Raised limit points and dead ground

[7db]

I've been thinking about cornering and using the limit point movement to trigger acceleration through and out of the corner.

Occasionally a raised limit point (ie at hedge / fence level) runs much sooner than the road limit point which may be obscured only by some small grass verge or local topology. This signals the lengthening of the radius (possibly not the end of the corner), but without providing visibility on the road.

In other words you may start the acceleration towards exit, as far as the corner is concerned, but there is dead ground that you need to pass through before exti, which might conceal an unseen hazard. Providing the hazard isn't a hole in the the road, then all is well and good.

I'm wondering what the risks are of using such a raised limit point rather than a pure roadsurface limit point and if anyone else has any views...

[Porker]

Hi Dave

I would be extremely wary of using a proxy for the limit point.

The Limit Point is defined as the furthest piece of road that you have an uninterrupted view of from your current position.

You may know the road, but you don't know what lies in the dead ground. (Dead ground = area you can't see into).

You may not know the road and the "simple bend" may contain a road deviation or worse.

Given the above, and knowing that you must only drive at a speed such that you can stop, comfortably, on your own side of the road in the distance you know to be clear (for other than single lane roads), you cannot safely use limit point proxies as a guarantee of safety because you have no idea what may be there to compromise your passage down that road.

By all means use all of the clues available to help you assess where the road *may* go, but they are never a substitute for knowing where it goes and what's on it.

So, you could sum it up as:

"Never put your car on a piece of road that you haven't looked at"

or as I tend to think of it:

"Mr c*ck-up is always ready to visit"

regards
Porker

[7db]

You're absolutely right, Nick. Or course you need to see the road before you drive into it.

Perhaps if you are hanging back from the limit point the raised limit point is more useful, providing you don't get too close to the true limit point...

Hmmmm.

[Prelude]

Just this evening, I was driving on an NSL country road which was on a slight downhill. I could see a long way off and knew there weren't any cars ahead, although there was a tight bend ahead which I knew could not be taken at very high speed. On this particular bend there was a wall on my side ( not sure why, as most of this stretch was just full of marker posts. Just coming out of the bend and starting to acclerate I noticed 2 tiny figures in the middle of the road......... (note) I was able to stop on my side of the road safely within the distance I could see to be clear..... and was greeted with the sight of an adult curlew and its little chick ( aww) in the middle of the road. Unfortunately, the mother and chick scampered off in opposite directions, yet I felt sure they would soon be reunited and that the chick woud get a surly telling off for 'jaywalking'!! And the moral of this story.... Well, no morals really, just that this low wall was low enough to be able to see any oncoming cars, but not careless curlews!!

[Nigel]

Soren made a good post on this, on the traffic answers forum.

I'm not sure if its just because I'm a bad advanced driver, or if its because we are constantly being critical of ourselves, but if I do a high speed self training run on twisty nsl's, practising cornering, I quite often wonder if I really could stop in the distance I can see to be clear.

On early morning runs, if you are sure no-one else is around, its quite often fun to do an emergency stop whilst cornering, its good to feel what the vehicle behaves like under these conditions, and its very surprising to feel the response of different vehicles.

Carefully though boys and girls, I'm not sure we'd get away with doing a "milton"

[TripleS]

Soren made a good post on this, on the traffic answers forum.

I'm not sure if its just because I'm a bad advanced driver, or if its because we are constantly being critical of ourselves, but if I do a high speed self training run on twisty nsl's, practising cornering, I quite often wonder if I really could stop in the distance I can see to be clear.

On early morning runs, if you are sure no-one else is around, its quite often fun to do an emergency stop whilst cornering, its good to feel what the vehicle behaves like under these conditions, and its very surprising to feel the response of different vehicles.

Carefully though boys and girls, I'm not sure we'd get away with doing a "milton"

The rule about always being able to stop in the distance you can see to be clear (and may reasonably expect to remain clear) is a very good one, but it is all too easy to let things slip and I'm not sure we do always comply with it -though of course we should try to.

As for doing an emergency stop while cornering - surely this is going to require more space than doing an emergency stiop on a straight road is it not? I wonder if we make sufficient allowance for the difference?

As for the Milton factor - let's not put it to the test, eh? I have this nasty feeling it might not work for us!

Best wishes all,
Dave.

[7db]

As for doing an emergency stop while cornering - surely this is going to require more space than doing an emergency stiop on a straight road is it not? I wonder if we make sufficient allowance for the difference?

Yes, but surprisingly not much more.

[Nigel]

in a car where everything is electronically taken care of for you...almost no noticeable diffence.

Interesting when you have several different vehicles to drive though...would you remember which one did what ?

[TripleS]

Thanks to 7db and Nigel.

I was expecting that while the ABS should help to prevent the car getting out of shape, i.e. going off line, spinning etc., it would still detract from optimum braking.

What you say is interesting and fairly reassuring.

Best wishes all,
Dave.

[7db]

Per Stressed Dave's calculation - if you have half a g used up for cornering (which is quite a bit), then on most road tyres, you still have 87% of available braking grip remaining.

Usually the tricky bit is making sure that you don't spin whilst using it, but traction control and ABS help an awful lot there. If you have them (turned on)

[rlmr]

Interesting thread... but I think we have to be careful .

I agree with the posts advocating seeing the actual road. Whilst walls, fences, hedges and the like can give us an indication of a relative limit point (I think the phrase used was "proxy" limit point) we have to be careful as verges can vary in size and consequently the "proxy" might not follow the actual road curve. A further point (I have lovely rural roads to drive on) is the fact that even when the proxy limit point is 100% we must see the road is clear. Its not the first time I have driven round a bend to be confronted with anything from a bale of hay to a dead sheep lying on the carriageway.

I am just home from 925 miles on the BMW around some of the finest roads in the North of Scotland . However I think the tongue is going firmly into one's cheek when some folk claim they are driving at a speed which allows them to stop in the distance they can see to be clear

[7db]

I'm guessing it's all a case of how proxy the proxy limit is, and how close ones stopping point is to the true limit point. If you're off the limit point and it's pretty proximate, it's a good indicator...

[Porker]

[quote="rlmr"However I think the tongue is going firmly into one's cheek when some folk claim they are driving at a speed which allows them to stop in the distance they can see to be clear [/quote]

Why so, Rennie?

Porker

[rlmr]

Why so, Rennie?

The limit point is the limit of one's observations and if the limit point "moves" it gives one an indication as to whether the curve is a) tightening b) as is c) opening up.

It is possible to go round the curve at an average speed and find the limit point moving away (bend opening) and still possible to add a good few mph to that speed on the approach and still find the limit point moving away. Remember that whilst "thinking distance" is a pure reaction time x speed, braking distance = 1/2 x available friction x time in seconds, squared. Check the Highway Code and you will see that if you double your speed, you do not just double your stopping distance . Consequently whilst you might have the available rolling friction to keep you on the road at speed you would use up a considerable % of available grip when braking. Irrespective of the previous posts on 87% grip available etc. we have to be realistic and remember there are two main causes for skidding. Too severe a course change or too severe a speed change for the available grip.

I have been fortunate enough to have conducted quite a few experiments, in controlled conditions (as a Police Accident Investigator) and matters such as "critical speed" start to factor in.

Suffice it to say, most folk understand the principle of keeping to a speed which allows them to stop in the distance they see to be clear, but when speed increases dramatically the distance required to stop increases by a factor which most fail to appreciate.

Driving round a bend quickly, having read the bend and factored in a speed which will allow the vehicle to continue around the curve, and being able to stop in the distance the driver can see, are two distinct aspects... not one in the same.

regards,

Rennie

[Porker]

Hi Rennie

If I have read your post correctly, the thrust of what you are saying is that the speed at which you can physically get around a bend may in the majority of cases be some way in excess of the speed that allows you to stop, comfortably, on your own side of the road in the distance that you can see to be clear.

This is - I think or at least hope for most of us on this forum - self evident.

Equally, I would hope that most on the forum appreciate that stopping distances (leaving aside reaction distances) are proportional to the square of the speed, or to put it another way, it takes four times as long to stop from 80mph as it does from 40mph.

What I don't follow is why you appear to believe that few folks on here understand that.

Apologies in advance if I've misunderstood your post.

regards
Porker