Safe Speed Forums

INverclyde Now hereSuch a clear case for the 85th %ile !!
What is this about extending lower limits with such flippancy ! To deliberately reduce a speed as if to aid motorists to slow is pointless and only devalues other speed settings and distorts hazard predictability.

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Safe Speed for Intelligent Road Safety through proper research, experience & guidance.

Ah the inexorably creeping zones/pointlessly low, reason defying limits.

The claims given are misleading, and in turn are bad for road safety.


This is misleading, for several different reasons:

- The average speed, assuming this is regarded as safe, is not an indication of the maximum safe speed.
- Drivers usually travel at, or less than, the maximum safe speed; more will be below than above. Therefore, the average speed will likely be below the maximum safe speed.

Why is the average (and below) considered to encompass the 'majority', when it is just as possible that the majority verdict could be above the average level?

There are various confounding factor that offset and skew (shape) the speed distribution curve:

Speed limits
Speed limits will be a strong factor in determining average speeds, simply because it is law - this is regardless of safety.
Setting to the 85th percentile will yield a much better measure of the average speeds that drivers consider safe.

Enforcement thresholds and other uncertainties
Those who exceed the limit generally won't do so by a great amount, because they risk 3 points/£60, or more.
Comparatively many more will be well below the limit, partly because of uncertainties of their determination and control of their speed, partly because of the uncertainties of the thresholds for speed enforcement, and because they are entitled to.
Again, this is regardless of safety.

Invasive measurements
Drivers will recognise that speeds are being monitored, and will reasonably conclude that enforcement is active. Therefore they will apply headroom and slow to speeds lower than they would otherwise have, thus again skewing the perception of what drivers consider a reasonable speed.

Speedo Over-read
Speedos are allowed to over-read above the true travelling speed of the vehicle, by up to 10% +6.25mph; however, speedos are not allowed to under-read. This, coupled with the tolerances, drift and ageing of the components in that system, results with speedos being set to over-read slightly such that they always remain within the allowed parameters of operation.
Therefore, those who believe they are doing exactly 60mph (as indicated by their dashboard speedo) will be doing anything between 50 to 60, most likely 56.
Those who aim for 60mph, but leave a few mph headroom for speed enforcement, and for uncertainties of exactly where the speedo needle is, as well as fluctuations of their speed, will inadvertently justify the reduction of the 60 limit to 50. This is not right, yet it is inevitable under these proposals.

Furthermore, many drivers don't know about the allowed speedo errors, so they assume speedos can under-read (some speedos are so much in error that they do under-read), hence they will leave additional headroom to cater for this uncertainty.

Slow Leaders
The slowest drivers are usually holding up many others who would have otherwise driven faster if not held up. Thus it does not follow that the held-up drivers believe these slow speeds they are forced to travel at, is also the limit for safety. At busy times (when the weighting is most significant), the number of those held up will significantly outweigh the number of slow leaders.
Even a very minor portion being slow movers (tractors and other slow pokes) can significantly reduce the average speed.
It would be wrong to base the speed limit from the effects of these slowed drivers, yet it is inevitable under these proposals.

Corners and bends (only valid if the measurements are taken over a distance)
Such features that slow traffic will affect the average speed throughout the length of the route; basing speed limits in one area on speeds in other areas is disingenuous.
Drivers on roads doing 60 safely along straights, but say 40 around bends, could be averaging 50mph, again inadvertently justifying the wholesale reduction of the 60 limit to 50 even on the straights. Again this is not right, yet it is again inevitable under these proposals.

Therefore, it is very possible, even likely, for the average speed to be well below what the majority of drivers consider safe. Under these proposals, a combination of the described factors would more than likely result with the limit being needlessly reduced.
All considered, the 85th percentile speed is a far more sensible basis to set limits.


These wholesale, as well as poorly justified, limit reductions of major routes will displace traffic to minor routes. We know that there the risk of a KSI accident is disproportionately higher on minor roads (link). So it may well be possible that the KSI on the reduced road will decreased, but the incremental increase of KSIs on surrounding routes will push the net (for all roads) KSI rate higher.


The respect for law will plummet - and with reason!!

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Absolutely & very well put indeed.
I have tried to contact the offices but they are all closed today until the new year !

I can email but I will consider my response and any PR for the New Year so that it won't become lost in the festive period.

Edited to Add :
I took a look for the Cloch Road and as I thought there are very few real alternative roads to this one especially if you are planning to catch the ferry! :
http://maps.google.co.uk/maps?q=Cloch+R ... m&t=m&z=11

I'll check the others.
A761 Port Glasgow to Kilmacolm : http://maps.google.co.uk/maps?q=A761+Po ... 6&t=m&z=12

B788 Greenock to Kilmacolm : http://maps.google.co.uk/maps?q=B788+Gr ... 0&t=m&z=13

Barely any real alternatives (without a very long way round!) ... but those that do exist are within their determination to reduce all the speeds on the roads.
This will add to greater risks as people become frustrated and take greater chances when overtaking or other manoeuvres.
Longer queues of close following traffic will make side junction pull out opportunities less, as traffic bunches in longer 'trains' too.
When one person stops or slows others will potentially collide before they have even realised that there is a problem. In harsh weather conditions this danger will increase.
Just slowing traffic does not reduce accident risk, nor does it encourage greater concentration or attention so potentially increasing accident risk.

If there are problems with many accidents in certain areas, then they may have an (or several) accident black spot/s, which can only properly be resolved with science and engineering solutions.

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Safe Speed for Intelligent Road Safety through proper research, experience & guidance.

This in itself is a misleading statement. The actual guidelines state that speed limits should NOT be set BELOW the mean speed, NOT around it. Many speed surveys on NSL roads show mean speeds to be around 51/52MPH, HENCE the speed limit should be 60MPH NOT 50MPH, just because this is around the figure of the mean speeds. Highway departments need to read the guidelines and apply them correctly.

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.