Safe Speed Forums

I sent a FoI request to the DfT on Roadworks crashes as follows:

-------- Original Message --------
Subject: Freedom of Information Act Request
Date: Tue, 05 Jul 2005 09:22:05 +0100
From: Paul Smith <psmith@safespeed.org.uk>
Organization: Safe Speed
To: DfT DfT <road.safety@dft.gsi.gov.uk>

Hi,

Please service the following request made under the Freedom of Information Act
2000.

I wish to understand in some detail the accident statistics relating to
roadworks on UK motorways.

For example, how many individuals were killed, seriously injured or injured in
motorway roadworks accidents for each year of the last 10.

Please be sure to break down the figures between those injured in the course
of their work and those injured by passing traffic.

What research has been carried out to justify the current crop of roadworks
speed limits?
======================================

Their reply has just been published here:

http://www.dft.gov.uk/stellent/groups/d ... 39530.hcsp

I've had it for a few days, but there's a lot to digest.

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

Mmmm . . .

Just taken a very quick look at the first item on the list at the link provided and that just leaped out at me. Particularly the first recommendation when taken in conjunction with the first of the list of contributory factors. They don't list speeding per se, just 'too fast for the conditions'.

And I don't see how extra rigorous speed enforcement is going to reduce aggressive driving - in fact, I'd rather tend to the idea that it might increase it!

Any other comments?

Cheers, Chris B

_________________
Stultus est sicut stultus facit - a known fact
Stercus accidit - a truism!

This is the crux of it - the problem is people driving too fast for conditions, yet their solution is not to discourage people from driving too fast for conditions, but rather to discourage people from speeding. I can only assume this is because it is easy to discourage people from speeding, but hard to discourage people from driving too fast for conditions. The question is whether discouraging people from speeding also discourages them from driving too fast for conditions, and even if it does, are there any negative side effects of doing so?

There have been at least two accidents at the M1 J21 roadworks covered by SPCS. The cameras are just passed the entry slip on the north bound section. I have seen people join the main carriageway then BRAKE when they see the SPECS...

Its a very complicated junction at rush hour.

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Speed camera policy Kills

Don't know if you've gone through all the documents yet, but in one of them they say they found no difference in accident rates when comparing roadworks with speed cameras to those without. They also found no difference in the accident rate at roadworks sites compared to non-roadworks sites. Of course, if people are slowing down at the non-camera roadworks because they're worried there might be cameras there, this wouldn't mean you could stop putting cameras at roadworks or eventually everyone would probably speed up again.

Hi I have read through the documents. Looking at the TRL document pages 75/76 total number of accidents = 403. Number where over speed limit a factor = 3. I.E. less than 1% of accidents caused by speeding. Wrong speed for conditions as a factor = 36 approx 9% of accidents.

Some reccomendations for slowing all traffic down and enforcement.

The Question I would like to pose is this :-

If the free flowing 85 percentile speed is higher than the posted limit this is causing drivers to drive at a MORE DANGEROUS allbeit slower speed. These speeds are also there when no road workers are present thus road works are more dangerous than normal BECAUSE OF THE REDUCED speed limit. Also in normal traffic flow tailbacks account for over 50% of the accidents. These tailbacks are made worse by the reduced speed limit.
Is there a way to quantify these factors with proven research ?

Richard

I believe this is a misinterpretation of the 85th percentile principle. The 85th percentile speed isn't a magic speed that is inherently safer, it is a speed which is safer in the context of the speeds that other drivers are driving at. Passing, or being passed by, other cars, is dangerous, so if you drive much slower than other traffic and are passed more, then you will increase your risk. Slowing down all the traffic doesn't expose vehicles to the same risk as driving at the same slower speed with lots of faster vehicles passing.


I would expect tailbacks to be reduced by the lower speed limit, as traffic throughput increases as speed is reduced, with maximum throughput occurring around 12mph. This is the principle behind the M25 variable speed limits. Of course if the roadworks reduce capacity, then they will cause tailbacks.

The 85th percentile speed is defined as the speed which would not be exceeded by 85% of drivers, in free-flowing conditions, in the absence of any statuory limits.
An analysis of the reasons why such a speed is the speed of choice for a particular road would probably indicate that that speed is in fact the most appropriate speed.



I've often wondered how they come to that conclusion - as there appears to be no logic behind it.
Assuming a 2-second gap, if vehicles were dimensionless, the rate of traffic flow would be 1800 vehicles/hour/lane, regardless of speed. But, as vehicles do have dimensions, the rate of flow decreases slowly with decreasing speed, until it's about a third less at about 12mph, and below that decreasing very rapidly to zero.
And even if it wore the other way around, as they say, the throughput per se would mean bery little, as a 12 mile journey along the motorway would take an hour at 12mph, rather than around 10 minutes at 70mph.

Cheers

Peter

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

You need to bear in mind that braking time increases with speed, as obviously it takes longer to stop from 60 mph than from 30 mph. I thought the optimum flow figure in terms of vehicles passing a given point per minute was about 18 mph, but it certainly is fairly low. Below that figure the length of vehicles begins to outweigh any further gains in braking time, and so the flow rate decreases.

The 2-second gap is only an average, it is not a fixed figure that does not vary with speed.

_________________
"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

This was recently discussed on another thread - suffice to say here that braking time will have little or no effect on gap times, therefore on throughput.



I know but, at the same time and for the same reasons, there is no quantifiable link between speed and throughput.

Cheers
Peter

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

I saw it derived on an Open University programme once, as an illustration of how to solve the relevant equations. It was based on people leaving a gap corresponding to their braking distance. The advice nowadays tends to be to leave a much smaller gap than this, e.g. the 2 second rule as you mention, which will result in a different speed to maximise throughput.

Well, they were wrong, wrong, wrong then.

The 2 second gap allows time to react. It's on the generous side to allow for unavoidably inattention (mirror checks for example) and for braking performance differential.

It's got absolutely nothing to do with braking distance/time.

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

Surely it bears a rough relationship to braking time, though. If vehicles took 20 seconds to come to a halt (think of trains) then a 2-second gap would be lethal.

_________________
"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

If the train in front takes 20 seconds to come to a halt too, you'll be fine.

Max braking in a car is a nice linear 20mph per second, so 2 seconds is worth 40mph speed reduction.

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

Only if braking performance was nonlinear with the better deceleration at the higher speed.

Taking it a bit further, the braking distance from 40mph is 17.8 metres (assuming zero reaction time) and a 2-second gap at 40mph is 35.6 metres.
So the two second stopping time equates to one second in terms of pre-braking time.

Cheers
Peter

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

But people don't leave a gap corresponding to their braking distance, and they don't even adhere to the 2-second rule a lot of the time - so I fail to see how they can possibly define any connection between speed and throughput.

Cheers
Peter

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

But I thought you agreed with the principle of only driving at a speed that means you can stop in the distance you can guarantee to be clear? If you have a multiple vehicle pile up, once cars start hitting other cars that have already stopped, they will stop in much less than their normal braking distance. I tend to like to leave a nice big gap to the car in front, getting close to it doesn't really advance my journey, the only thing that makes me get closer than I'd like is that people pull into the gap, so you end up forced to drive at a distance from the car in front that avoids having car after car dive in front of you.

By way of a complete aside, I find it rather bizarre to quote a speed in imperial units and the distance in metric units - both in the same sentence. I'm not having a go - many people make that choice, but I personally find it incongruous.

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

In that context, it's quite understandable, as scientific concepts such as deceleration are usually expressed in metric, but speed limits, and concepts of speed, on UK roads, are Imperial.

_________________
"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