It is generally accepted that, in comparing like-for-like accidents, a vehicle occupant wearing a properly fitted seatbelt has a significantly lower chance of death or serious injury.

The effects of such laws are disputed, stemming from the observed fact that no country is able to demonstrate a reduction in road fatalities due to passage of a seat belt law, though deaths have in some cases been migrated from drivers to other road users. This has influenced the development of risk compensation theory, which says that drivers adjust their behaviour in response to the increased sense of personal safety wearing a seat belt provides. In one trial habitual wearers and non-wearers were asked to drive round a course a number of times under the pretence of testing different seat belt materials for comfort. It was found that non-wearers drove consistently faster when belted than when unbelted (similar responses have been shown in respect of ABS braking and, more recently, airbags). It is also possible that the types of injury modelled in the trials were only a subset of potential serious injuries - for example, oblique impacts may produce twisting forces on the head leading to diffuse axonal injury, a particularly serious type of brain injury.