Perception response time (PRT) is commonly known as reaction time and can be defined as the time that elapses from the instant that the driver recognises the existence of a hazard in the road, to the instant that the driver takes appropriate action, for instance, applying the brakes. The response time can be broken down into four separate components; detection, identification, decision and response.
Driver reaction times first appeared in the Highway Code in its 3rd edition 1947.
In that edition it appeared on the inside of the back cover and suggested that a vehicle would travel 1 foot for every mile per hour that the vehicle was travelling, while the driver reacted (‘thinking distance’).
Since then the same distances, albeit now in metric units, have appeared in each edition up to and including the current 2007 15th edition.
It is worth noting that even with the introduction of modern braking systems and better tyres that the stopping distances have not changed.
Maybe it is time the Highway Code was updated to reflect the stopping capabilities of the modern motorcar and the research that is now available relating to perception response times.
There has been a great deal of research in an attempt to establish a Perception Response Time (PRT), however, to date there have been only a limited number of studies that reflect the circumstances/hazards presented to drivers in a real-world situation.
Most research has taken place with volunteers on the road or in simulators in situations where they were aware that they were being tested and knew what their response had to be, i.e. push a button or tap the brake pedal. These tests produced stopping times/distances close to those in the Highway Code.
Accident Reconstructionists tend to cite research carried out by Olson (Driver Perception Response Times; Olson, P.L. Society of Automotive Engineers, 1989) who suggests a time of between 0.75 and 1.5 seconds, with 85% of the sample responding in 1.5 seconds.
This figure of 1.5 seconds appears time and time again in reports without any explanation of how it was obtained or any consideration of the circumstances or complexities of the particular accident that is under investigation.
Marc Green, in an article [Perception-Reaction Time: Is Olson & Sivak All You Need To Know? published in Collision, (2009), 4, 88-95], is critical of Olson’s research, highlighting the ‘actual study reveals that the methodology was biased to produce short PRTs (perception/response times)’.
In 1981 Heikki Summala tested un-expectant drivers and produced the paper, “Driver/Vehicle Steering Response Latencies” in which he states:
A steering manoeuvre was induced among unaltered drivers by the sudden opening of the door of a car parked near the path of travel, lateral displacement of 1326 cars was measured unobtrusively. When plotted as a function of the time available for it, the average lateral displacement started at a latency of about 1.5 seconds, reached its halfway point at 2.5 seconds and its maximum at somewhat less than 4 seconds.
On the basis of these and consistent earlier results from related experiments, it was recommended that, for safe operation, at least 3 seconds should be reserved for drivers to respond, by steering, to changes in the road environment.
It may be worth to noting that the AASTO (American Association of State Highway and Transportation Officials) handbook 1973 states:
Whenever the driver is confronted with a complex or highway situation and is required to make choices, judgements, and decisions, his response time may increase to 2, 3 or even 5 seconds.
The Olson, Dewer and Forber book ‘Forensic Aspects of Driver Perception and Response’ provides an overview of the current PRT research and states;
In an emergency situation, in which the hazard is relatively conspicuous and first appears directly ahead or nearly so, available research suggests that most drivers will respond in about 1.5 to 2 seconds.
These are simple or straightforward situations. Many situations are complex and must be dealt with as such.
The time between the appearance of a hazard and the response can be sub-divided into the following stages:
Detection: The detection interval starts when the hazard enters the driver’s field of view and ends when the driver has become aware that it is present.
Identification: Having become aware of something the driver must next obtain sufficient information to decide what action, if any, must occur.
Decision: After identifying there is a hazard the driver must decide what the appropriate action must be.
Response: At this stage the brain sends instructions to the necessary muscles to carry out the intended action. The perception-response time ends when the action is completed, i.e. when the foot lands on the brake pedal.
The time to detect and identify the hazard is the largest variable and can be dependent upon its size, orientation, whether it is easy to identify, the contrast between it and its background, luminance levels, the colour of the object, the location and any sources of glare, such as oncoming lights from another vehicle, complexity of the junction, movement of other pedestrians and/or vehicles, etc.
In all likelihood, 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. If we can agree on that, it is difficult to understand why so many professional investigators seem willing to assume that PRT can be expressed as a single value. The value quoted varies widely from one investigator to another in the experience of the authors, ranging from 0.5 to 3.5 seconds, and sometimes longer. By far the most popular value seems to be 1.5 seconds. One might argue with some apparent justification that they cannot all be correct, but depending on the situation under consideration, the PRT of a reasonably competent driver could be found anywhere within that range.
It should be appreciated that, because one driver may take a little longer to respond to a potential hazard than another driver, it does not mean that driver was not reacting in a reasonable time.
Therefore, investigators must be extremely careful when providing perception response times. All aspects of the collision should be taken into account and, importantly, investigators should not suggest that one figure suits all circumstances.
Jim Keenan, Accident Investigator, Keith Borer Consultants