Since the 1970s, progress has been made in reducing the number of fatalities and injuries on our nation’s roads, but the numbers are still significant. From 1975 through 2001, annual fatalities decreased from 44,525 to 42,116, or about 5 percent. During the same period, the fatality rate per 100 million vehicle miles traveled, a common method of measurement, dropped from 3.35 to 1.51, or about 55 percent. This reduction in fatalities was considerable, given the growth in the number of drivers and vehicles on the road. For example, from 1975 through 2001, licensed drivers increased from about 130 million to about 191 million, and the number of registered vehicles increased from about 126 million to about 221 million. Figure 1 shows the yearly number of fatalities and the rate of fatalities per 100 million vehicle miles traveled. Injury and property- damage-only crashes also fell, going from about 6.8 million in 1988, the earliest year of available data, to about 6.3 million in 2001. The fatal, injury, and property-damage-only crashes have significant economic cost. The National Highway Traffic Safety Administration (NHTSA) recently calculated the economic costs for motor vehicle crashes in 2000 at more than $230 billion, or the equivalent of over $800 for every person living in the United States. NHTSA’s estimate of economic costs includes productivity losses, property damage, medical costs, rehabilitation costs, travel delay, legal and court costs, emergency services, insurance administration costs, and costs to employers. One of the most significant studies to date on the factors that contribute to motor vehicle crashes was the Tri-Level Study of the Causes of Traffic Accidents, conducted in the 1970s by the Indiana University at Bloomington Institute for Research in Public Safety. Referred to as the Tri- Level study, it investigated how frequently various factors contributed to traffic crashes. According to NHTSA officials, the Tri-Level study has been the only study in the past 30 years to collect large amounts of on-scene crash causation data. To provide researchers with insight into the factors that contribute to traffic crashes, collision data were collected on three levels, each providing an increasing level of detail, including 13,568 police- reported crashes; 2,258 crashes investigated by on-scene technicians; and 420 crashes investigated in depth by a multidisciplinary team. The study assessed causal factors as either definite, probable, or possible. The study found that crashes were caused by human (or driver-based) factors, environmental (roadway or weather-related) factors, or vehicle-related factors. As shown in figure 2, the study concluded that the human factors were definite or probable causes in about 93 percent of crashes, while environmental and vehicle factors contributed to about 33 and 13 percent, respectively. See appendix II for a more detailed discussion of the Tri- Level study. NHTSA’s mission is to reduce deaths, injuries, and economic losses resulting from motor vehicle crashes. As part of this responsibility, NHTSA conducts or sponsors research into the causes of motor vehicle crashes. NHTSA also conducts research on driver behavior and traffic safety to develop more efficient and effective means to improve safety. Three principal databases provide information about traffic crashes: the Fatality Analysis Reporting System (FARS), the Crashworthiness Data System (CDS), and the General Estimates System (GES). The FARS database contains information provided by the states on all vehicle crashes that result in the death of an occupant or nonmotorist within 30 days of the incident. The CDS database contains information from a detailed sample of about 4,000 minor, serious, or fatal tow-away crashes, annually. To obtain this information, teams of trained crash investigators visit the crash site and collect data on such elements as the damage to the vehicle and interior locations struck by the occupants. The GES database contains information from a nationally representative sample of police accident reports. This is NHTSA’s largest crash database, with information collected on over 50,000 crashes each year. The Federal Highway Administration’s (FHWA) safety mission is to reduce highway fatalities and injuries through development and implementation of a program of nationally coordinated research and technology innovations. Research is conducted in areas that address FHWA’s highway safety goals related to roadway departure, intersections, and pedestrians. FHWA is also conducting research in a number of areas that will partially focus on crash causation, including rollovers, speed management, intersection safety, and pedestrian and bicyclist safety. FHWA annually produces a highway statistics report, which consists of data on motor fuel, motor vehicles, driver licensing, highway-user taxing, state and local government highway finance, highway mileage, and federal aid for highways. FHWA also maintains a database, called the Highway Safety Information System. The system uses data on crash, roadway, and traffic variables collected by eight states to analyze a number of highway safety problems. These analyses range from identifying basic problems, to identifying the size and extent of a safety issue, to modeling efforts that attempt to predict future crashes from roadway characteristics and traffic factors. Motor vehicle crashes are complex events that rarely have a single cause. For example, it would be challenging to identify a single cause of a crash that occurred on a narrow, curvy, icy road when an inexperienced driver, who had been drinking, adjusted the radio or talked on a cell phone. It would likely be the combined effect of a number of these factors that contributed to the crash. In examining the causes of motor vehicle crashes, a number of experts and studies identified three categories of factors that contribute to crashes— human, roadway environment, and vehicle factors. Human factors involve the actions taken by or the condition of the driver of the automobile, including speeding and other traffic violations, as well as the effects of alcohol or drugs, inattention, decision errors, and age. Roadway environment factors that contribute to or are associated with crashes include the design of the roadway (for example, medians, lane width, shoulders, curves, access points, or intersections); roadside hazards (for example, poles, trees, or embankments adjacent to the road); and the roadway conditions (for example, rain, ice, snow, or fog). Vehicle factors include vehicle-related failures and vehicle design issues that contribute to a crash. In general, human factors are considered to be the most prevalent factor contributing to crashes, followed by roadway environment and vehicle factors. Although this report discusses these categories separately, they should be viewed in terms of how they can concurrently contribute to an unstable situation that results in a crash. Human factors involve actions taken by or the condition of the driver of the vehicle. They are considered the most prevalent factors by data, experts, and studies in traffic crashes. Human factors that can contribute to crashes include speeding and other traffic violations, as well as the effects of alcohol or other drugs, inattention, driver decision errors, and age. Driving either faster than the posted speed limit or faster than conditions would safely dictate can contribute to traffic crashes. Speeding reduces a driver’s ability to steer safely around curves or objects in the roadway, extends the distance necessary to stop a vehicle, and increases the distance a vehicle travels while the driver reacts to a dangerous situation. According to our analysis of NHTSA’s databases, from 1997 through 2001, speeding was identified as a contributing factor in about 15 percent of all crashes and about 30 percent of all fatal crashes. In addition, almost 64,000 lives were lost in speeding-related crashes. As shown in figure 3, we found that for every age category of drivers involved in fatal crashes, males were more likely than females to be involved in a fatal speed-related crash. In addition, younger drivers, regardless of sex, are the most likely to be involved in a speed-related fatality. From 1997 through 2001, 36 percent of male drivers and 24 percent of female drivers 16 to 20 years old who were involved in fatal crashes were speeding at the time of the crash. The percentage of speeding-related fatal crashes decreases with increasing driver age. A 1998 study by NHTSA and FHWA indicates that fatal crashes increased in states that raised speed limits. When Congress enacted the National Highway System Designation Act of 1995 (P.L. 104-59), which repealed the national maximum speed limit, the Secretary of Transportation was required to study the impact of states’ actions to raise speed limits above 55 and 65 miles per hour. The study found that states with increased speed limits in 1996 experienced approximately 350 more Interstate fatalities than would have been expected based on historical trends—about 9 percent above expectations. Concurrently, the Interstate fatalities experienced in states that did not increase speed limits in 1996 were consistent with pre-1996 trends. The Insurance Institute for Highway Safety also assessed the effects of speed limit increases. Its researchers found an increase in fatalities for a 9-month period in 1996 on Interstate highways and freeways, as compared with the previous 6 years—about 16 percent in 12 of the states that had raised maximum speed limits to at least 70 miles per hour by March 1996. In contrast, occupant fatalities increased only 4 percent on Interstate highways and freeways in the comparison group of states that did not raise speed limits. However, both of these studies are limited because they cover short time periods. According to a Transportation Research Board official, studies have confirmed a direct relationship between speed and crash severity. Once a crash has occurred—that is, a vehicle has hit another vehicle or a stationary object—the vehicle undergoes a rapid change in speed. While the vehicle decelerates rapidly, its occupants continue to move at the vehicle’s speed prior to impact until they are stopped by striking the interior of the vehicle, by impact with objects external to the vehicle if ejected, or by being restrained by a safety belt or an airbag that deploys.