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Suggested Citation:"Chapter 4. Trends in risk and exposure." National Academies of Sciences, Engineering, and Medicine. 2019. Identification of Factors Contributing to the Decline of Traffic Fatalities in the United States from 2008 to 2012. Washington, DC: The National Academies Press. doi: 10.17226/25590.
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Suggested Citation:"Chapter 4. Trends in risk and exposure." National Academies of Sciences, Engineering, and Medicine. 2019. Identification of Factors Contributing to the Decline of Traffic Fatalities in the United States from 2008 to 2012. Washington, DC: The National Academies Press. doi: 10.17226/25590.
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Suggested Citation:"Chapter 4. Trends in risk and exposure." National Academies of Sciences, Engineering, and Medicine. 2019. Identification of Factors Contributing to the Decline of Traffic Fatalities in the United States from 2008 to 2012. Washington, DC: The National Academies Press. doi: 10.17226/25590.
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Page 9 Chapter 4. Trends in risk and exposure Figure 4-1 shows trends in fatal crashes, traffic fatalities, and crashes of all severities over the period. All crashes are estimated from the National Highway Traffic Safety Administration’s (NHTSA’s) General Estimates System (GES). Fatal crashes and fatalities are derived from NHTSA’s Fatality Analysis Reporting System (FARS) file.1 The trends are shown normalized to 2001. In other words, the number of crashes and fatalities for each year are shown as a ratio to the number in 2001. Normalizing in this way facilitates comparisons of trends in frequencies of significantly different magnitudes—there were around six million crashes per year but only around 38,000 fatal crashes. Four notable things can be observed about the trends illustrated in the figure: 1. The number of fatal crashes and fatalities in traffic crashes from 2001 through 2007 was relatively stable. In 2001, there were 37,862 fatal crashes, resulting in 42,196 deaths. In 2007, there were 37,435 fatal crashes with 41,259 deaths. 2. In contrast, the total number of traffic crashes, as estimated from GES, declined by 11%, from 6.3 million to 5.6 million. 3. Between 2007 and 2011, which was the low point in the series, the number of fatal crashes and fatalities dropped by about 20%, from 37,435 crashes and 41,259 fatalities in 2007 to 29,757 fatal crashes and 32,479 fatalities in 2011. 4. Though the pattern for fatal crashes and fatalities are very similar, their curves increasingly diverge over time, and this divergence is observed over the whole period. In addition, there was a greater drop in fatal crashes than in crashes of all severities. 1 In this report, all crash statistics are derived either from the FARS or GES. All traffic crash and related statistics from FARS or GES. See the data discussion in section 5 for more information about FARS and GES.

Page 10 Figure 4-1 Trends in crashes, fatal crashes, and fatalities, 2001-2012 Changes in the volume of exposure to fatal crashes, as measured by VMT or vehicle registrations, were not sufficient to account for the drop in fatalities, as is clear from Figure 4-2. The figure shows traffic fatalities, VMT, and vehicle registrations normalized to 2001. VMT and vehicle registrations increased steadily from 2001 to 2007, while the number of fatalities increased more slowly and tended to fluctuate slightly. VMT declined slightly from 2007 and then remained relatively constant through 2011, while traffic fatalities decreased sharply. Vehicle registrations plateaued in 2008, declined slightly to 2010, and then increased slightly. Thus, exposure, measured by VMT or vehicle registrations, either declined slightly or stayed relatively flat over the period, while traffic fatalities dropped dramatically. Figure 4-2 Trends in traffic fatalities, VMT, and vehicle registrations, 2001-2012

Page 11 That fatalities declined sharply while exposure remained relatively stable or only slightly declined implies that there was a change in the risk of travel. This is the necessary implication of equation 1: Fatalities = Risk × Exposure. Figure 4-3 shows the trends in VMT, traffic fatalities, and traffic fatality rates by VMT, relative to 2001. The rate of traffic fatalities per mile traveled declined relative to the rates in 2001. In the period from 2001 through about 2007, the decline was gentle, reflecting the overall reduction of risk reflective of the steady pressure to increase traffic safety, i.e., the first process of the analytical approach (section 2). The figure shows the ratio of the rates for each year to 2001, and by 2007, the ratio for fatalities per VMT was down to 0.90. The line for normalized fatalities is shown for comparison; the ratio to 2001 remained around 1.0 through 2007, when it was 0.98, before plunging to a low of 0.77 in 2011. This suggests the second process which is the target of the analytical approach. In contrast, VMT normalized to 2001 was 1.08 in 2007 and 1.06 in 2011. In effect, the reduction in the risk of travel pulled down the number of traffic fatalities, despite the relative stability of exposure as measured by VMT. This pattern is consistent with the hypothesis that the fatality risk of travel contributed to the significant decline in fatalities over the period. The decrease in exposure due to the recession and subsequent slowdown in economic activity contributed less. Figure 4-3 Fatality rates by VMT and vehicle registrations, and fatalities, normalized to 2001

Next: Chapter 5. Data »
Identification of Factors Contributing to the Decline of Traffic Fatalities in the United States from 2008 to 2012 Get This Book
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Between 2005 and 2011, the number of traffic fatalities in the U.S. declined by 11,031, from 43,510 in 2005 to 32,479 in 2011. This decline amounted to a reduction in traffic-related deaths of 25.4 percent, by far the greatest decline over a comparable period in the last 30 years.

Historically, significant drops in traffic fatalities over a short period of time have coincided with economic recessions. Longer recessions have coincided with deeper declines in the number of traffic fatalities. This report from the National Cooperative Highway Research Program, NCHRP Research Report 928: Identification of Factors Contributing to the Decline of Traffic Fatalities in the United States from 2008 to 2012, provides an analysis that identifies the specific factors in the economic decline that affected fatal crash risk, while taking into account the long-term factors that determine the level of traffic safety.

A key insight into the analysis of the factors that produced the sharp drop in traffic fatalities was that the young contributed disproportionately to the drop-off in traffic fatalities. Of the reduction in traffic fatalities from 2007 to 2011, people 25-years-old and younger accounted for nearly 48 percent of the drop, though they were only about 28 percent of total traffic fatalities prior to the decline. Traffic deaths among people 25-years-old and younger dropped substantially more than other groups. Young drivers are known to be a high-risk group and can be readily identified in the crash data. Other high-risk groups also likely contributed to the decline but they cannot be identified as well as age can.

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