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Suggested Citation:"Chapter 2. Analytical approach." 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 2. Analytical approach." 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 2. Analytical approach." 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 2. Analytical approach." 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 3 Chapter 2. Analytical approach Fundamentally, two processes were at work over the period. The first process set the baseline level of safety that influenced the number of traffic deaths each year. This baseline level was the product of long- term trends in the factors that are known to affect traffic safety, such as safety belt use, improvements in the crashworthiness of cars, the spread of crash-avoidance technologies, alcohol consumption, speed limits, highway infrastructure, traffic enforcement and safety campaigns, driver license laws, and other efforts to reduce the number of fatalities on U.S. roads. The second process consisted of the factors that precipitated the sharp decline in fatal crashes and deaths in 2008-2011. The major event that occurred in this period was the recession that started in December 2007 and ended in June 2009 (NBER 2010). At the same time, the factors that influenced long-term trends in safety continued, such as incremental improvements in safety belt use, the introduction of more crashworthy passenger vehicles into the fleet, safety campaigns to improve driver behavior, infrastructure improvements, and other factors. Explaining the drop between 2008 and 2011 is a major goal of the project, but the explanation is undertaken within the context of overall trends in traffic safety over the period. 2.1 Risk vs. exposure The number of fatalities and injuries in crashes is the product of risk times exposure: Fatalities = Risk × Exposure Eq. 1 Risk is expressed as the number of fatalities (in this case) per some unit of exposure (vehicle miles traveled (VMT), the number of drivers or registered vehicles, etc.). Therefore, fewer traffic fatalities can be the result of lower risk of travel or less exposure to travel or some combination of the two. The risk/exposure equation suggests several possible general and high-level explanations for the drop. The dramatic decline 2008 through 2011 coincided with and followed a significant economic recession, so naturally the economic slowdown is one possible explanation, though not the only one. Possible explanations for the decline in traffic fatalities include the following hypotheses: 1. The economic decline resulted in reduced vehicle miles traveled (VMT). The reduction in exposure to traffic crashes in turn resulted in fewer fatalities. 2. The economy had a differential impact on travel on certain types of risky roads. For example, rural areas tend to have higher fatal crash rates than in urban areas for similar classes of roadways. (See Table 6-1 for fatality rates over the period.) 3. The economic decline led to changes in driver behavior such that they may have driven more cautiously. For example, drivers may have slowed down to conserve fuel, or they may have reduced leisure and discretionary driving, and thus reduced the risk of driving.

Page 4 4. The economic decline may have reduced travel by high-risk drivers. Younger drivers and other high-risk drivers may have reduced their travel disproportionately, thus reducing the exposure to travel of particularly risky drivers. 5. Other factors, operating independent of the recession, may have reduced the risk of travel. These factors include increased penetration of electronic stability control (ESC), improved crashworthiness, state programs to improve highway safety and reduce drunk driving, as well as other safety efforts, may have combined with the economic factors to reduce the risk of travel. 2.2 Use of the Haddon Matrix Factors that affected the incidence of fatal crashes and fatal crash injuries over the period were organized using the Haddon Matrix (Williams 1999). The Haddon Matrix provides a framework to cover comprehensively the factors related to crash risk (Table 2-1). Within the Haddon Matrix, factors are disaggregated into pre-crash, crash, and post-crash factors, and by human, vehicle, and environmental factors. The pre-crash row of cells affect the probability of crashes; the crash row includes factors that affect the severity of crashes; and the post-crash row covers severity outcomes. Human factors include who drives and how they are prepared and licensed (pre-crash); how they drive and what occupant protections they use (crash); and their resilience to injury (post-crash). Vehicle factors cover design and performance; how well or poorly they protect occupants in crashes; and post-crash systems such as event notification and technologies to reduce the probability of post-crash fires. The environment component includes roadway design and conditions that affect the probability of crash involvement; designs and structures that reduce the severity of crashes that occur; and the post-crash infrastructure to treat crash injuries quickly and effective. The utility of the Haddon Matrix is that it ensures that all aspects of what might be called the crash system—vehicle, occupants, and environment—are considered. Table 2-1 Haddon Matrix to organize factors related to crash risk and severity Human Vehicle Environment Pre-crash Factors that affect who drives and how they drive, such as graduated driver licensing and public information campaigns. Factors related to the condition and performance of vehicles, such as crash avoidance technologies. Factors that affect the road and built environment to reduce crashes, such as rumble strips, wide shoulders, etc. Crash Choices occupants make to protect themselves, such as safety belt use. Factors that affect how vehicles protect occupants in crashes, such as crush spaces and restraint effectiveness. Roadway design & structures to reduce crash severity. Post-crash Factors that affect how occupants respond to crash injury, such as age, physical condition. Vehicle design and technologies to reduce post-crash harm, such as crash notification. Better/faster access to crash scenes, improved EMS.

Page 5 2.3 Economic framework Drivers, vehicles, and the infrastructure environment are all embedded within current economic conditions. VMT is in part a response to pure economic demand. Increased economic activity is supported by increased travel (especially for commercial motor vehicles). The state of the economy also influences private drivers’ decisions about travel. Increases in gross domestic product (GDP) and higher incomes allow more discretionary travel. Increases in the unemployment rate, to the contrary, tend to reduce income and thus reduce travel and exposure to crashes. Reduced income might also affect the decision to buy newer vehicles, which are generally more crashworthy and which may be equipped with crash avoidance technologies. Similarly, increased fuel costs raises the price per mile of travel and would be expected to lower the amount of travel, and thus exposure to crashes. Economic contraction might also slow the funding of infrastructure improvement that could reduce the number and severity of crashes. The recession in this period officially began in December 2007 and ended in June of 2009. The period of relative stability in the number of traffic fatalities corresponds to the period up to the beginning of the recession. The initial period of the decline in the number of traffic fatalities occurred at the same time as the recession was occurring. The National Bureau of Economic Research defines a recession as “a significant decline in economic activity spread across the economy, lasting more than a few months, normally visible in real GDP, real income, employment, industrial production, and wholesale-retail sales.” (NBER 2013) Each of the changes identified in the definition of a recession would tend to affect traffic safety by affecting the amount of exposure as well as, more subtly, by decisions about who drives and where they drive. Referring to the initial discussion above, the problem consists of two parts. The first part is to identify the factors and trends that contributed to the underlying level of safety with respect to fatal injuries in traffic crashes. These are the factors that have contributed to the overall reduction in fatal crash rates over the past fifty years. The focus in the current project is on the years from 2001 to 2012, as a way of constraining the problem to a manageable timeframe, but the factors that have contributed to the overall downward slope to fatal crash risk are reasonably well-known. They include such things as (to work through the HM) graduated driver licensing, improved head-lights, improved road design with wide lanes and predictable geometry, increased safety belt use, more crashworthy vehicles equipped with airbags, increased use of safety belts, more effective guardrails and crash cushions, and quicker responses from EMS with more effective trauma care. 2.4 Study outline The next section outlines the international parallels, in which countries with advanced economies experienced a decline of similar magnitude in traffic fatalities coincident with the economic recession. Chapter 4 frames the discussion of evaluating the relative contribution of changes in risk and exposure to the drop in traffic fatalities. Chapter 5 discusses the data sources used in the analysis. This is followed in Chapter 6 by an analysis of the most important trends in fatal traffic crashes and VMT in the U.S. over the

Page 6 period 2001-2012. Chapter 7 provides the results of statistical modeling to understand the contribution of different factors to the drop in traffic fatalities, 2007 to 2012. Chapter 8 provides a complete discussion of the results. Chapter 9 outlines the major limitations of the present study and Chapter 10 describes needed future research and the data necessary to support it.

Next: Chapter 3. International parallels »
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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