172mph 'on a rural road'...

[Gareth]

I was more interested in thoughts about visibility at these speeds, daytime versus nighttime. I accept a PC1 or equivalent is likely to be a much safer driver than a relatively untrained driver, everything else being equal.

[James]

I said at the time of the 159 thread that I had serious concerns over the visibility issues of night driving. It brings a new definition of driving within a distance you can see to be clear. During the day this distance will be flexible to the degree of the horizon, by night there will be a maximum speed limit which is fixed (this being the furthest distance the naked eye can see).

The presence of street lighting will negate this theoretical maxium speed limit one would have to obey to fit in with the rule highlighted above. However there will be patches of black between street lighting and peripheral vision will still be limited.

[ipsg.glf]

I said at the time of the 159 thread that I had serious concerns over the visibility issues of night driving. It brings a new definition of driving within a distance you can see to be clear. During the day this distance will be flexible to the degree of the horizon, by night there will be a maximum speed limit which is fixed (this being the furthest distance the naked eye can see).

James

Just a very minor point. We must always remember that the limit point is not the furthest point we can see but the nearest where a [yet unseen] hazard could emerge from.

[James]

The limit point is the point where the road either meets the horizon or goes out of view around a bend, hence using the "limit point" to judge corners... I would not bring the limit point forward because of a side junction, driveway or other form of hazard, they would be other hazards that would be mentioned seperately...

[Gareth]

I think ipsg.glf's oblique point is that when driving so that you can stop within the distance you can see to be clear, the limit point is often further away than the nearest hazard that must be fully accounted for within the driving plan.

[James]

Yes, agreed. But in darkness and in the absence of any other hazards, the point at which vision runs out is much less than in daytime hours...

I don't mean limit points as in bends / horizons but the basic eyesight required to stop within the distance you can see to be clear, i.e in darkenss at 40mph your lights should give more than sufficient vision to enable you to stop within their beam, however, what lights extend so as to iluminate the stopping distance required at say 90, 140 or even 172mph?

[7db]

Just a very minor point. We must always remember that the limit point is not the furthest point we can see but the nearest where a [yet unseen] hazard could emerge from.

I recognise that formulation, and approve!

[TripleS]

Just a very minor point. We must always remember that the limit point is not the furthest point we can see but the nearest where a [yet unseen] hazard could emerge from.

I recognise that formulation, and approve!

Is there a bit of doubt as to what limit point means? I have always taken it to refer to bends and summit situations. At least what I've read on the subject always seemed to work on that basis.

Best wishes all,
Dave.

[ipsg.glf]

Just a very minor point. We must always remember that the limit point is not the furthest point we can see but the nearest where a [yet unseen] hazard could emerge from.

I recognise that formulation, and approve!

Is there a bit of doubt as to what limit point means? I have always taken it to refer to bends and summit situations. At least what I've read on the subject always seemed to work on that basis.

Best wishes all,
Dave.

In my viw, the issue is whether something like a side junction could mask a potential hazard.

Imagine you are on a single carriageway A road which is a NSL. You can see for a good mile ahead. The carriageway is empty. On the left is a junction which is open and you have excellent visibility. You can see that there are no vehicles to emerge and a vehicle could not cross come into conflict with you.

So you can dismiss that particular junction as being free from hazarsd and move on to the next hazard.

Now, imagine that junction as having a restricted view, maybe a pub building on the corner, or abardn, or lots of trees. If it is difficult for you to see cars at (or approaching) the junction then the reverse will generally also be true.

There is much more potential danger here and I would be adjusting my drivng accordingly.

The Roadcraft video covers a similar scenario. (Junction with restricted view on the left, on a right hand bend).

[James]

I think we are covering three very valid points and confusing them all together.

Limit Points are the last points of view, as StressedDave has said, and can be tarmac meets horizon (ahead) or road meets hedgerow/other feature (bend).

Junctions, Pedestrians, Other Cars and all other hazards are Observation's, aka Hazard Perception. Depending on their activity one will react to them in the drive giving due regard to their urgency. Some will require no action (an empty junction), some a slight reaction (on a 2 laned DC a car emerges at junction on the right and waits, mirror's, lane change moving away from the point of danger) and some a major action (pedestrian steps out in front of you).

Stopping within the distance one can see to be clear is based solely on what you can see in front of you in terms of visibility. I.e not much in fog, alot more in sun. It them turns out that limit point can be married to this rule, although they are two seperate theories (they just compliment each other).

My earlier darkness comment was based on the latter point...

[James]

Its gone quiet - I wasn't trying to kill the thread - just thought we needed to clarify all the AD points... please continue if you so wish - it's a very topical issue and well worthy of debate...

[TripleS]

Its gone quiet - I wasn't trying to kill the thread - just thought we needed to clarify all the AD points... please continue if you so wish - it's a very topical issue and well worthy of debate...

Suppose you have a situation where you're driving along a straight road and ahead of you is a junction on your left, and there are buildings right up to the corner, so you have no means of knowing if a vehicle is travelling along that side road towards the junction.

We're not suggesting that we should apply full limit point thinking in that case are we? That would mean we have to be able to stop by the time we arrive at that junction, in case there is a vehicle on the side road and it comes out in front of us. It hardly seems realistic to cater for that at every blind junction. Some added wariness and restraint of speed would seem appropriate, but not, I would suggest, a complete stopping ability.

Best wishes all,
Dave.

[Gareth]

Suppose you have a situation where you're driving along a straight road and ahead of you is a junction on your left, and there are buildings right up to the corner, so you have no means of knowing if a vehicle is travelling along that side road towards the junction.

I have a few specific thoughts but no definitive answer.

First and given the opportunity, I would off-side to some degree, increasing the lateral separation between myself and potential danger.

Second is that I should try to approach the junction carrying no more speed than a driver coming to a stop on that side road might reasonably expect. From what I've seen, most drivers expect cars to be approaching at no more than about 50 mph in an NSL, so that might be my guide, modified by the degree of lateral separation I've managed to gain in the first step.

My last thought is that an emergency stop is exactly that, so I should be aware of stopping distances at various speeds, and that should also guide my approach. It doesn't have to be tidy as long as it's effective.

Thinking more generally now, many junctions have some visibility across their entrance, and that can give a driver enough confidence to approach at a higher speed than would otherwise be the case.

It's urban situations where the scenario outlined by TripleS is more readily apparaent, as there are many drive-ways and footpaths hidden by the corners of buildings, in exactly the same way that small children and dogs can be hidden by parked cars or vans.

It is for this reason that I think that most drivers, advanced or otherwise, drive too quickly through built-up areas. I should be clear about this - I don't expect drivers to crawl through urban areas, but instead to vary their speed according to hazard density, paying careful attention to lateral separation from those potential hazards.

[James]

Gareth has summed it up nicely. Again Dave it would not be Limit Point Analysis used for scenario's like this, the junction is a hazard and will remain so until you pass it. The question of how much of a hazard it is depends on its characteristics and will therefore determine your Position Speed and possibly Gears as you might slow to such a speed as to warrant going down the box once passed the junction.