Human factors (driver perception and reaction) expert
Expert class library · Transportation · Either side
A human factors specialist, often a psychologist or engineer, who explains how drivers and pedestrians perceive hazards, how long they need to react, and how signs, lighting and road design shape what they expect. Courts admit the science of perception and driver expectancy, but exclude reaction-time and avoidance opinions that rest on a figure the expert cannot source or on facts the record does not supply, such as how much time and space a driver actually had.
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What this expert is retained to answer
- Where does the perception-reaction time come from: published research matched to this kind of driver and hazard, or the expert's habit in past cases?
- Does the avoidance opinion rest on record facts about distance, speed and sight lines, or on assumptions?
- Is the expert explaining how people perceive and react in general, or claiming to know what this driver saw and thought?
- Do the opinions about signs or road design connect to what the driver actually experienced?
- How does the analysis fit with the reconstruction of speeds and positions?
Methods
- Perception-reaction time analysis
- Driver expectancy and positive guidance analysis
- Visibility and conspicuity assessment
- Time-distance and avoidance analysis
- Attention, memory and distraction analysis
How the testimony is attacked
- Unsourced reaction time. A reaction time chosen from experience rather than research is a common target. In M.B. v. CSX Transportation the court excluded an engineer's train-operator reaction time, used in hundreds of prior cases, because he had never studied operators' reaction times and gave no foundation for the figure.
- Avoidance opinions without record support. Saying a driver could have braked or swerved requires facts about time, distance and space. In O'Connor v. Fairfax Taxi the Fourth Circuit affirmed excluding a reconstruction opinion that a cab driver could have avoided a head-on crash, because nothing in the record showed a clear lane or enough time to react.
- The driver does not remember the signs. Defendants argue sign and road-design opinions are irrelevant when the driver cannot recall the signs. In O'Brien v. Ohio Department of Transportation the appeals court held that human factors testimony on memory, expectancy and reaction time was still relevant to causation.
- Common knowledge. Opponents argue that how drivers react is within jurors' everyday experience, so expert testimony does not help.
- Speaking for the driver's mind. An expert may explain how people generally process information but should not claim to know what a particular driver perceived or intended.
What the public record shows
Courts have filed at least 14 opinions since 2015 that address this class of testimony under Daubert or Rule 702, as of October 4, 2026. Three that show how courts handle this class of testimony:
| Outcome | Case | Court | Year | Why |
|---|---|---|---|---|
| Excluded | O'Connor v. Fairfax Taxi, Inc.No. 25-1699 | 4th Cir. | 2026 | The Fourth Circuit affirmed excluding an accident reconstruction expert's opinion that a cab driver could have swerved or braked to avoid a head-on collision, both as an untimely disclosure and under amended Rule 702, because the opinion depended on facts the record did not supply, including a clear lane and enough time to perceive and react; summary judgment followed. |
| Exclusion reversed | O'Brien v. Ohio Department of Transportation2019-Ohio-724 | Ohio Ct. App. | 2019 | The appeals court held the Court of Claims abused its discretion by limiting a human factors psychologist's testimony on how a driver processed the signs approaching an intersection, including memory, positive guidance and perception-reaction time, because it was relevant to causation even though the driver did not remember the signs. After hearing the full testimony on remand, the Court of Claims again entered judgment for the department in 2022. |
| Excluded | M.B. v. CSX Transportation, Inc.130 F. Supp. 3d 654 | N.D.N.Y. | 2015 | In a case of a child struck by a train, the court excluded an engineer's opinion that the operator should have braked within 1.0 to 1.2 seconds, because the reaction time came from his experience in prior train cases rather than any study of train operators. |
Each case links to the free opinion text on CourtListener.
Under amended Rule 702
Since December 1, 2023, Rule 702 says expressly that the party offering an expert must show the court it is more likely than not that the testimony meets each requirement: that it rests on sufficient facts or data, uses reliable methods, and reflects a reliable application of those methods to the case. Questions about the basis of an opinion are no longer automatically matters of weight for the jury. For how the circuits have applied the amendment, see the Rule 702 tracker, which follows each court of appeals; for what that means for preparing or attacking this class of expert, see the guide on amended Rule 702.
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Last reviewed October 4, 2026. How this page is built: rulings are found by searching court opinions on CourtListener, and each one is read in the opinion before it is summarised here; outcomes are labelled by what the court did with the expert's testimony. No individual expert is named. This page summarises public decisions for orientation and is not legal advice; read the opinion before relying on any ruling.