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Commercial truck driver Hours-of-Service (HOS) rules are periodically revised to reduce driver fatigue and improve driver health in costefficient ways. HOS research must demonstrate causal relationships between HOS parameters and important safety outcomes. Thus, two scientific requirements are internal validity (demonstration of true cause-effect relationships) and external validity (generalizability to important real-world consequences). HOS rules ostensibly act by mitigating driver fatigue; thus, dependent measures in most HOS studies must verifiably capture and measure alertness/fatigue. That is, dependent measures must have construct validity. This paper examines these basic scientific validity requirements and finds significant threats to them within the designs of major U.S. HOS studies. Lessons learned apply to many other areas of behavioral research. Improved designs and compensatory methods are suggested for addressing validity threats and thereby increasing internal, external, and construct validity. Improving scientific validity would in turn raise the likelihood that HOS changes based on research would be safety-effective in the real world of truck transport on our nation’s highways.

Threats to Scientific Validity in Truck Driver Hours-of-Service Studies