Research Project #3
Develop common denominators for safety measures

The problem: Each of the modes uses a different set of denominators for evaluating changes in safety risk.

• Some programs (e.g., general aviation, recreational boating) use primarily a count of deaths or injuries, since they currently lack a reliable measure of "exposure." The Federal Motor Carrier Safety Administration (FMCSA) and the Federal Highway Administration (FHWA) also use a count, by choice, although some exposure data are available.
• Some (National Highway Traffic Safety Administration (NHTSA), FRA) use primarily a rate based on vehicle miles, although the meaning and magnitude of a "vehicle" varies, and the numerator may be viewed as inconsistent with the denominator (e.g., person-deaths divided by vehicle-miles).
• One program (USCG maritime safety) uses the number of workers as a denominator for occupational safety, to enable comparisons with other occupations.
• One program (commercial aviation safety) uses the number of fatal accidents divided by flight (i.e., "vehicle") hours.
• One cross-modal program (grade-crossing safety) uses a composite denominator—the product of two "vehicle" mile counts—to attempt to account for the risk associated with the intersection of two modes.

This variety makes aggregation or comparison unworkable, which limits our ability to find differences in risk. It can mask important safety risks, and limits the public's understanding of the relative risk associated with different activities or transportation modes. Without good information, choices may be misdirected, and the public may be exposed to unnecessary risk.

What we need: We need some set of common denominators that can be used to characterize transportation safety in a comparable way for comparable circumstances. It should be possible to compare the risk of recreational boating, for example, to the risk of recreational flying or recreational driving. Similarly, the risk of working on a ship should be comparable (even if not equal) to the risk of working on a train, truck, bus, taxi or airplane.

Benefits: Denominators allow us to express safety in terms of risk, and comparable measures of risk allow us to determine the riskiest operating environment. That may help reveal risks we were not aware of. It may help focus effort on more serious safety threats, and it will provide useful information to the public. It will also help inform policy and resource decisions, and it may offer a better basis for justifying resource requests.