Safe Speed Forums

In another thread warwickshire police the old thing about "perceptions of speed" came up.

Many folk (and I don't mean the folk around here) apparently haven't understood that "numerical speed" is fundamentally different from "appropriate speed". Our perception of each sort of speed is quite different and our management of each sort of speed is quite different.

Here's are two experiments:


Experiment 1:

Required: Wide urban road preferreably 40mph limit near a motorway and regularly visited.

Part a. Drive along the road after urban driving. Try to set your speed to 30mph without reference to the speedo. Cover the speedo over if necessary. When satisfied, uncover the speedo and record your speed.

Part b. Drive along the road after a long motorway drive. Try to set your speed to 30mph without reference to the speedo. Cover the speedo over if necessary. When satisfied, uncover the speedo and record your speed.

Compare results from part a to part b. Expected result: Perception of numerical speed altered by high speed run.


Experiment 2.

Required: Quietish wide roundabout near a motorway.

Part a. Drive around the roundabout as fast as you feel comfortable without reference to the speedo after urban driving. Record speed.

Part b. Drive around the roundabout as fast as you feel comfortable without reference to the speedo after a long motorway drive. Record speed.

Compare results from parts a and b. Expected results: Speeds from part a and part b are the same - unaffected by motorway driving.


Drawing conclusions:

If the expected results are achieved, we have proved that two sorts of speed perception operate independently. You'll probably be 15mph too fast on the urban road after the motorway run, but spot on for the roundabout.

Does anyone find their speed on the urban road UNAFFECTED by the high speed run?

Does anyone find their speed on the roundabout AFFECTED by the high speed run?

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

I think most people are aware of the perception of speed after coming off a motorway.

For going round a roundabout, especially as you've said about using a speed you're comfortable with, I wouldn't be surprised if the speed would be roughly the same in both examples, as you couldn't go round much above 40 or so without you sliding to the left in the seat!

I haven't carried out the experiment but I would not be surprised to find you're correct. I would theorise that the explanation may be that drivers instictively use the 'g' force sensation when cornering as a guide to cornering speed. Obviously different drivers' 'comfort' perception of 'g' forces will vary but I suspect that any individual driver will not select a speed when cornering which produces a 'g' force greater than he is comfortable with.

I haven't fully analysed this but it seems to me this is both good and bad. Good because I think that most drivers comfort 'g' force will be reached at a speed generally lower than the limit of adhesion speed and bad because it doesn't take account of road conditions.

My reason for this is based solely on analysing my own driving (so it could be wholly inapplicable across a population of drivers) on a motorway slip road (for anybody who lives around London, it's the M25 clockwise slip onto the eastbound M40 - J16). This is a long, constant radius curve of (guessing here) about 220 degrees. Some years ago, I realised that I consistently choose to drive it (in my car) at 60mph (traffic permitting). I then realised that I drive it at the same speed almost regardless of the weather conditions. So I concluded that my speed selection was not determined by available grip (I'm certain the curve could be taken significantly faster in the dry and somewhat faster in the wet) but by comfort.

Is this all rubbish? Does it mean anything?

[edited to add]
It happens that 60mph also approximates to the appropriate speed determined by limit point analysis, but I'm not sure whether I chose that speed before I learned about limit points or after.

That won't do.

We can use the "roundabout judgement" to set a turn in speed before we experience g forces (on that occasion).

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

True. But if you turn in a bit too fast, you will scrub off a fair bit of speed quite easily if the 'g' force is building too fast/above your comfort level. And if you turn in a bit more slowly, it's easy to accelerate up to the comfort 'g' force level. Also incorrect initial speed selection (i.e. that which does not produce the comfort g' force) could (partly) explain why we often see drivers making multiple steering corrections on a constant radius curve.

I think you're highlighting a shortcoming in the design of my experiment.

Unfortunately the road layout required for a revised version of the experiment may not be so commonplace. I guess we'd need a straight road with a single bend. We attempt to set an appropriate turn in speed after urban driving and after a high speed run.

It'd be oh so easy with a test track.

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

Thinking about this some more, perhaps it's be better to substitute (or add) a braking distance test.

My contention is that judgement of braking distances is unaffected by the high speed run.

In this case we'd choose a wide quiet road near a motorway and pick a stopping point. As we approach the stopping point we brake firmly and steadily with the intention of stopping precisely at the stopping point.

Th difficulty I see with this version is that we may subconsciously (or even consciously) adjust the braking effort during the braking phase to refine our performance. I'm not at all sure how we could guard against that one...

Perhaps, however, we have no need to do the experiement since we can all normally brake to a stop without difficulty when we reach the roundabout at the end of most motorway journeys. If our perception of appropriate speed was much affected by the high speed run this would be impossible.

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

A bit off-topic, but what you said leads to something else I've been thinking about for a while - steering corrections.
Ever since someone else pointed it out to me, I've been conscious of the fact that I hardly ever make more than the slightest steering correction while going around a bend - even those I'm not familiar with.
I couldn't tell you why this is, I don't consider myself to be a better driver than the next person.
Is it because I've subconciously developed a good cornering technique, or is it simply because I've been driving for more than three decades?
Or is it something else?

To get back onto topic, I hardly ever look at my speedo when approaching a bend, and certainly not while I'm in the bend.

Regards
Peter