I really didn’t give my answer the thought it deserved

On reflection, I would say there is no easy answer to Brookwood’s question because drivers have differing abilities, but I think ‘going with the flow’ is likely to be the safest option for the majority of drivers (some thinking they need to faster than others is akin to competition and easily leads to frustration and undesirable manoeuvres) – so long as one’s mind isn’t prone to wandering in such un-stimulating situations. However, speed isn’t the most important factor when considering road safety

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Clearly complete safety is to have no speed at all. So lets ground all aircraft, order all shipping into port and trains to their depots, and bar all vehicles from the roads.

Issue everyone with a horse and cart and go back to 1830. No deaths due to accidents and the economy at a complete standstill.

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Good manners maketh a good motorist

Who says that in 1830 there were no deaths due to accidents with a horse and cart?

Absolutely. Horses are really dangerous. There are some figures around somewhere, and if I remember correctly, deaths in London alone from horse accidents were over 2,000 a year in about 1860. And that's before you factor in the health consequences of manure...

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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

But it's a lot better for your rhubarb than old sump oil!

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"Politicians are the same the world over... We build bridges where there aren't any rivers." - Nikita Kruschev

You only have to get behind a jam sandwich doing 69mph on the motorway to see how risky same-speed driving can be.

We've all been there - trafpol going just on the limit, cars bunched behind as far as the eye can see, some cursing under their breath, the rest pretending to be good, eyes riveted to speedo.

Then some brave soul will make a dash for it (rather, a crawl) and get past, followed by a trickle of others. Now they're clear ahead and well spaced, driving in generally safer conditions.

Sorry, they can't be - they're breaking the speed limit.

It would appear that nobody else has seen the report I am thinking off so I will try some research and get back to you. In the meantime if anbody else finds something I am sure we would all be interested.

I have seen some huge bunches on motorways caused by police car doing 65 in the inside lane but that is all about driver education.

I once got pulled by the police on a motorway for taking too long to overtake. When I said I didn't want to exceed the speed limit because I had a police car right behind me the he explained it was safer to complete the manoeuvre quickly. You can forget that when they start putting cameras on motorways.

Anyway I thought it was agreed elsewhere on this forum that everybody travelling at the same speed was actually quicker because of the 'wave effect'.

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Shooting is good for you and too good for some people.

You are right, but I don’t think that’s the same situation.
I think there is a great difference between wanting to ‘go with the flow’ and forcing everyone to slow down to the same speed.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

True. On second thoughts, we have two different scenarios; one where there's bunching due to sheer traffic volume, the other bunching caused by a clearly identifiable moving obstacle (the police car). The latter is more frustrating because you can see what's causing it but dare not do anything about it.

In those conditions, same-speed is worrying, especially when you know many others will have their eyes on the speedo instead of the road ahead.

(EDIT: Smeggy, you beat me to it!)

In contrast there's a stretch of A1 between Cambridge and Peterborough, four very wide lanes, dead straight, and rarely running to capacity or even close to it. Quite often you'll find the entire block of traffic travelling at 85-90mph but if feels like the safest place on earth. With so much space around, no-one is tailgating or jockeying for position. Everything just glides along smooth as silk. It's how long-distance driving should be. I don't know what the accident rate is on this stretch but there's a noticeable lack of those tell-tale rubber scorch marks showing near-misses (or hits) of the past.

On wave theory, I wonder if there is more to it than the author (can't find the original paper just now) made out. He drew some natty animations showing how the wave effect causes traffic to slow or stop in a bottleneck. Something bothered me - the net throughput of traffic was the same. In reality it probably isn't. I guess it's because the cars at the front do not clear as quickly as his animation showed.

I’m beginning to think I’ve misunderstood your original point; did you mean everyone at the same speed for all lanes? (just like America)


Not everyone need travel at the same speed to remain safe, that’s why we have the keep left rule for multilane roads, if only drivers abided by it Tailgating is caused by driver attitude only. The wave effect is caused drivers who execute poor lane manoeuvres and/or under-accelerate and/or over-brake (usually the tailgaters). Look far enough ahead when on a motorway during peak times and you can sometimes see the cause of the ‘domino effect’ the resulting propagation/amplification.

Examine the German example; thanks to their driving standards and culture, it’s perfectly possible for them to travel safely at hugely differing speeds!

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

What I think the orignal report was about and the thing I agree with, is that if you are in a line of traffic travelling at 35mph in a 30 limit it is safer to go with the flow rather than reduce to below the speed limit and cause bunching and tailgating.

I also think the same applies on motorways, better to maintain the 80mph with everybody else and if you don't want to travel that fast then get out of the way.

I certainly don't like the idea of everybody in every lane doing the same speed unless you have some very low blanket speed limit of 55mph for example.

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Shooting is good for you and too good for some people.

As I see it the wave effect is caused by reactions being slower than gaps.

Suppose you're one second behind the car in front when it slows fractionally. If you wait (say) 1.5 seconds before you slow gradually, you'll find you have to slow a little less gradually to maintain your (inadequate) gap.

In this way a gap that is smaller than the response time of the following driver functions as a 'braking amplifier'. After 20 or 50 such undersized gaps we've suddenly got 70, stop, 70, stop.

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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

This is a classic and inherent symptom of lazy drivers who don’t look beyond the rear bumper of the vehicle in front, who then must compensate by over-braking when an unexpected event occurs. These drivers then realise the fact they had to overcompensate so next time they’ll leave a much bigger gap when building up speed, hence under-accelerating, thereby amplifying the wave.

I honestly believe these domino effects would cease if everyone applied at least some level of C.O.A.S.T. some decent motorway instruction would be a good start!

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

From the JJ Leeming page on the ABD website.



Deaths per million population.

Exactly Paul. That is why congestion would reduce if all drivers left sensible gaps. The only other point I'd add (which I think youl allude to) is that it is driver response times that are important, which may be much longer than the average physical reaction time. The 2 seconds minimum gap allows a decent 'comfort' margin, on top of the physical reaction time, which will soak up a brief lapse of concentration without adverse effect.

Yeah - but which driver?