Safe Speed Forums

Is there any hard data to back this up?

It's something I've thought about a few times, and whilst on the one hand I can see that increasing traffic density could have the effect you mention, on the other hand it also increases the chances of a driving error leading to an accident. In other words say I misjudge a left hand bend and exit on the wrong side of the road, if there is twice as much traffic on the road then the odds of me hitting an oncoming car double.

Also, increasing traffic density means more following other traffic, which means (a) more boredom-induced crashes due to the soporific effect of following slow traffic, and (b) more frustration leading to ill-judged overtakes.

And of course this compounds: more of the ill-judged overtakes will result in crashes with oncoming vehicles because there are more of them.

The more traffic the lower the risk of unexpected vehicles.

Take care with this line of argument. I seem to recall reading that increased traffic density leads to reductions in average speed. The possible indications that accidents do not increase in proportion to density could be used by others as an argument that decreased average speeds result in proportionally fewer accidents? The reducto ad absurdum argument (very few accidents in traffic jams) certainly "works".

There were a couple of articles in Accident Analysis and Prevention looking into congestion and accident risk. I have one of them downloaded at work along with a few other interesting ones such looking at why people speed and aggregation bias in statistical analysis.

If you're interested I can zip them up and email them to you.

Abstract
The aim of this work is to be a starting point for a more thorough description and analysis of safety related road user behaviour in order to better
understand the different parts forming the traffic safety processes. The background is that it is problematic to use analysis of crash data and conflict
data in the everyday traffic safety work due to low occurrence rates and the focus on rather exceptional and unsuccessful events.
A new framework must consider the following aspects: (1) The importance of feedback to the road users. (2) Inclusion of more frequent events,
“normal” road user behaviours and the possibility to link them to a severity dimension. (3) Prediction of safety/unsafety based on the more frequent
events.
By constructing severity hierarchies based on a uniform severity dimension (Time to Accident/Conflicting Speed value) it is possible to both
describe the closeness to a crash and to get a comprehensive understanding of the connection between behaviour and safety by both considering
unsuccessful and successful interactive situations. These severity hierarchies would make it possible to consider road users’ expectations due to
feedback and estimate its safety relevance.

I see your point, but of course Safe Speed would never shy away from good information because it didn't fit our objectives or ideas. The care that we need to apply is care against abuse of information.

One of the papers mentioned earlier establishes that congestion tends to reduce crash risk. My 'instinct' would be to map this effect to the psychological domain via habituation rather than the physics domain via benefit of lower speed. This is amply aided by the small print in TRL511 which establishes that faster roads are safer as a sound general principle.

I read several times recently that if you double the traffic in a given situation you only expect crashes to rise by about 40%. I'm not sure where I've read this, but of course it makes good sense. If vehicles are unexpected then crashes are more likely. The more traffic the lower the risk of unexpected vehicles.

Putting this is the wider context of national road safety, more traffic should mean more familiarity and more habituation - and crashes may rise, but shouldn't rise in proportion.

So in my opinion higher volumes would not reduce the accident toll unless the all roads were full 24 hours a day.

Higher volumes would increase the accident rate by a factor that was not directly proportional to the added traffic. If the mid day density was still higher you could see a significant benefit in the centre of town. But every town has suburb where the opposite would happen? accidents would rise.

There must be an optimum traffic density where more serious accidents happen. The higher the amount of roads at this density for more hours of the day, would affect the overall accident rate.