Safe Speed Forums

Mostly we are agreed that 85th percentile based speed limits are a reasonable best compromise.

But even the best set speed limit covers places, circumstances and times where it is entirely inappropriate.

Imagine for example:

- A twisty 'B' road with a sensible 40mph speed limit. But there's also a 0.5 mile perfect straight section frequently used safely for overtaking, where 80mph is reasonable in good conditions.

- A wide 30mph surburban feeder road, highly busy during the day with both traffic and pedestrians. Yet during the day it does safely flow at about 30mph. In the middle of the night conditions are entirely different and 50mph is commonplace and safe.

- A twisty 'A' road across open moorland. Speed limit 60mph. Perfect road surface. At 45mph a well laden Transit is teetering on the limit of grip with a novice driver at the wheel. Two minutes later in identical conditions a skilled driver in a Porsche is cornering at 80mph and touching the ton on the straighter sections in perfect safety.

The point is, that however well we set the speed limit, there will still be circumstances within in where it will be exceeded fairly wildly by normal folk behaving safely.

Hence discretionary enforcement is essential if we are to

a) have speed limits and
b) preserve the vital link between the law and safety.

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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

Yet still, this is where ALL the enforcement occurs.....

Enforcement logic in its current form is incredibly flawed as this straight section is where the "speeding problem" would exist, it doesn't matter that all of the accidents would occur on the bends.

Is the 85 percentile taken from the speed that drivers would choose if left to their own devices, or when there is already a speed limit in force? The former seems more sensible but I don't see how that would ever be determined on a public road.

It also seems to me that whatever speed this analysis comes up with would probably be the speed at the slowest point along the relevent section of road, and would then be rounded down to the nearest 10 mph, so the resulting limit is certain to be lower than the 85 percentile speed, possibly substantially lower in some situations.

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Peter Humphries (and a green V8S)

Not sure about that. The serious crashes are possibly badly judged overtakes? Possibly exacerbated by side effects of 40mph LGV limit?

One would hope that a B-road would only be given a 40 mph speed limit due to the level of development along it, not simply because it was twisty. If the half-mile straight had a significant amount of development, then a 40 mph speed limit would be appropriate, if not, then it is long enough for a stand-alone stretch of NSL.


The specific way in which it would be applied is, if a lower speed limit is being considered for a road, that speeds should be measured in free-flowing conditions, and the speed limit should be set at a multiple of 10 no more than 6 mph below the 85th percentile speed.

In most cases, ensuring the speed limit is set above the mean speed would give the same result; the exceptions would tend to be on roads such as mountain passes where the greater skill of the more capable drivers would allow them to achieve a higher speed differential over the mean.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Sorry, I was talking in a very hypothetical sense just to highlight that they tend to enforce speed limits where it is safe to exceed them.

I am seeing many more of these bad overtakes you describe, as more and more drivers get frustrated by very slow moving vehicles.

Yes, one would indeed hope. The example was to illustrate that conditions are not continuously similar along the length of a road. The example also works if the road is NSL.

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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