There are several ways to measure traffic safety: the number of highway fatalities; the number of serious injuries from crashes; the economic loss to people involved in crashes; and the social cost of emergency response and accident-induced traffic delays. To understand these numbers in context, other measures are often applied to produce rates such as number of events per million miles traveled, per million registered drivers, and per million persons in the total population. Similar measures can reveal trends for narrower categories such as vehicle occupants and bicyclists. The fatality rate per 100 million vehicle miles traveled (VMT) is the most commonly cited measure of traffic safety, due in part to the seriousness of that outcome and in part to the fact that fatalities are closely tracked and unambiguous. Nearly every crash involving a fatality is investigated and clearly identified as a vehicle-related incident. In contrast, crashes involving injuries or property damage may be reported inconsistently by local public safety agencies or may not come to the attention of authorities; reported data on the numbers of serious crashes and of crashes involving injuries are estimated based on sampling. As Figure 1 indicates, the fatality rate has improved significantly since the passage of the first federal highway safety legislation in 1966. However, the improvement has not been continuous. The fatality rate dropped rapidly, from around 5.5 per 100 million VMT to around 3.2, in the first decade of federal oversight. As the fatality rate has become lower, reductions in that rate have necessarily become more modest: over the past 20 years it dropped from around 1.8 to 1.1. There have been four periods of abrupt declines in the fatality rate since 1970, all of which coincided with recessions; in each instance, the decline was followed by a period of little improvement or even an increase in the fatality rate as the economy emerged from recession. The number of people killed in traffic-related crashes dropped from a high of around 55,000 in 1972 to around 33,000 in 2013. This decline took place in the context of significant increases in both the number of licensed drivers and annual VMT (see Table 1 ). Although the fatality rate from motor vehicle crashes has fallen to a record low, other measures indicate that improvement in traffic safety has stalled. The rate of crashes resulting in serious injuries was significantly higher in 2013 than in 2008, and the overall crash rate has been rising ( Figure 2 ). Highway safety rates and trends vary by state. Fatality rates tend to be lower in more urbanized states. In addition, some states have relatively strict laws concerning safety matters, such as enforcement of mandatory seat belt use and requirements that motorcyclists wear helmets, while other states have fewer safety laws or enforce such laws less vigorously. The fatal crash rates for passenger cars, light trucks, and large trucks have fallen steadily since the 1980s. The same is not true for motorcycles. The fatal crash rate for motorcycles doubled between 1997 and 2005, then fell sharply in 2006 and 2007. Since then, there has been no further improvement. The motorcycle fatal crash rate in 2013 was slightly higher than in 1996 (see Figure 3 ). Motorcyclists are somewhat more likely to be involved in a crash than other drivers. However, a motorcyclist is much more likely to die as a result of a crash than is a driver of a car or light truck; in 2013 the likelihood of a motorcyclist dying in a crash was more than 26 times that of a passenger car occupant. Figure 4 compares the fatality rate for occupants of motorcycles and passenger cars. The causes of the recent trend in the motorcycle fatality rate—rising, then falling back to near the level reached in the mid-1990s but no lower (see Figure 5 )—are not clear. Motorcycle registrations increased during this period, but at a lesser rate than the increase in the fatality rate, and registrations continued to increase even as the fatality rate declined after the recession. The median age of motorcyclists has increased from 27 in 1985 to 41 in 2003 —a change that would be expected to reduce fatality rates, as older drivers are generally less likely to be involved in crashes than younger ones—but the average age of motorcyclists dying in crashes has also increased. The proportion of fatally injured motorcycle operators on a bike with an engine size greater than 1,400 cubic centimeters has risen from 1% in the mid-1990s to around 30% today, suggesting that the combination of older riders and larger, heavier bikes may be a factor in rising fatality rates. One factor that appears to be important in motorcycle deaths is alcohol. While 23% of passenger car drivers involved in fatal crashes in 2013 were legally intoxicated, the figure for motorcyclists was 27% (though this was down from 33% in 1995). Large trucks—vehicles with a gross vehicle weight rating greater than 10,000 pounds—represented only around 4% of registered vehicles in 2013. But the average large truck is driven far more than the average passenger vehicle, and large trucks are involved in crashes at a rate proportionate to the distance they are driven rather than their proportion of vehicles on the road. Large trucks accounted for 9% of total vehicle miles traveled in 2013 and represented 9% of vehicles involved in fatal crashes. The crash involvement rate of large trucks, like that of most other types of vehicles, has fallen significantly since 1988 (see Figure 6 ). Large trucks can be divided into two groups: single-unit trucks and combination trucks such as "semi" tractor-trailers. Only around 25% of all large trucks are combination trucks, but these account for 61% of total truck mileage. Combination trucks experience 1.7 fatal crashes per 100 million VMT, compared to 1.1 for single-unit trucks. This is noteworthy, since drivers of combination trucks do most of their driving on the Interstate System (see Table 2 ), on which accident and fatality rates are far lower than on the local roads on which most single-unit truck mileage is logged. Fatal large truck crashes often involve multiple vehicles. Over 90% of the persons killed in such crashes are occupants of the other vehicle(s) rather than occupants of large trucks (see Table 3 ), so the decline in fatalities in crashes involving large trucks is likely due at least in part to improved crashworthiness of cars and light trucks (see Table 3 ). Driver intoxication is a far less significant factor in large truck crashes than in crashes involving passenger vehicles. Of greater concern with respect to drivers of large trucks is fatigue. Where the average noncommercial driver might drive for a couple of hours, divided into two or more periods, on a typical day, a commercial driver might spend up to 12 hours driving. One survey of commercial drivers found that 25% reported having fallen asleep while driving at least once during the previous year. However, fatigue is not a risk confined to commercial drivers; in a 2006 study of crashes involving both passenger vehicles and large trucks, fatigue was listed as a contributing factor for 8% of the commercial drivers—and for 15% of the passenger vehicle drivers. Commercial drivers and vehicles are subject to random roadside inspections by law enforcement personnel checking for compliance with federal and state regulations. Each year safety groups sponsor a "Roadcheck," a three-day "blitz" of commercial vehicle inspections. The dates are announced months in advance, so while the inspections during that period are still random, drivers and trucking companies could be aware of a heightened possibility of being inspected. During the 2015 Roadcheck, roughly 70,000 vehicles and their drivers were inspected; 5% of the drivers and 18% of the vehicles were found to have violations so serious that they were placed out of service. The most common cause of out-of-service orders for drivers was violations of hours-of-service regulations; for vehicles, it was brake problems. The trend over time for vehicle condition has been improving; in 1991 around 5% of drivers and 35% of vehicles were placed out of service during the Roadcheck period. Miles traveled exposure data are not available for pedestrians, so pedestrian fatality rates are typically reported as a proportion of the total population. The pedestrian fatality rate fell by more than half between 1975 (7,516 pedestrians were killed, a rate of 4 per 100,000 population) and 2013 (4,735 pedestrians were killed, a rate of 1.5 per 100,000 population). How much of this reduction is due to safety measures (such as additional sidewalks, pavement markings, and lower speed limits) and how much to a reduction in exposure (as the proportions of workers walking to their workplaces and students walking to school have declined) is not known. In 2013, 14% of pedestrians killed were children under the age of 15. Consistent miles traveled exposure data is not available for bicyclists, and highway safety organizations including the National Highway Traffic Safety Administration (NHTSA) within the U.S. Department of Transportation (DOT) tend to report the number of bicyclist fatalities rather than calculating a rate based on population. The number of bicyclists killed in traffic crashes has generally declined since 1975, and has risen following the Great Recession. Data indicate that 722 cyclists were killed in 2004 (0.25 per 100,000 population), 628 in 2009 (0.20), and 741 in 2013 (0.24). The recent increase may simply be tracking the general increase in vehicle-related accidents and fatalities following the 2007-2009 recession, but it may also be related to increased bicycle usage. Although there are no reliable data on bicycle VMT, the proportion of American workers who bicycle to work rose from 0.4% in 2000 to 0.6% in the 2008-2012 period. The average age of bicyclists killed in crashes increased from 24 in 1988 to 44 in 2013. At least three in five bicyclists killed in crashes were not wearing helmets.