A driver who fails to avoid a roadway hazard, such as a pedestrian, cyclist or other car, could have his pre-crash responses closely scrutinized if the crash is investigated. Driver responses are generally scrutinized in three ways: 1) response speed, 2) response choice, and 3) response magnitude.
The time it takes a driver to respond to a hazard is called the perception/response time. This time begins the instant a hazard becomes visible and ends when the driver begins to steer or brake. It captures everything from figuring out what the hazard is doing, deciding how to respond to the hazard, and then contracting the muscles needed to either steer or brake the vehicle.
Differences in individual drivers, hazards and environmental conditions affect the length of perception/response times. By adjusting for these differences, a driver’s actual perception/response time can be compared to those measured in detailed studies of driver behaviour in similar situations. This comparison can then establish if a specific driver’s response was normal or unusually delayed.
When faced with a complex hazard, a driver may not know if braking, steering or doing both is the best way to avoid a crash. For instance, a driver might brake and steer to the left to avoid a child who runs onto the road from their right. This would be the right choice if the child sees the car and stops, but could be the wrong choice if the child continues running (see figure). In the latter case, not steering at all would have avoided the collision.
This sort of after-the-fact analysis of a driver’s choice is affected by hindsight bias1, which means that, with the benefit of hindsight, the analyst can clearly see the best choice because he knows what the child actually did. At the time the driver had to choose a response, the driver did not know what the child would do.
For this reason, some legal jurisdictions acknowledge that drivers facing the “agony of collision” should not be held to a standard of perfection, but rather to a standard of what a “reasonable” driver would do in similar circumstances. Thus, information about how typical drivers respond to a situation can help judges and juries see that a specific driver’s response was reasonable, despite the unfortunate outcome.
Figure 1: With hindsight, we can see that steering left away from the incoming child, contributed to the collision, whereas not steering would have avoided an impact.
Most drivers do not apply their vehicle’s brakes maximally in an emergency. A study of drivers who encountered a child dummy suddenly crossing their path found that 86% of drivers had a two-stage braking response: they quickly applied their brakes to an initial submaximal level and then paused to assess the situation before braking harder2. Drivers who used two-stage braking were 17% more likely to strike the dummy than drivers who braked hard immediately. Here again an analysis that assumes a driver braked hard immediately might show the collision could be avoided when the driver’s less perfect braking behaviour was the same as a typical driver.
Response speed, choice and magnitude need to be considered carefully in the context of a driver’s specific collision conditions to properly assess whether their response was typical of most drivers or atypical and possibly responsible for the collision.
 Dilich M, Kopernik D, Goebelbecker J (2006) Hindsight judgment of driver fault in traffic accident analysis – Misusing the science of accident reconstruction. Transportation Research Record: Journal of the Transportation Research Board, No. 1980, 1-7.
 Prynne K & Martin P (1995). Braking Behaviour in Emergencies, 950969. Society of Automotive Engineers.