By Chris McCahill
U.S. traffic deaths increased by 14 percent in the first half of 2015 compared to the first half of 2014, to nearly 19,000 deaths through June, according to the National Safety Council. If the trend holds, this year will be the deadliest for road users since 2007.
At the same time, traffic volume as measured by vehicle-miles traveled (VMT) also reached its highest six-month level since 2007—roughly 1.54 trillion miles in the first half of the year—according to the Federal Highway Administration.
The NSC connects the two trends, singling out the growth in VMT as a likely cause of the increase in deaths. However, NSC President Deborah A.P. Hersman also pointed out that the increase in driving is much smaller than the rise in traffic deaths and injuries. Hersman speculates that other factors, such as higher speed limits in some states and distracted driving due to mobile phone use, may be to blame as well.
Yet viewed over a period of 20 years, the association between deaths and VMT appears to be strong, albeit not at a 1-to-1 rate. Traffic safety appears to be highly sensitive to changes in the amount of vehicle travel taking place. As shown in Figure 1, traffic deaths dropped by more than 20 percent between 2007 and 2011, when VMT decreased by around 3 percent. The projected growth of each in 2015 shows a remarkably similar pattern, with both variables increasing but the death toll growing at a faster rate than VMT.
Why would a small change in VMT be related to a big change in deaths? One reason could be that discretionary travel likely plays a key role in VMT fluctuations. Discretionary trips often cover long distances on unfamiliar roads or they are made at night and on weekends, when serious crashes occur most often.
That is only one theory. If you have a thought on this relationship, please share it. If we receive good ideas we will summarize them in a subsequent post.
Whatever the cause, these findings suggest that U.S. traffic fatality rates—the number of deaths per million miles driven—are not following a constant downward trajectory, but instead are related to the amount of travel taking place. As shown in Figure 2, fatality rates were decreasing relatively gradually through 2007, when they dropped sharply, at the same time that VMT fell. Now with VMT growing in 2015, those past safety gains are also receding.
Several states, including California, Massachusetts, and Washington, have policies in place aimed at reducing VMT. Those policies are typically framed as solutions for congestion and/or greenhouse gas emissions. But stakeholders should be aware that VMT reduction also appears to be a powerful way to reduce injuries and save lives.
Chris McCahill is a Senior Associate at SSTI.