Five Reasons to Retain a Traffic Safety Human Factors Expert

This article was first published on the Expert Institute's website and is reposted here.


Human factors experts focus on how people interact with their environment and consider principles of psychology, design, and usability to optimize safety and performance.

Traffic safety human factors experts strive to answer crucial questions regarding road user behavior, expectations, capabilities, and limitations. For example:

  • Where do drivers look while they drive?
  • What work zone layout will minimize collision risk?
  • How much time do drivers need to change lanes or merge onto a freeway?
  • How do drivers change their behavior when they see pavement markings or signage?

Given their extensive knowledge of road user behavior and road design, traffic safety human factors experts are often called upon to consult as expert witnesses on motor vehicle collision cases. These experts are invaluable counsel in their ability to identify and explain the contributing factors of what caused a collision.

This article discusses five reasons to retain a human factors expert as you pursue traffic safety cases.

1) The Case Centers on Decisions or Actions of People

In the aftermath of a vehicle collision, the most common questions concern what a driver saw and when they saw it. This is known as perception-response time—the amount of time a person needs to detect, identify, decide, and respond to a situation. Perception-response time varies widely by situation and person. Age, impairment, experience, and expectations all influence a person’s perception-response time. An opposing expert who treats perception-response time as a constant value leaves a door wide open for a highly qualified human factors expert to set this record straight and strengthen your case.

If the opposition offers opinions on what a driver should do in a given situation, you should call a human factors expert to confirm the accuracy of those claims. A human factors expert will, instead, explain what an alert and capable driver would be expected to do. A “should do” approach is prescriptive and offers nothing more than how drivers should behave per the Driver’s Handbook. A human factors approach is based upon studies of observed human behavior and describes how people actually perform under various real-world scenarios.

2) The Case Relies on Eyewitness Statements

Human memory is not like a video recording where we can rewind the tape and review past events. Memories change over time and even the act of recall can change the memory. A person’s memory of the event can even be changed by the questions that are asked.

In a 1974 psychology study, Loftus and Palmer sought to understand whether leading questions or subsequent information could impact eyewitness accounts. [1] In the study, participants viewed films of automobile collisions and then answered questions about what they saw. Loftus and Palmer found that the verbs used in the questions influenced the responses. For example, “About how fast were the cars going when they smashed into each other?” Larger impact words – like smashed – elicited higher speed estimates than lower impact words – like bumped or contacted. One week later, participants who were asked with the verb smashed were more likely to affirm that they saw broken glass, although broken glass was not present in the film.

Human factors research has also shown that people are very poor at judging speed, distance, and time. Eyewitness statements based upon memory are likely to be even worse, especially for a traumatic event. [2] Expert opinion reports relying heavily on eyewitness testimony regarding speed, distance, or the timing of events cannot be considered reliable for the basis of forensic reconstruction.

3) Opposing Counsel has Retained a Human Factors Expert

You can count on the opposition’s human factors expert to opine on their assessment of the case event’s contributing causes. However, the scope of their opinion may be limited to issues that are least favorable to your argument.

Take, for example, a 2018 case of a train-vehicle collision.[3] The defendant rail operator’s human factors expert conducted sound measurements inside a similar vehicle under the same conditions as the collision. Even with the engine, fan, and radio all turned on, the expert was able to demonstrate that the train horn would have been audible to the driver. However, this expert had been tasked only with looking at the impact of the audible warning of the train horn.

With the help of their own human factors experts, the plaintiff was able to identify additional contributing factors to the collision, including uncovering issues related to insufficient sightlines, similarities to abandoned crossing locations, and the proximity of the rail crossing to a controlled intersection. With their own human factors experts on the case, the plaintiff was able to counter the defendant expert’s opinion and provide a more robust representation of the many interacting components at play.

4) Opposing Counsel has Retained an Engineer to Opine on Human Factors

Unlike many professional fields, there is no formal association that regulates who can call themselves a human factors expert. But even without an official governing body, the title of human factors expert is not necessarily transferable to neighboring fields, such as engineering.

In a 2020 case, for example, an engineer was able to qualify himself as an expert in human factors, even though his expertise consisted only of “a two-day course in 1998 and a week-long course in 2004”. [4] On the stand, when asked to explain the human factors modeling theory taught in the 2004 course, the engineer admitted he did not understand the basis for the modeling. Though most human factors experts have taken courses from engineering departments, most engineers have never taken a course in psychology.

In another case, an engineer knew enough about the human factors concept of inattentional blindness (when an observer fails to notice something that is fully visible) to attempt to apply it to a pedestrian collision. The collision occurred at a crossing on an onramp to a freeway in an urban area. The engineer opined that the “No Pedestrians” signage contributed to the inattentional blindness of the driver and, therefore, the city was liable.

But as a human factors expert, I disagreed and explained that’s not how attention works. You cannot make someone more likely not to notice a person in an environment by telling them that the person shouldn’t even be there. Any reference to pedestrians can only increase a person’s awareness of pedestrians, not decrease it.

5) Opposing Counsel’s Engineering Expert Conducted a Field Re-enactment

A common collision scenario is one where a pedestrian is struck by a vehicle at night. Engineering expert witnesses for such cases will often conduct a re-enactment of the event, complete with a similar vehicle, pedestrian, and nighttime conditions. Usually, engineers will measure sight detection distances for the driver to the pedestrian, potentially leading to the conclusion that there was sufficient time for the driver to hit the brakes and avoid the collision. This approach, however, gives an overestimated sight detection distance and a reduced perception-response time. The re-enactment was conducted by a driver who knew what to look for, where to look, and when to look.

The importance of expectation holds true for the driving task and this has been demonstrated in multiple studies going back to Roper and Howard in 1938.[5] Researchers Roper and Howard had participants drive a vehicle at night. The drivers then encountered an unexpected pedestrian in their path (a man-sized mannequin in dark clothing). The experimenters noted the distance to the pedestrian when participants first took their foot off the accelerator. In the second part of the study, when the drivers knew what they were looking for, the participants were instructed to repeat the drive and identify when they could first detect the pedestrian. On average, the drivers detected the pedestrian at twice the distance as before. Comparing the performance of a re-enactment observer who is expecting to see a pedestrian to the performance of a driver who is not expecting to see a pedestrian is not a fair comparison.

Closing Thoughts

When taking on a traffic safety case, don’t overlook the importance of traffic safety human factors expertise. From analyzing collision re-enactments to assessing eyewitness accounts, human factors experts are able to break down the factors that contributed to the event more acutely than an engineering expert.


[1] Loftus, E. F., & Palmer, J. C. Reconstruction of auto-mobile destruction: An example of the interaction between language and memory. Journal of Verbal Learning and Verbal behavior, 13. 1974. Pages 585-589.

[2] Green, M. Roadway Human Factors: From Science to Application. Lawyers & Judges Publishing Company, Inc. 2018. Page 44.

[3] Huang v Canadian National Railway Company, 2018 BCSC 1235 (CanLII). File Number S150260. Retrieved on 2020-10-09.

[4] R. v. Tabanao, 2020 ONSC 3501 (CanLII). File Number CR-18-097. Paragraph 304. Retrieved on 2020-10-09.

[5] Roper, V. J., and Howard, E. A. Seeing with motor car headlamps. Transactions of the Illuminating Engineering Society. Vol 33. Pages 417-438. 1938.