Error, as James Reason and others have reminded us, is invariably a highly complex cocktail of failures: in cognition, in correct interpretation of data, in judgment and so on - all of which in turn can interact and compound one another. All those concerned with the management of safety should be familiar with James Reason's taxonomy of error: slips and lapses, mistakes (risk and rule-based) and violations (exceptional, routine and situational). The worrying thing about much safety practice however - and in road safety in particular - is that the case for things being deemed acceptably safe is based on assuming high levels of human reliability, that is human beings performing reliably all the time. With the management of high consequence risks (major hazard plants, etc.), relying on one or two people so that things always go right is never acceptable. The old adage "If it matters it must not happen, If it happens it must not matter" is a good starting point here. Human beings of course are incredibly smart but with all their manifold frailties and deficiencies they are also living breathing error machines. Much of the time we all depend for our safety on one another but we have a terrible tendency to over-estimate not only our own safety performance but that of those around us.