The evidence with respect to seat belts suggests that the law had no
effect on total fatalities but was associated with a redistribution of
danger from car occupants to pedestrians and cyclists.
"In Britain, as in Australia, and as in the eight European countries
examined by Isles, in the year that the wearing of seat belts became
compulsory the numbers of pedestrians and cyclists killed increased,
by 8% and 15% respectively. The numbers of pedestrians and cyclists
killed by heavy goods vehicles and public service vehicles (categories
not covered by the law) decreased following the law."
Isles, J.E. 1981. Seat belt savings: implications of European
Statistics. London, UK Department of Transport. This report was
suppressed, i.e. not published, and not available to Parliament when
it debated the seat belt law. A leaked copy was reported four years
later in New Scientist (7 February 1985). The contents of the report
and its conclusions have never been denied.
The study is available here: http://john-adams.co.uk/wp-content/uploads/2007/01/isles%20report.pdf
This actually already contains some interesting comments:
the variation in trend between countries is greater than a 'seat belt
law effect' ... Any beneficial effect could be concealed by rejecting
So basically, by choosing the right countries, any effect could be proven, and the discussion which countries have to be chosen is not very strict: what if a law is not enforced? Or partially enforced (only some users, eg with recent cars, only some areas, eg outside the city centre).
This comment should also apply to Isles own study. Here is the graph that should support the fact that there is a jump in pedestrians killed the year the legislation is passed:
It shows the number of pedestrian casualties, scaled to 100 3 years prior to the law. This means that there is no confidence interval that year (everything scaled to 100). Interestingly enough, one country (Belgium) was excluded in the rest of the analysis of Isles, because their model did not fit well (!)*, this already means that all other confidence intervals are underestimations of the true uncertainty.
But even disregarding these facts, it is clear from the remaining confidence intervals that their width is a lot bigger in the 'law year'. which makes the above comments more relevant again: if one county is included/excluded, the effect prior to the law would be small, but after the law would be big. The reason that the interval suddenly gets large is due to 2 things: the fact that scaling was done 3 years before (and autocorrelation is becoming less important), but also the fact that car usage was seriously changed in some in the same years due to the oil crisis.
I also don't think the presented model fits the data nicely after the law year. The large jump in the 'exponential with jump' line seems too large in the law year, but it is that effect which is used to prove that there is a 8% increase.
(* Being Belgian, I can imagine that they somehow changed the way that statistics were gathered and that that caused the strange numbers. If however no reason is found it would be plainly wrong to exclude the data because they don't fit an exponential trend seen in other countries.)
Apart from that, and that is the core of his message:
The claims made for seat belt laws in all these other jurisdictions
rested on a deduction which assumed no risk compensation effect.
It is true that this assumption was not proved. But the reverse, that the risk compensation effect is not negligible is also not proven. Even if a correlation would be present proving that more fatalities occur after introduction of the law, it still does not prove a causal link.
So let's look at articles that cite him:
A study of driver behavior before and after a mandatory seat belt use
law in Newfoundland found that the benefits of such legislation are
not reduced by riskier driving, as has been suggested by some
A time-series analysis of Japanese data for the period 1966-85
estimates the effectiveness of seat belt usage and government safety
policies on various types of traffic-related fatalities. The results
indicate that these policies reduce the risk of death for both motor
vehicle occupants and non-occupants.
The efficacy of government safety policies on traffic-related fatalities: Empirical estimates from Japan
Applied Economics [APPL. ECON.]. Vol. 25, no. 3, pp. 409-412. 1993.