Safe Speed Forums

PeterE certainly seems to suggest it's a way of dealing with idiots who don't keep left on motorways, why, be an even bigger idiot!

Why else would he ask "So you don't agree with the principle of "keep to the left unless overtaking" after I point out tailgating is a method people use to communicate to others that they want them to move out of their way?

(BTW, I don't think it needs saying that I think people should keep to the left when driving).

It appears I owe you an apology Weepej: I gave you too much credit.

I thought you had answered/agreed with my earlier 4-option question. Instead it appears you, yet again, evaded giving an answer and instead responded with an irrelevant question (as JTB rightly pointed out). Is there any reason why you never give a straight answer?

Let's try again:

Do you agree that the two options you showed (both the 0.5secs) are the risky ones: Yes or no?

If your answer was yes, then Does the risk follow the speed, or the gap?

If "both" then How can you possibly say that you didn't show the "A motorway driver doing 80mph and leaving a 3 second gap " as risky?

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

Clearly leaving a bigger gap is less risky.

Now how about answering mine?

Which of these is more risky and why?

- A motorway driver doing 80mph and leaving a 0.5 second gap
- A motorway driver doing 40mph and leaving a 0.5 second gap

I've said before I'd have no problem with motorway limits being raised to 80 where it's suitable, as long as it was subject to a pre and post implmentation assessment, and that the original punishment structure remains in place (i.e. getting caught doing 105 on a 80 limit motorway would have the same effect as getting caught doing 105 on a 70mph motorway).



Because if you say that hight speeds are more risky somebody comes along and says "but motorways our are fastest roads as well as our safest".

Because it's not relevant to the point: tailgating (both your examples show it, thus making your question redundant).
Do you want to answer my other pertinent questions that preceded yours, that you have been repeately evading?


To do for you which you've still not yet done for me (even though this is utterly irrelevant):
The driver tailgating doing 80 is at greater risk of injury when striking a stationary object.
The driver tailgating doing 40 is at greater risk of fartigue and not reacting to events ahead (non-free-flow speeds); in free-flowing speeds that driver has a greater speed differential tending towards unreasonable and unpredictable.

Yet again: "Which effect wins"? There is no clear answer.

They're both very risky compared to the other 2 options (with the 3 second gap).



So are you, at last, agreeing that the risk follows the gap and not the speed? (or at least the effect of the gap dominates over the effect of the 'speed')

Yes? No?

Are you beginning to understand the difference between 'travelling too fast for the conditions' and 'travelling too close for the conditions'?

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

Rightly so because it leads to the following:
"Clearly motorways are the fastest roads. They're also the safest even though they are the fastest.
Does this not show that other factors besides 'speed' are at play, and that these other factors are dominant?"

Thus far it seems to have gone totally unchallenged!

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

That would make 99.9% of your posts "speed related" then.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Playing devil's advocate, there is really no way anyone can force someone ahead of them to go any faster, but, OTOH, it's all too easy for a slow driver to force everyone behind them down to their speed. All they have to do is just sit there. And they do - all the time. And then complain about the occasional tailgater

Pot, Kettle, Black!

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

It's like we're reading a different thread! I can't see any way that PeterE suggests that.

He does suggest the tailgating is a reaction to bad lane discipline but that's not the same as saying "it's a way of dealing with idiots who don't keep left on motorways". Tailgating is ultimately futile as (unlike poor lane discipline) it can't directly alter the behaviour of other drivers.

In fact IME it's counterproductive. Drivers with poor lane discipline - those that are aware of people behind them - seem to be less likely to pull over if they are being intimidated. I find the best method to be to sit 2 seconds back with a look of saintly patience on my face.

I would disagree Steve, since the vehicles involved must at some point have been travelling at the same speed prior to an impact event. Both are almost identically dangerous, the impact differential speed difference will be practically negligable. However, the cars at 70 mph are much closer together than those at 40mph, so the impact speed differential would actually be lower, so in fact the accident at 70 would be less severe than the 40mph accident.

[edited - oops]

My argument would be that as the time separation is the same, then there is no/negligible difference in risk*.

However, at 80mph the gap would be twice as big. So as you say "Clearly leaving a bigger gap is less risky" you clearly it's clearly less risky at 80.

You surprised me there, weeps.




(* - can any engineers answer me a question? Do brakes generate equal force at different speeds? What I can remember of my MEng makes me think that brakes would exert a greater force at higher speeds, and consequently a gap of equal time at a higher speed would allow for more braking to be applied. But ! could be wrong ).

How do you figure that? I have it the other way around.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

But what they're saying is true! The reason it's true is, however, what your objecting to, I think? Clearly, a great deal of their "safeness" is to do with junction design, carriageway separation etc - all points you've already raised.

BUT, doesn't that only serve to illustrate that speed is actually only one, relatively small, factor in all this?

Of course, you can mitigate the effects of ANY accident by reducing speed. In fact, you can avoid any accident, ever, if you reduce the speed to zero, but that's my big problem with your particular argument. A motorway with a zero (or even 30) limit would be no use as a motorway. It would be like taking all the blades off kitchen knives - sure, you'd reduce kitchen knife accidents, but they wouldn't be much use as kitchen knives! Given that we need them (and motorways) we just have to work round the problem from other angles. So if you really believe that 80MPH is a reasonable motorway speed limit, you need to forget any arguments like "but they'd be safer still at 30". Yes, everyone knows that, but they wouldn't be "motorways" then!

More geek...

Back to the half second gap thing

Given the car in front manages to a braking rate of 20mph/second instantly and the following car does not brake at all.

From 80mph it will take 2 seconds for the impact to occur so the speed differential will be 40mph

From 40mph it will take 1.414 seconds for the impact to occur so the speed differential will be about 28mph

This may not appear correct, however do not forget that the half second gap is reduced not at the travelling speed but at the differential speed.

The differential speed increases with time at the same rate in both cases, i.e by the rate of deceleration.

The gap at 80mph is twice the size so it is hardly surprising that it take longer to close it.



The maths-

S = u.t + 1/2.a.t.t

Where S is distance travelled, u is initial speed, a ia acceleration and t is time.

If the gap is G seconds then the lead car is u.G ahead so S = u.t + u.G + 1/2.a.t.t

The impact occurs when, ignoring the length of the cars, when both cars are the same distance from the point the rear car occupied when braking began.

So if the rear car does not brake

u.t = u.t + u.G + 1/2.a.t.t

cancelling the common terms

0 = u.G + 1/2.a.t.t

rearranging

t = squareroot(2.u.G/a)

Since a is negative we do not have to use imaginary numbers.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Because I didn't read through what I'd posted, I worded it wrong, but a brilliant explanation is in the geek explanation above from toltec

Other little gems that fall out of the above maths.

Providing you can start braking at least as hard as the car in front and in no more than your gap time you will not hit the car in front.

Think of it this way, the lead car starts braking at a distance of u.t in front of you and will travel the distance as calculated above, 1/2.u.t, before it stops, so providing you can brake to a stop in the same distance and start doing so before you reach the position on the road where the lead car started braking you will stop before you hit it.

If you are going to follow a car at a gap lower than your reaction time then you are better being really close to it as this will minimise the differential impact speed. NOT what you should do on the road, however consider racing and how close they are in the BTCC and how seldom a minor nudge causes the cars to crash.

If the lead car does not brake to a stop but simply drops to a lower speed then the rear car can start braking slightly later, i.e the gap can be slightly smaller than the drivers reaction* time and there would still not be an impact.

*By reaction time I mean the amount of time it takes to start braking not the usual how long it takes to react after noticing and thinking about what is happening ahead.


So what comes out is that theoretically if you have equivalent or better brakes than the car in front you can drive at a gap larger or equal to your reaction* time and not hit anything. I know there is much more to this so please do not think this is how I am suggesting we drive, it is just a tool for thinking about what is actually happening. Unfortunately I really have to head off to work now so I will try to catch up later.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

I agree with the maths, but it all pre-supposes that the car in front is only going to stop as a result of it's braking effort and not as a result of hitting the back of another vehicle that forms the back of an existing pile-up! It certainly works most of the time for truckers though. You see them happily trucking along at 56 with a gap betwen them too small to even park a mini in!

Just the kind of thing I meant.

What I was trying to show was that speed is not the issue, it is the way drivers use space that causes the problems. In your example it is about being able stop in the distance you expect to remain clear. This can include the space in front of the vehicle in front of you. If you cannot see past the vehicle then you need a much larger gap.

Before weepej chimes in, this also includes the possibility of something slower moving into the gap or moving from a concealed location into the gap.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Ah yes, I remember now I was trying to recall what Eric Idle said in The Meaning Of Life.

That’s what we’ve been trying to tell weepej but he’s being a 'very naughty boy'. He still thinks speed kills. How I lived to tell the tale of my 140mph jaunt’s on Germany's Autobahn’s I'll never know. Luck I guess...

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

I think increased speed (given the same environment and level of driving skills) increases your chances of being killed/seriously injured or killing/seriously injuring somebody else.