EXAMPLE 4 Measuring highway safety

Roads got better. Speed limits increased. Big SUVs and crossovers have replaced some cars, while smaller cars and hybrid vehicles have replaced others. Enforcement campaigns reduced drunk driving. How did highway safety change between 2007 and 2012 in this changing environment?

We could just count deaths from motor vehicles. The Fatality Analysis Reporting System says there were 41,259 deaths in 2007 and 33,561 deaths five years later in 2012. The number of deaths decreased. These numbers alone show progress. However, we need to keep in mind other things that happened during this same time frame to determine how much progress has been made. For example, the number of licensed drivers rose from 206 million in 2007 to 212 million in 2012. The number of miles that people drove decreased from 3031 billion to 2969 billion during this same time period. If more people drive fewer miles, should we expect more or fewer deaths? The count of deaths alone is not a valid measure of highway safety. So what should we use instead?

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Rather than a count, we should use a rate. The number of deaths per mile driven takes into account the fact that more people drive more miles than in the past. In 2012, vehicles drove 2,969,000,000,000 miles in the United States. Because this number is so large, it is usual to measure safety by deaths per 100 million miles driven rather than deaths per mile. For 2012, this death rate is

The death rate fell from 1.4 deaths per 100 million miles in 2007 to 1.1 in 2012. That’s a decrease—there were 21% fewer deaths per mile driven in 2012 than in 2007. Driving became safer during this time period even though there were more drivers on the roads.