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Sydney Lenssen The hazards of risk analysis

The Hazards Forum is 10 years old. Sponsored by all the major engineering institutions and scientific bodies to learn lessons from disasters, it has a new chairman - ex-ICE president Stuart Mustow. The forum has done sterling work at professional level. Now it is time to make an impact on the man in the street.

Mustow sees the forum as the best available vehicle for independent judgement on the levels of risk that should be reasonably acceptable to society, when faced with natural or man-made threats.

Civil engineers have always dealt with risk assessment in design and construction, more often than not unknowingly. All of us judge risks, for example from smoking, BSE or GM foods, and such assessments are not made easier when assurances of safety by interested parties are treated with growing suspicion. Hence Mustow's vision of an educated and unbiased mentor.

I'm a fan of the forum's newsletter, which stimulates and exposes the intellectual challenge posed by disasters. In the latest issue, Odd Nordland from Trondheim analyses risk tolerance and reveals differing national characteristics.

Risk in France is judged on the GAMAB principle - globalement au moins aussi bon. Assuming that a presently acceptable solution exists, then any new solution should be at least as good in total. In total is important for it allows one part of a system to get riskier provided that another more than compensates.

Britain uses the ALARP principle - residual risk shall be as low as reasonably practicable. It assumes the level of risk acceptable to the public is known, and that anything new should have a risk at least below that. This translates, according to Nordland, into cost. Since infinite effort could reduce risk to infinitely low levels but would be infinitely expensive, what we need is an actual level of risk which the public accepts as not worth spending more to reduce.

Germany uses the MEM - minimum endogenous mortality - principle. Each age group in society has a death rate and some portion of that rate is caused by technical systems. Any new system should not significantly increase that portion.

Analysis has shown that the age group with the lowest technologically caused death rate is 5-15 year olds and, in practice, that has become the reference level. Its value is 0.0002 fatalities per person per year, and the 'not significant' increase is limited to 5% of that. All very Teutonic.

But some human reactions are shared by all nationalities. People who drive themselves accept a high level of road deaths far more readily than when they travel on public transport where someone else is driving. When 100 people are killed in a single accident, it is taken more seriously than 10,000 deaths on the roads each year.

Such characteristics are accounted for in any mathematical approach with a DRA factor - differential risk aversion. This factor also differs between countries. In Britain and Germany, the DRA is linear which means the decrease in risk acceptability is proportional to the increase in the potential death toll.

In Holland, the DRA follows the square of the potential death toll. Perhaps that is why most Dutch housing estates have a 20km speed limit and you wait forever at traffic lights.

Nordland goes on to derive a formula relating risk levels with casualty rate, the accident rate and the benefit provided by the system concerned. The thresholds of acceptability remain dependent on social, political and geographical factors.

As an illustration - by the time this appears in print, I shall be in Romania representing the UK in the European F3J model glider championships. Sadly this year the junior team will not fly. The British Model Flying Association has refused to ratify the entry of three under-18 year olds because of the perceived risk of visiting Deva, a small town 200km from the Yugoslav border.

Parental permission had been given for the three boys to travel. Hostilities have ceased for the time being. But the national official body, aware of the possibility of an unfortunate incident and the potential headlines when a young person is injured or killed when travelling in parties, refused the risk.

Who can blame them when risk and hazard are such emotive issues? I wonder what the Hazards Forum would suggest.

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