Road Safety News
 

Danish experiment suggests higher speeds can cut crashes

Tuesday 25th February 2014

A trial being conducted in Denmark appears to be showing that increasing the speed limit on certain rural roads is reducing the number of collisions (Copenhagen Post).

The Copenhagen Post report says that two out of three road fatalities occur on Denmark’s country roads where the speed limit is 80 kilometres per hour.

Since 2011, Vejdirektoratet (the Danish road traffic directorate) has upped the limit to 90 km/hour on some stretches, which has resulted in fewer collisions due to a “reduction in the difference between the cars obeying the speed limit and those exceeding it”.

The trial runs until 2015 and as such cannot as yet be called a success, but the signs are encouraging at the halfway point.

Rene Juhl Hollen, a Vejdirektoratet spokesperson, said: “If there is a large difference between speeds then more people will attempt to overtake, so the more homogeneous we can get the speeds on the two-lane roads, the safer they will become.”

Nine years ago the speed limit on certain motorways in Denmark was increased from 110 km/h to 130 km/h and this resulted in fewer traffic fatalities on those stretches of road.

The police were initially sceptical about increasing the speed limit, fearing that people would drive even faster, but have changed their position based on the results from the ongoing experiment.

Erik Mather, the head of traffic police in South Zealand and Lolland-Falster, said: “The police are perhaps a little biased on this issue but we’ve had to completely change our view now that the experiment has gone on for two years.”

Brian Gregory, ABD joint chairman said the findings “vindicate what the ABD has been saying for years”.

Mr Gregory said: “Raising unreasonably low speed limits improves road safety by reducing speed differentials and driver frustration.

“It is now time for the Government to push ahead with raising the motorway speed limit to 80mph. It must also change its guidance to local authorities on setting speed limits, so that they are once again set at a level that commands the respect of drivers. 

“This means reinstating the 85th percentile principle - setting limits that 85% of drivers would not wish to exceed."

Click here to read the full Copenhagen Post news report. 

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Eric
You have already said that it is derived from taking the speed limit off a road and then measuring the speed of the drivers. I detect certain practical difficulties with this!
Rod King, 20's Plenty for Us

Agree (2) | Disagree (4)
-2

Rod
The 85%ile is not plucked from the air, it is derived scientifically based on driver behaviour. It is not a single number and will vary along a stretch of road. Its properties, hardly magical, are based on analysis over many years.

In contrast, I suggest that 20mph has been plucked out of the air; it is expected to be applied to miles of road of varying styles and hazard density, and you evidently believe it has magical properties.
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (3) | Disagree (1)
+2

Eric
You are once again pulling the 85% figure out of thin air and giving it magical properties. We have never said that a speed limit tells people what to drive at it merely tells them the maximum speed that is legal. You haven't addressed my point that if your 85%ile is based on no speed limit then in the UK no such reference points exist anyway.
Rod King, 20's Plenty for Us

Agree (2) | Disagree (3)
-1

Rod
You say "Of course it (the 85%ile speed) takes no account at all of the visibility of a hazard".

Not true. It is based on the principle that 85% of drivers are good at looking for hazards and recognising where hidden hazards may be. It's why they will drive down a narrow street with cars parked on both sides at, say, 20mph in a 30, or at 30 on an icy national speed limit open single carriageway. Telling drivers what speed they should drive at is a very poor way of trying to improve road safety. They need to be alert to the conditions, and the potential for hazards, and their speed adjusts automatically.
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (5) | Disagree (1)
+4

Eric
You have demonstrated my point. You maintain that the 85%ile is a figure based on not having any speed limits because "drivers are influenced by the sign (ie limit)".

In the UK it is therefore a hypothetical figure as for the past 40+ years all roads have had a speed limit. Of course it takes no account at all of the visibility of a hazard. Where hazards are not obvious is the very place that limits are most valuable.

Speed limits should be set on the basis of reasonable and objective judgement taking into account the needs of all road users rather than the subjective and often erroneous judgement of selected road users (ie drivers). And that's why the DfT abandoned its use in 2006.
Rod King, 20's Plenty for Us

Agree (2) | Disagree (5)
-3

This is just the sort of news to get the Daily Mail on a campaign. I agree with other correspondents that there is insufficient disclosure of evidence that allows a meaningful analysis in the report. But I do notice the higher quality of their vision for cyclists and pedestrians in the photograph used to illustrate the trial road section.
We do have a very unsatisfactory culture in the UK of tailgating to put pressure on the vehicle in front to speed. We saw the results of this in the Sheppey pile up of around 100 vehicles that miraculously had no fatalities. I understand that the Swiss have effective ways of deterring tailgate bullies.
Malcolm Whitmore Leocestershire

Agree (2) | Disagree (2)
0

Let us not forget that when the existing speed limits became enforceable many, many years ago there were no scientific trials and it was an arbitrary speed limit. Who is to say that it or they were right?
bob craven Lancs

Agree (5) | Disagree (0)
+5

Rod:
The 85%ile speed is the speed which 85% of drivers would not exceed if there were no speed limits, and has long been recognised as a good guide to what a speed limit sign should say. Changing the value on a speed limit sign and measuring 85%ile speed is measuring something else, which has been influenced by the sign and there is no reason to draw any conclusions from the resulting 85%ile speed. The "85%ile hypothesis" remains valid.

It's worth considering the purpose of speed limits, which I believe are:
1) To firmly guide inexperienced and under-skilled drivers away from exceeding safe limits by wild margins
2) To provide a ready means of prosecution of those who use speed dangerously
3) To provide a "standard warning" of expected hazard density

It is thus essential that they are set with these considerations in mind, and not with an arbitrary aim of slowing the traffic down.
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (7) | Disagree (4)
+3

You can test the hypothesis of the 85%ile being optimal for anything fairly easily.

1) Assume that the 85%ile produces lowest casualty numbers. let it be x
2) Change the speed limit, either up or down
3) Does the 85%ile change? If so let's call this speed y.

4)Well you cannot have two different speeds which are both safest and provide best traffic flow.

Hence unless you believe that speed limits do not effect driver speeds (and that the 85%ile does not change) then any idea that an 85%ile speed is safest, best, or unique in any way is impossible because it keeps on changing when you change the speed limit.

There is no magic associated with the speed that the 16th% fastest driver drives below at.
Rod King, 20's Plenty for Us

Agree (4) | Disagree (5)
-1

If the Danish result (and it is only provisional pending a final report in 2015) were unique, then it would be right to be cautious. But it confirms evidence from the US, UK and elsewhere, dating back decades in some cases, that setting speed limits at the 85th percentile leads to the smoothest traffic flow, the lowest spread of speeds, and the lowest accident frequency.
Malcolm Heymer, Dereham, Norfolk

Agree (15) | Disagree (4)
+11

Perhaps one reason why there appear to be more accidents when speed limits are lower, is that complacency sets in. Following the herd and repeatedly checking the speedometer for compliance distracts from the road ahead, and what is in it.
Derek Reynolds, Salop.

Agree (11) | Disagree (4)
+7

Yes care needs to be taken about the confidence in this conclusion but it does seem that the blood-bath often predicted by the "speed kills" camp if cameras are deactivated or speed limits increased never actually happens.
Dave Taylor, Guildford

Agree (11) | Disagree (6)
+5

Dave Finney is right to be cautious about interpreting the results without sufficient data - we have seen far too much of that in relation to speed cameras. However the point raised above that speed differentials can be more important that average speeds is valid, and has been recognised for a very long time. It has also been my experience here in the A272 ("the longest lane in England") where I am frequently tailgated on 30/40mph roads that had been 40/50 or even 60mph roads for at least 30 years, with no such problems. That demonstrates the asburdity of predicing risk on the basis of average speeds - lowering limits normally lowers avearage speeds but increases differentials.
Idris Francis Fight Back With Facts Petersfield

Agree (12) | Disagree (6)
+6

Of course you could just see this as a charter to reward intolerance and the failure to exercise the skill of anticipation. In any other setting such a thing would be reprehensible. Before you reach for the "disagree" button, give some thought to the number of collisions which involve a single vehicle in which some unfortunate has lost control after self-imposing a speed limit they cannot cope with. Safety generally involves inducing a margin for error into risky situations, not pushing them to their limits. And then there's the concept that speed limits are sometimes set with the safety of road users other than motorists in mind. Of course, if cyclists and pedestrians are frightened away they will not be injured, so casualties will go down, all in the name of the sacred 85th percentile. OK, now you can push the button.
Tim Philpot, Wolverhampton

Agree (15) | Disagree (13)
+2

The Danish authorities have not said what the actual reduction was i.e typically, how many crashes per year previously and how many to date. Is it a minimal reduction or a significant one and if and when the crashes revert to their previous level, what then?
As there's no clear reason why inceasing a speed limit by a whopping 6 mph (roughly) would bring about a reduction in crashes, then it must be a fluke. The reduction was despite the speed limit increase and not because of it.
Hugh Jones, Cheshire

Agree (5) | Disagree (12)
-7

Natural speed limits have always been the safest and the unnatural ones the most dangerous. Don't need a trial to tell us that.
Duncan MacKillop, Stratford on Avon

Agree (15) | Disagree (12)
+3

We must be very careful in interpreting these results. If, as must surely be the case, speed limits were raised on those roads that had had a higher crash rate, then crashes would likely have “regressed towards the mean” even had the speed limits remained the same. In fact the same results might have been obtained had the speed limits been lowered, or speed cameras deployed, or red balloons released.

Were the Danish speed limit increases performed within scientific trials? Was RTM measured?

If neither of these was done then the results might be worse than inconclusive. If we in Britain are to increase speed limits (or reduce them) we must do so only within scientific trials otherwise £millions might be wasted and more people killed or seriously injured on our roads.
Dave Finney, Slough

Agree (15) | Disagree (2)
+13

I perhaps have an example of where a speed increase would assist in road safety. On the way out of Blackpool is a road called Yeadon Way - its where the old south rail link came in and has been tarmacked and is about one and a half miles long. If I ride or drive along it at the maximum speed limit which is 40 mph I will get tailgated and there will form a collection of traffic behind me. All far to close. If however I speed up to 45 mph then the car behind stays a fairish distance behind and there is no build up of traffic as before.

I have been on other roads at up to or just below the speed limit and had a contingent of other vehicles catch up and follow but if I speed up to two or three mph over the limit it sometimes seems to spread itself out and reduce the problem. Strange isn't it. Human nature that is.
bob craven Lancs

Agree (18) | Disagree (4)
+14