Accident Reconstruction Evidence: Perception Reaction Time

March 3, 2021

David Sandiford

In this article, David Sandiford reflects on one element of accident reconstruction – perception reaction time – and offers some useful thoughts and perspectives to consider when working with experts.

In higher value liability disputes arising from road traffic accidents it is essential to get the most out of your accident reconstruction expert. That means getting your expert to think about the case in the right way, helping you to maximise your client’s chances of a favourable outcome whilst looking for weaknesses in the other side’s position.

Perception reaction time (PRT) is the time that elapses from the moment a driver (or rider – although for ease I will refer to driver) recognises a hazard to the moment the driver starts to take perceived appropriate action. From that first moment of recognition, a driver is forced to embark upon an often complex series of decisions and actions.

As long ago as 1973 the American Association of State Highway and Transportation Officials Handbook advised that whenever a driver is confronted with a complex traffic situation and is required to make choices, judgements and decisions, response times may increase to 2, 3 or even 5 seconds.

Over the years a significant amount of research has been done into PRT. Expert reports are often accompanied by extensive lists of references. So when you are considering any expert report it is essential to look carefully at the references being pulled on, to master them and to drill down into the detail of the work – the methodology, the outcomes and any peer reviews. Does it support the conclusion contended for, or is it simply sitting within a myriad list of other references hoping never to be opened up and challenged?

By way of just one example, a report may still cite research by Olson & Sivak (1986) in which a large percentage of volunteer drivers had a PRT of 1.5s. It can be an attractive reference, but if one really gets into the detail of the methodology it arguably inevitably had a tendency to produce a relatively shorter PRT.

Olson did acknowledge that any individual engaged in the investigation of accidents at a professional level would agree that the events with which drivers are confronted vary greatly in their complexity and it was difficult to understand why some investigators were willing to assume that PRT could be expressed as a single value.

That, taken with the fact that one academic review of Olson & Sivak’s work identifies no less than seven factors which would arguably add time to the optimised PRT , serves to illustrate the wide range of possible disagreement around this one issue.

What could influence PRT in your particular case? If a situation confronting a driver requires conscious thought, that will slow response time down. Consider this scenario: a driver travelling at night becomes aware of something ill-defined in the distance. Once identified, the driver must then determine the distance, search his or her memory for previous similar experiences, try and decide what is going to happen if he or she responds or doesn’t respond, choose what that response should be and then choose how hard to execute that response if it is to be a brake or swerve….or will simply lifting off the accelerator be enough? In all of this a driver is continually trying to assess their ability to control the vehicle, its speed and direction.

In your case, does the hazard – be that a pedestrian, vehicle or otherwise – first appear in peripheral vision? Once detected in peripheral vision, an observer must deliberately shift their vision to dwell upon the hazard & make sense of it. If the hazard in the peripheral vision is missed the first time, further rapid eye movement is required to get back and pick up the hazard second time around.

In your case, is the driver quite reasonably shifting their gaze from the rear view mirror back to the road ahead – a process requiring adjustment and further time?

This is all a question of physics, speeds, times, distances, perceptions and human behaviour, all viewed long after the event through the lens of research and published papers. Has your expert taken all of the variables affecting PRT into account? Do the references cited stand up to scrutiny and support the conclusions? Is your expert looking at the case in the right way? Has the expert on the other side missed something or perhaps just plumped for a PRT without much in the way of analysis?

So we need to work with our experts with a constructive yet critical mindset. This approach, with the unavoidable hard (but nevertheless rewarding) graft that goes along with reading and mastering the references and published works on PRT, will stand you in good stead.

David Sandiford is a member of the Personal Injury: Defence team at Exchange Chambers. David has a large and busy higher value defendant personal injury practice. He brings over 25 years of experience of fighting and settling high value liability and quantum disputes in all types of personal injury and civil police actions.