Problems Caused by Crash Count Reflecting Both Crash Frequency and Crash Severity

Nothing is known about unreported crashes; the information we have is of reported crashes only. Whether a crash gets reported depends on its severity. It follows by logic that, using only data about reported crashes, it is impossible to say whether a change or difference in crash counts reflects a change or difference in crash frequency or in crash severity. This indeterminacy has practical implications. Examples discussed are of the misattribution of overrepresentation in reported crashes of older drivers and of trucks to causal factors related to the frequency of crash involvement, and the misinterpretation by researchers of findings about the roll-over propensity of SUVs.