Safe Speed Forums

Split from: http://www.safespeed.org.uk/forum/viewt ... 3746#73746



Don't you guys find you tend to have less pressure / more time at higher speeds? I tend not to be going faster unless the road is clearer.

To put that another way, shouldn't the margin of error be greater in faster places?

See these graphs:





And see http://www.safespeed.org.uk/braking.html

Those graphs tell me that I need 2.5 seconds of certain knowledge at 70mph but I want to ensure ~4.3 seconds continuously at such a speed. Surely at higher speed we MUST have more time? (Or else we're doing it wrong?)

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I've had this idea floating around for some time. But suddenly it's come clear in my mind.

If we are driving with an appropriate attitude and sense of responsibility, faster speeds actually deliver more time to react don't they?

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

To some extent my post was intended to be a humourous way of suggesting that I don't need the gadget to nanny me - if I don't have the available time, then I don't look at it, if I do then I shall. As for faster speed delivering more time to react; I think you may have correlation and causation a bit about face - better conditions deliver the opportunity to travel faster without dangerously compromising the space and time needed to react to any foreseeable situation.


ETA

Hmmm..... I have now reread your post and considered the graphs and see where you are coming from - that seems so counter-intuitive that I'm going to have to mull it over for a bit.

or perhaps?

If we are driving with an appropriate attitude and sense of responsibility, we have more time to react at any given speed than someone who isn't driving with the same attitude.

Paul, you might want to replot those graphs on the same time axis, at the moment the slope appears to be very similar, so perhaps the point is lost.

I think I get Paul's point, and why his derivation is the right way round.

As speed increases, the time differential between "normal" braking and "emergency" braking increases too. Thus if we are selecting our speed based on our "normal" braking response then the faster we are travelling at any given time the longer we have to react before we run out of "emergency" braking.

For example, when we are driving somewhere where it is only safe to do 30mph then if we miss our "normal" braking point we only have 0.7 seconds before we miss our emergency braking point too; whereas in an 80mph situation we have a full two seconds delay which we can recover through emergency braking.

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

Clearer if the graphs were plotted on the same time axis though, no?

Yep. That's it. I'm amazed, actually, that it's so simple and so compelling.

Obviously I'll write it up properly in due course.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Early days, Einion, early days...

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

But is 1.3 seconds of braking time enough to bring the vehicle from 80 to 30?

_________________
What makes you think I'm drunk officer, have I got a fat bird with me?

???

The point was that if you are driving at 80mph according to your "normal" braking distance, then you have the capability to gain 2 seconds simply by braking harder. In other words if you take an extra 2 seconds to react to something you can regain the time simply by braking harder.

But at lower speeds this buffer is proportionately slower, ie at 30 seconds if you dither for more than 0.7 seconds it's already too late, even if you perform a full emergency stop.

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

But if you are going faster, then you need more distance to stop surely?

And if you need more distance, then you are going to need those extra 2 seconds to cover that distance.
In other words, the 2 second gain isn't really a gain at all because you are travelling over 150% faster than the original 30mph, and in theory need an extra 150% more space in order to stop at the same point (if you see what I mean).

_________________
What makes you think I'm drunk officer, have I got a fat bird with me?

I don't see the point that this is making? If the road is clear then you have more time to react? Isn't that a bit obvious? What have higher speeds got to do with it? If you drove at 30mph on a motorway and a crash happened about 300 yards in front, you'd have about 10 seconds to react.

Eerm, the faster you go the more space you need to stop perhaps.


And if I drove at 60mph and an accident occured 300 yards in front of me, I'd only have around 5 seconds to react because I am covering the distance twice as quickly as what I would have been at 30.

_________________
What makes you think I'm drunk officer, have I got a fat bird with me?

But you'd still have 5 seconds to react - which, when you think about it, is plenty of time.
It's when you haven't got plenty of time that you have a problem - like, when something happens 20 yards ahead while you're doing 30mph.

The corollary to Paul's hypothesis is equally valid - in that when you're doing lower speeds things happen faster and there's less margin for error. If this wasn't the case then accidents wouldn't occur at lower speeds.

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

I still don't get the point of this!

I think what is being said is that, assuming the driver has chosen a sensibe speed that means they wont have to do a full on emergency stop as soon as the car in front puts the brake lights on for some reason, it means you have more time to spend picking your nose or drinking coffee or otherwise not paying attention and still avoid a crash, albiet by doing an emergency stop after all.

It's my fault for posting a few hints rather than explaining the whole thing.

A constant cry from the 'speed kills' advocates is that higher speeds give you less time to react. That claim runs counter to my own real world experience and I've been wondering why.

In the real world I definitely feel that I have more time to react to a hazard, (say) on a motorway compared to (say) around town.

I've noticed that if we set speed according to the Safe Speed recommended conservative version of the safe speed rule, we do indeed have more 'time in reserve' at higher speeds.

The Safe Speed recommended rule is: "Always ensure that you can stop comfortably, on your own side of the road within the distance that you know to be clear."

It's that word 'comfortably' that helps. It means we're not setting speed based on a full emergency stop, but instead setting speed based on comfortable braking effort. This brings comfortable driving and of course also provides a substantial margin for error because we always have the option of full emergency braking.

The new information is the realisation that such an approach to speed setting inherently provides a margin for error that INCREASES with speed. However counter-intuitive this may seem, it is actually correct.

Now the only assumption required is that real world drivers set conservative safe speeds. I'm perfectly satisfied that they do because we see an awful lot of modest braking in a day's driving and usually we see no emergency braking at all. Almost every time drivers stop for traffic, or traffic lights or brake for a bend or a roundabout they do so with only modest braking effort. They MUST be setting conservative safe speeds for known hazards or they simply could not do this at all. But it's normal. It's routine. It's everywhere.

So there you have it. If we set a conservative safe speed according to the conditions and the hazard environment, greater speed means MORE time to react and MORE margin for error.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

It's the unknown hazards that get most people especially around town where excessive speed seems to be the norm. This 40mph 'comfort zone' (ie 40mph everywhere) seems to becoming more and more common.

Yes it is. But 'not knowing' a hazard is a driver quality issue.



Yes. That's dumbing down in action.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety