Two main paradigms have been proposed to explain why some states have a higher fatality rate than others. The "speed kills" paradigm assumes that the likelihood that an individual driver will suffer a fatal accident increases with speed. It relies on the physical concept that the damage suffered by a colliding body is proportional to its kinetic energy and, thus, to the square of its speed. An alternative to this traditional approach assumes that it is the variability of speeds among drivers, and not speed itself, that causes highway fatalities. The rationale for the "variance kills" paradigm is equally simple: more speed dispersion results in more passing, thus increasing the probability of a fatal collision. Current speed laws contain elements of both paradigms. For example, the imposition of a minimum speed limit on some highways is consistent with the "variance kills" paradigm, but not with the "speed kills" paradigm. A maximum speed limit is consistent with both paradigms.' To determine which paradigm better describes reality, Lave [10] tested the relationship between the fatality rate, the average speed, and the standard deviation of speeds for the crosssection of states. Using data aggregated by type of highway for each state, Lave found the standard deviation of speeds to be positively related to the fatality rate. In contrast, no significant effect of the average speed on the fatality rate was found. He concluded that "variance kills, not speed" and suggested that the focus of speed laws should be changed so that they coordinate speed, rather than limit it. This conclusion has recently been challenged by Fowles and Loeb [7], Levy and Asch [12], and Snyder [14]. Their empirical results using data aggregated by state support the notion that speed is a relevant variable in the determination of the fatality rate. Significantly, however, these papers also confirm Lave's [10] main discovery, namely, the positive relationship between speed variance and the fatality rate. In a reply, Lave [11] claims that aggregating data by state, rather than by type of highway, may spuriously raise the average speed coefficient.2

Car Accidents: Environmental Aspects

Citations

Speed, Speed Dispersion, and the Highway Fatality Rate

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Chapter 18 – Road accidents