What's a human-factors expert? Glad you asked. A human-factors expert mixes psychology and engineering to explain how humans interact with their environments. They explain how things can be designed more safely, why a particular person might have been unable to perceive a particular risk, and much more. They might be hired to testify in many types of personal injury cases, especially premises and product liability cases.
If you're interested in learning more, look here: "Big Benefits in Using a Human Factors Expert," by A.C. Macris.