People adjust behaviour to hold perceived risk at a target level, offsetting safety improvements.
Strong forms are contested; partial offsetting is well documented. Wilde's version — that the target level is fixed — implies safety interventions cannot work at all, which the evidence does not support.
In practice
- A fleet fitted with collision warning sees accidents fall, and by less than the manufacturer's figures implied, because following distances shortened.
- A resort that grooms a run more thoroughly finds skiers taking it faster, and the injury rate settles between the old figure and the promised one.
- Possible explanations
- The explanation is a target level of risk: people adjust behaviour until perceived risk returns to what they are willing to bear, so a safety improvement is spent rather than banked.
- In its favour
- Partial offsetting is well documented, and the general point, that safety measures change behaviour as well as outcomes, is now standard in transport research.
- Against it
- Wilde's strong version, in which the target is fixed and offsetting complete, implies that safety interventions cannot work at all, which is contradicted by large measured reductions in deaths.
- Standing
- Strong forms are contested; partial offsetting is well documented.
- Subject
- Risk & Reliability
- Essay
- How accidents actually happen