Safe Speed Forums

Hi all,

While writing up the "why do drivers speed?" thing for the new web page:

http://www.safespeed.org.uk/why.html

I paused to consider the new question: "What are the units of safe speed?"

Clearly any speed can be expressed in miles per hour, but that's of no use at all. I've been thinking around ideas like:

* "relative size of safe braking zone" - if you can stop in half the distance you know to be clear then that might be expressed as 50%.

* time to react expressed in seconds: If the nearest obstruction is 2 seconds further away than our stopping distance then we could be said to have a "safe speed margin of 2 seconds".

Or is there a better way? If we can come up with a useful, driver oriented, numerical expression of the safety of a speed in a given set of circumstances that in itself might help us to commuincate the Safe Speed rule to a wider audience, and it might help counter the present "mph mania."

Any thoughts?

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

Hmmm, I can see what you're getting at here...I think You are suggesting a unit of measure that would always indicate whether a driver was travelling at a safe speed or not, i.e. it relates to the circumstances around them at the time.
So 30mph would translate into 35 'safespeed units' in one scenario, and 25 'units' in another.
Or have I got the wrong end of the wrong stick?

You've certainly got the right stick. Since posting the first post in this thread, I think I've decided safe speed should be measured in "risk units".

Clearly if you've going too fast to stop in time for a hazard ahead you have zillions of risk units. Quite how we'd ever get to a number, I don't know. And the other problem is that a lower speed would always seem to tend to lead to a lower risk which isn't very realistic, especially in L3 of a motorway.

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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 that's very interesting. I've read comments that assume that if you disagree with speed cameras you want to drive, for example, at 36mph on residential roads, past schools at 3.35pm etc. etc. In terms of safety, that is not the same speed at all as 36mph on a wide suburban main road or even some out of town roads which have a 30 limit.

But isn't it slightly confusing to say that safe speed can't be measured in miles per hour? I thought that the point is that the "safe speed rule" is better than speed limit enforcement for choosing an appropriate and safe absolute speed (measured in mph). Or is what you posted just another way of expressing the safe speed rule? Talking about the "relative size of the braking zone" not exceeding, say, 50% seems to me to be a simple, common sense argument, which could be used to counter the apparent common sense of speed kills.

If it is confusing, I apologise. Technically I believe I am correct when I say that a safe speed can be EXPRESSED in miles per hour, but that it cannot be MEASURED in miles per hour. After all if the value (e.g. 36mph) is always the same, if it were a measurement of safe speed then the safety of 36mph would always be the same, which clearly it isn't. But that doesn't stop us from saying, quite correctly: "his speed of 36mph was safe in the circumstances".

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

Driving too slowly in lane 3 might reduce risk for that one driver, but would probably increase it for everyone else on the motorway. Surely "risk units" should take account of drivers creating a hazard for others, as well as dealing with ones that affect themselves. I mean, driving around with four bald tyres would increase the risk for everyone, not just the plank who dosesn't bother with basic maintenance. Mind you, all that makes defining and quantifying "risk units" even harder.

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Make everything as simple as possible, but not simpler - Einstein

Thank you, I see the distinction now.

Units of safety would be a function of hazard density and escape routes I suppose. If you had the option of braking to avoid a problem or altering course then you have twice the options where you have a situation where only braking would help. The escape routes could also be affected by their outcome dangers eg swerving has the risk of losing control of the car or conflicting with another road user. The escape routes would also be affected by the amount of time taken to execute them and the timing of when you had to do it and the margin of error between it avoiding the accident and doing it at the wrong time or too late to make a difference.

I am not sure you can express it it in units easily as I think most of it is down to gut instinct. I think you could do some interesting experiments in a simulator where you changed all these paramaters to see whether there was a relatively fixed formula drivers were using unconciously to decide what speed they were going to be travelling at. Eg road width, number of oncoming cars, parked cars at side or street, junctions etc. Speed is the easiest indictor to measure, road position (defensive or neutral etc) might be another thing to look at.