Chapter 5 - Transportation Performance: System Reliability, Safety, Energy Use, and Environmental Impacts

  • Traffic congestion costs a commuter, on average, 34 hours in delay and 14 extra gallons of gasoline per year—roughly double the costs of two decades ago.
  • All modes of freight transportation experience substantial congestion. Truck congestion alone cost $23 billion in wasted fuel and hours of delay in 2010.
  • While fatalities have declined over the last four decades, transportation still accounts for nearly 35,000 lives lost and over 2.2 million injuries each year.
  • The transportation sector accounts for 70.2 percent of total petroleum consumption in the United States. It is the second largest consumer of energy, next to the industrial sector.
  • Almost all transportation pollution emissions have decreased by 50 percent or more, despite an increase in vehicle use since 1990; only ammonia has increased.
  • Transportation is the single largest sector generating greenhouse gas emissions, accounting for about one-third of the U.S. total; transportation's 1.8 billion metric tons of carbon dioxide emissions in 2010 is down from 1.9 billion metric tons in 2005.

Performance of the U.S. transportation system includes how reliably and safely the system serves travelers and shippers and how the movement of people and goods on the system affects the economy and the environment. The elements of performance are highlighted in the USDOT Strategic Plan and the Moving Ahead for Progress in the 21st Century Act (Public Law No. 112-141) under the topics of safety, state of good repair, economic competitiveness, environmental sustainability, and livable communities. State of good repair and economic competitiveness are already covered in chapters 1 and 4, respectively. System reliability (a component of economic competitiveness), safety, and environmental consequences are discussed below. Livable communities will be discussed in a future edition of this report.

System Reliability

Transportation network capacity has not kept pace with growth in travel and commerce. The resulting congestion makes travel times longer and arrival times less predictable for both passengers and freight shippers. The Federal Highway Administration has identified seven root causes for transportation system congestion:

  1. Physical Bottlenecks—insufficient capacity in the system to handle the volume without delays.
  2. Fluctuations in Normal Traffic Volume—especially in regard to highways, some days of the week see much higher traffic volumes than others.
  3. Incidents—breakdowns or accidents on the highway, rights-of-way, and runways can delay traffic.
  4. Weather—can lead to changes in driver behavior that affect traffic flow.
  5. Special Events—create surges in demand on the system that are significantly greater than usual.
  6. Work Zones—repair or maintenance work on highways, railways, airports, or port facilities can reduce the ability of the system to handle normal traffic volumes.
  7. Traffic Control Devices—interruptions from railroad crossings, drawbridges, poorly timed traffic lights, etc. can cause delays that impact travel reliability [USDOT FHWA 2005a].

For the majority of travelers, the most frequently experienced form of transportation system delays occur on highways. Highway travel demand, as measured by vehicle-miles traveled, increased by 38.3 percent over the 1990–2010 period, while highway capacity, as measured by lane-miles, increased by about 6.6 percent [USDOT RITA BTS 2012a tables 1-35, 1-6]. About half of highway system delays are recurring; resulting from two of the root causes identified above—physical bottlenecks and recurring fluctuations in normal traffic volume [USDOT FHWA 2005b].

Road congestion, measured in terms of costs and hours, has increased in the past 20 years. The recession that began in December 2007 led to a significant drop in 2008 road congestion levels, which started to partially rebound in 2009 (table 5-1). In 2010, road congestion costs totaled $101 billion dollars in wasted time and fuel compared to $46 billion (2010 dollars) in 1990. Over the same period, total annual hours of delay grew from 2.4 billion hours to 4.8 billion hours while the amount of wasted fuel increased from about 0.9 billion to 1.9 billion gallons [TTI 2011].

Impact of Congestion and Delay on Passenger Travel

The Texas Transportation Institute (TTI) estimated that in 2010 the average commuter spent an additional 34 hours annually (the equivalent of 4 workdays) and wasted 14 gallons of gas sitting in traffic. Together, delay and wasted fuel cost $713 more per car commuter (driver and passengers) in 2010. For commuters in some areas, congestion costs were much higher: $1,495 in the Washington, DC metro area and $1,568 in the Chicago metro area [TTI 2011]. TTI has not quantified the air quality effects of congestion.

Drivers experienced the biggest bottlenecks at 173 highway interchanges, which alone create an estimated 250,000 truck hours of delay annually [USDOT FHWA 2005b]. In the largest urban areas, recurring congestion can last for up to 6 hours with the evening hours having the worst congestion and the highest percent of delay by time of day. In addition, congestion builds as the week progresses: Sunday is the best day and Friday the worst [TTI 2011]. Roadway congestion is no longer considered only an issue in very large urban areas [TTI 2011]. According to TTI, highway-sector operational improvements and high-occupancy vehicle lanes saved 327 million hours of delay and 131 million gallons of fuel in the 439 urban areas analyzed in 2010. By keeping additional cars off the road, availability of public transportation services is estimated to have saved commuters an additional 796 million hours of delay and an additional 303 million gallons of fuel [TTI 2011, pp. 14 and 15].

Congestion and delay are not limited to roadways. Amtrak reports that sharing track with other passenger and freight rail service providers is a major source of delay. This resulted in 21.9 percent of all Amtrak trains arriving late at their final destination in 2011 [Amtrak 2011a]. Seventy-two percent of the miles traveled by Amtrak trains are over lines owned by freight railroads [Amtrak 2011b]. The host freight railroads are responsible for 72.6 percent of the minutes of delay to Amtrak service in 2011 [Amtrak 2011a].

Airline flight delays are triggered by bad weather and heavy passenger flight volumes, among other factors. Approximately 20 percent of flights arriving at their scheduled destination are delayed by at least 15 minutes, although the percentage has declined every year since 2007. Between 2007 and 2011, late departures1 decreased from 21.1 to 17.3 percent (for all carriers and all airports), while late arrivals declined from 24.2 to 18.2 percent [BTS 2012c]. The average length of arrival delay for late flights is over 50 minutes (table 5-2). According to BTS, weather was responsible for nearly 38.67 percent of total delays in 2011 [USDOT RITA BTS 2012b]. These arrival and departure delays often ripple throughout the U.S. system and sometimes extend to airports overseas.

Freight Reliability

The Federal Highway Administration estimates that about 11 percent of the National Highway System (NHS) experienced recurring peak-period congestion in 2007 that resulted in passenger and freight traffic slowing below posted speed limits or encountering stop-and-go conditions [USDOT FHWA 2011] (figure 5-1). Highvolume truck segments of the NHS—sections of the NHS that carry more than 8,500 trucks per day and where trucks account for more than 25 percent of total traffic—experienced substantial congestion, particularly near metropolitan areas. In 2007, recurring congestion on high-volume truck segments slowed traffic on 4,700 miles of the NHS (out of the 4.1 million miles of public roads) and created stop-and-go conditions on an additional 3,700 miles (figure 5-2). Between 2007 and 2040, those congested miles are projected to increase by nearly 366 percent, assuming no change in network capacity and predicted increases in truck and passenger traffic [USDOT FHWA 2011].

According to TTI, truck congestion alone cost $23 billion in wasted fuel and hours of delay in 2010. While trucks accounted for approximately 6 percent of vehicle-miles traveled in urban areas in 2010, they shouldered nearly 26 percent of congestion costs [TTI 2011, pp. 1, 5, and 8]. According to the Association of American Railroads (AAR), congestion on freight railroads is becoming a widespread problem. AAR indicates that 30 percent of the rail network will experience service breakdowns and unstable flow conditions in 2035 if current capacity is not increased [AAR 2007].

On inland waterways, congestion is caused by several factors, including aging infrastructure and weather-related events (e.g., flooding,drought, and ice). In 2011, nearly 795,928 vessels passed through Federal and state locks. Of that total, about 37 percent experienced delays that averaged 1 hour and 29 minutes in 2011 for tows, compared to 1 hour and 10 minutes in 2005 [USACE 2012]. More than 50 percent of locks are over 50 years old, and many are too small for today's larger vessels. Because of their age, older locks are more susceptible to closure for maintenance and repairs [USACE 2009].

Transportation Safety

The rates and actual numbers of both injuries and fatalities in the transportation system have declined in recent decades. Transportation fatalities in 2011 were down about 22.5 percent from 2000 and 27.4 percent from 1990 (table 5-3). In 2011, the 2.3 million injuries were 30.9 percent less than in 2000. In contrast, the number of injuries in 2000 was down only slightly more than 1 percent from 1990 (table 5-4). These declines in the actual number of fatalities and injuries were made despite U.S. Census numbers that show a 24.9 percent increase in the U.S. population—from 249 million in 1990 to nearly 312 million in 2011 [USDOT RITA BTS 2013]. The majority of transportation fatalities and injuries take place on the Nation's highways, which carry most of the passenger and freight traffic in the United States. In 2011, 32,367 of the 34,387 transportation fatalities were highway related, accounting for 93.9 percent. Over 98.3 percent of transportation injuries occurred on the Nation's roads, accounting for 2.2 million out of the overall 2.3 million. That means that even though 2011 was the safest year since 1949 on the highways in terms of the number and rate of traffic fatalities [USDOT NHTSA 2011a], 89 people per day died and over 6,074 per day were injured on the highways. Transportation, including highways and the other modes, accounts for about one-third of the accidental deaths in the United States and is the leading cause of death for people between the ages of 5 and 24 [USHHS CDC 2012].


In 2011, 32,367 people died in highway motor vehicle crashes, including pedestrians and bystanders struck down by vehicles. The death toll in 2011 was 11,143 fewer than in 2005 [USDOT NHTSA NCSA]. Some of the 25.6 percent decline in fatalities may be due to fewer vehicle-miles of travel (VMT), which can be attributed, in part, to the 2007 economic downturn and rising fuel prices.2 However, the drop in the rate of fatalities outpaced the drop in the rate of VMT. As a result, the fatality rate per VMT, a key indicator used to measure the risk of death on the road, was the lowest on record. In 2011, there were 1.10 fatalities for every 100 million VMT on our highways. This compares to 1.46 fatalities per 100 million VMT in 2005 [USDOT NHTSA 2012].

The recent declines in fatality and injury rates are part of a longer trend. Over approximately the last four decades, the number of highway vehicle fatalities has declined by about one-third despite much higher volumes of highway traffic. There were over 50,000 motor vehicle fatalities annually between 1966 and 1973. In 1970, more than 4 fatalities per hundred million miles of vehicle travel were reported—nearly four times the 2010 rate. By 1990, the fatality rate had fallen by half, but this was still twice the 2011 rate [USDOT NHTSA 1994, table 2; and 2009, table 2]. Table 5-5 shows the distribution of fatalities among different users of the U.S. transportation system in 2011. Figure 5-3 shows how the fatality risk has changed since 1985 for selected modes of transportation. As shown in table 5-5, 94.1 percent of all transportation fatalities were due to highway motor vehicle crashes in 2011. Passenger car and light-truck occupants accounted for 61.8 percent of all the transportation fatalities, while motorcycles accounted for another 13.4 percent. In contrast to other highway users, both the share and number of motorcycle fatalities have gone up over the last decade during a time of significant increase in motorcycle use [USDOT RITA BTS 2012, table 2-4]. Motorcycle fatalities have increased 59.2 percent from 2000 to 2011. However, motorcycle fatalities per VMT have declined in recent years after peaking in 2005. Pedestrians, bicyclists, and bystanders struck by motor vehicles accounted for nearly 15.5 percent of transportation fatalities.

Even though crashes involving large trucks claimed 3,757 lives, only 635 of transportation fatalities were occupants of those trucks, with most of the remaining fatalities occurring among occupants of other vehicle types or bystanders (table 5-5). Accordingly, large-truck occupants accounted for about 1.9 percent of transportation fatalities.

Of all transportation related fatalities, 6.1 percent occur on modes other than highways. The 758 recreational boating and the 444 general aviation fatalities account for most of the other transportation fatalities in 2011. In addition, 505 people died trespassing on railroad tracks or highway-rail crossings, not involving motor vehicles. No commercial airliner crash occurred in 2011, although 41 people died in air taxi crashes (table 5-5).

Unlike the large and commuter U.S. air carriers that had no fatal accidents in scheduled passenger service in 2011, general aviation has accounted for at least 400 fatalities annually over the last 10 years. The general aviation fleet consists of 223,000 aircraft of various types. The 444 fatalities in general aviation accidents for 2011 is about one-third of the 1970 toll of 1,310 deaths when the general aviation fleet consisted of 131,743 aircraft [USDOT RITA BTS 2012, table 1-11].


An estimated 2.2 million people were injured in highway motor vehicle crashes in 2011. The percentage of injuries in highway crashes reported to be incapacitating has ranged from just under 10 percent to 12 percent in recent years, accounting for 175,000 injuries in 2010 (7.9 percent).3 Highway crashes represent over 99.1 percent of transportation injuries, with all other modes combined accounting for about 20,029 people injured in 2011. As with fatalities, the number of injuries from transportation crashes and accidents has gone down over time. In 1990, there were an estimated 3.2 million injuries related to highway vehicle crashes and about 38,799 injuries for the other modes (table 5-4)—about a million more injured people than in 2011.4

The Centers for Disease Control determined that the medical costs and productivity losses arising from injuries incurred in motor vehicle crashes eclipsed $99 billion in 2005. Motorcyclists accounted for 6 percent of the deaths but 12 percent of the costs, and pedestrians accounted for 5 percent of the injuries and 10 percent of the costs [USHHS CDC 2010]. The $99 billion in injury-related costs may understate the full economic impact of these crashes because it does not allow for non-quantifiable costs, such as future loss of earning power for those injured, costs for treatment of ongoing residual physical or emotional pain, or the long term costs for treatment of resulting disabilities.

Total Crashes and Accidents

Highway crashes account for the vast majority of transportation accidents [USDOT RITA BTS 2012, table 2-3]. However, the total number of highway crashes has decreased by about 16.3 percent over the past two decades. Air, railroad, and transit all reported significant declines during this time. Waterborne accidents decreased by 11.2 percent. Only pipeline, mostly hazardous material, reported an increase in the number of accidents.

Hazardous Material Transportation

Transporting hazardous materials requires special precautions, handling, and reporting, with separate safety regulatory systems in place for pipelines and vehicles. These special requirements recognize that incidents involving the transportation of hazardous materials can affect the environment in addition to creating the potential for risk of injury and death to persons. Table 5-6 shows 15,614 hazardous materials incidents in 2011. Of these, only 373 were related to highway and rail accidents, resulting in 13 fatalities; most incidents involve loading, unloading, package failure, and human error. In regard to personal safety, there were 13 fatalities arising from gas pipeline incidents, 12 fatalities related to hazardous materials accidents involving highways, and 1 fatality involving oil or hazardous liquid pipelines.

Occupant Protection Equipment

Improved occupant protection equipment and its use are a major contributor to reduced fatalities and injuries. The National Highway Traffic Safety Administration estimates that occupant protection devices, such as safety belts, air bags, and motorcycle helmets, saved the lives of 16,566 people on the highway in 2011 (table 5-7). While such devices are available for all vehicles, their success in improving safety depends on vehicle operators and passengers using them.

Safety belt use by occupants of cars, vans, and sport utility vehicles (SUVs) now averages about 86 percent. Only about 77 percent of pickup truck occupants wear safety belts. USDOT certified helmets offer some protection to motorcycle operators and riders in accidents and crashes, but only about two-thirds of people on motorcycles use helmets (table 5-8). At one time, most states required use of helmets by all riders, but the number of states that require all riders to wear helmets decreased from 26 in 1997 to 19 in 2012 [GHSA 2012].

Seventy percent of the 758 people who died in recreational boating accidents during 2011 drowned (table 5-5), and 84 percent of those who drowned were not wearing a life jacket [USDHS USCGS 2012, p. 6]. Boating safety classes are offered to boaters, but 65 percent have not taken one, according to a U.S. Coast Guard survey [USDHS USCG 2002].

Alcohol Use

There has been a 24.6 percent reduction in motor vehicle-related fatalities arising from drunk driving between 2005 and 2010 [USDOT NHTSA 2012]. However, the percentage of fatal crashes involving some presence of alcohol in the blood (either the driver or a person struck outside the vehicle) has remained in the 36.3 to 37.6 percent range between 2005 and 2010 and in the 31.1 to 31.8 percent range for blood alcohol levels of 0.08 percent and above (table 5-9).

Alcohol impairment is also a factor in many of the fatalities associated with recreational boating. These fatalities include not just operators of boats but also inebriated passengers falling into the water and drowning. The U.S. Coast Guard estimates that alcohol was a leading contributing factor in 16 percent of the 758 deaths in recreational boating accidents in 2010 [USDHS USCG 2012, p. 6].

Distracted Operating of Vehicles

The National Highway Traffic Safety Administration (NHTSA) estimates that 9.4 percent of the highway fatalities in 2011 involved drivers who were distracted by such activities as using a cell phone, texting, eating or drinking, using navigation systems or a map, or grooming themselves (table 5-10). As of December 2012, 39 states and the District of Columbia had laws banning texting while driving, and 10 states prohibit drivers' use of handheld cell phones (figure 5-4). No state had banned all cell phone use by drivers, although 33 states do so for novice drivers and 19 states for school bus drivers [GHSA 2013].

There is widespread awareness that distracted drivers are more likely to have accidents. While more than one-half of drivers queried in a 2010 survey said that their own driving performance was not adversely affected by using a hand held cell phone while driving, over 85 percent of these respondents said they would feel "very unsafe" riding as a passenger in a vehicle where the driver talked on a hand held device, texted, web surfed, or e-mailed while driving. Large majorities supported bans on handheld cell phone use and texting while driving [USDOT NHTSA 2011b, p. v1]. In January 2013, BTS released a special report that shows result from the 2009 Omnibus Household Survey, which indicate that the public is open to a ban on hand-held cell phone use and texting.5


Fatigue is a key safety concern, affecting both vehicle operators, such as truck and bus drivers, pilots, and railroad engineers, as well as workers responsible for safe system operations, such as air traffic controllers. Many highway crashes involve drowsy or otherwise fatigued drivers. Among commercial vehicle drivers in crashes, an estimated 12.8 percent were considered fatigued [USDOT FMCSA 2007, table 2]. A 2010 survey found that 3.9 percent of all drivers admitted to having nodded off or fallen asleep at the wheel in the prior month [AAA Foundation, p. 4].

Transportation Energy Use

Transportation system reliability affects energy consumption, as noted in the previous section, and energy consumption in turn affects the economy and the environment. The transportation sector of the U.S. economy is the second largest consumer of energy in the United States. In 2011, the transportation sector accounted for about 27.9 percent of total U.S. energy consumption (figure 5-5), compared to 31.4 percent for the industrial sector [USDOE EIA 2011a]. About 94 percent of the transportation sector's energy needs are supplied by petroleum, of which 44.8 percent of petroleum was imported in 2011 [USDOE EIA 2012a figure 3.3a]. The transportation industry accounts for about 70.2 percent of total U.S. petroleum consumption of 19.1 million barrels of oil per day (figure 5-6). The United States is the world's largest oil consumer [USDOE EIA 2011b].

Between 1990, when transportation accounted for 64 percent of U.S. petroleum consumption, [USDOE EIA 2011a] and 2010, transportation petroleum use increased by about 24 percent. In comparison, the commercial and residential sectors reduced petroleum consumption by 24 percent and 2 percent, respectively; while industrial petroleum consumption increased by only 2 percent over the 1990–2010 timeframe [USDOE EIA 2011a, tables 5.13a-d]. Other transportation energy sources include natural gas in a gaseous state that powers pipeline compressors, liquefied natural gas for some vehicles, electricity for transit rail and oil pipelines, and alcohol for blending with gasoline.

Energy Consumption by Mode

The highway mode accounted for more than 84.2 percent of total transportation energy consumption in 2010. Cars and other light-duty vehicles, such as pickup trucks, SUVs, and minivans, accounted for 60.9 percent of the sector's energy use in 2010, while single-unit/ combination trucks and buses accounted for 23.3 percent (table 5-11). Between 2007 and 2010, all highway modes registered a decline in energy consumption per mile, but single-unit trucks reported the biggest percentage decrease (7.6 percent).6 In addition to improvements in new vehicle efficiency in terms of miles per gallon, the drop in energy consumption for the highway mode can also be attributed in part to a 2.1 percent decline in vehicle-miles driven, the economic downturn, and rising fuel prices.

Among nonhighway modes, air transportation is the biggest energy user, accounting for approximately 6.8 percent of transportation energy use, followed by water, pipeline, and rail. From 2007 to 2010, the air carriers reduced their energy consumption by 14.9 percent, while water and rail reduced their energy use by 29.6 and 14.0 percent, respectively. Over the same period, transit and natural gas pipelines increased energy use. Natural gas pipelines, which used about 2.7 percent of the transportation sector energy, are the only mode that does not depend on petroleum for fuel. Natural gas pipelines use natural gas to fuel compressors, and oil pipelines use electric pumps to maintain flow. Oil pipeline pumping stations may use fuel oil to generate electricity for their own use.

Vehicle Efficiency

Energy consumption is affected by vehicle fuel efficiency. During the last 20 years, the average fuel efficiency of the total U.S. car and light-truck fleet improved very little while new vehicle efficiency increased nominally. The fuel efficiency of new cars rose by 14.1 percent, from 31.2 miles per gallon (mpg) in 2007 to 35.6 mpg in 2012 (table 5-12). New imported cars were more fuel efficient than domestic brands (table 5-12). During the same period, new light truck mpg increased by 8.2 percent, from 23.1 mpg to 25.0 mpg. Light truck is a category that includes vehicles such as pickup trucks, minivans, and SUVs.

Light truck sales nearly doubled between the 1990 and 2000 [USDOT RITA BTS 2012a]. However, after peaking at 52 percent in 2004, the light truck share of the market has declined to about 40 percent [USEPA 2011, table 1 and figure 1]. Because new passenger cars are 42.2 percent more efficient in terms of miles per gallon (35.6 mpg for new cars compared to 25.0 mpg for light trucks), an increase in the passenger car share of the market results in improved overall fuel efficiency simply because the more efficient vehicle represent a larger share of passenger cars and light trucks. Ongoing improvements in fuel efficiency for both segments of the market can be tracked to the Corporate Average Fuel Economy (CAFE)7 standard, established by Congress in 1995 to improve vehicle efficiency. The 27.5 mpg standard for cars was unchanged until new laws were enacted in December 2007, beginning with model year 2011.8 These standards established a fuel economy standard of 35 mpg by 2020 for all passenger cars and light trucks. Newly proposed CAFE standards are projected to require an average combined fleet average of 40.1 mpg in 2021 and 49.6 mpg in 2025 [USDOT NHTSA 2011].

Alternative Vehicles and Fuels

Initially spurred by the Energy Policy Act of 1992 (Public Law 102-486) and the Clean Air Act Amendments of 1990 (Public Law 101- 549) and more recently by higher fuel prices, the use of alternative and replacement fuels and the number of alternative-fuel vehicles has risen. Alternative fuels include liquefied petroleum gas, compressed natural gas, liquefied natural gas, E859 , biodiesel, and hydrogen as shown in table 5-13. Between 2000 and 2010, alternative fuel use, including biodiesel, nearly doubled from a yearly total of 332 million gallons to 693 million gallons (table 5-13). Even so, alternative and replacement fuels displace a small share of total gasoline consumption. In 2010, the United States consumed about 138 billion gallons/year, or 378 million gallons/ day of gasoline [USDOE EIA 2012b]. Thus at the 2010 level of use, total annual alternative/ replacement fuel use would replace about 2 days of total U.S. gasoline consumption. The use of fuel additives or replacements such as alcohols and ethers (oxygenates) far exceeds alternative fuels usage. Oxygenates are blended with gasoline to promote a more complete combustion of motor fuel, which reduces emissions. Between 1995 and 2009, oxygenate consumption doubled, from 3.6 billion gallons to 7.3 billion gallons [USDOE EIA 2011a, table 10.5].

Although not considered alternative-fueled vehicles as defined in the Energy Policy Act of 1992, hybrid vehicles have become very popular since the early 2000s as a replacement for traditional gasoline- and diesel-fueled vehicles. Powered by both gasoline/diesel and electric engines, hybrids provide more fuel economy, lower emissions, and fuel flexibility. Sales increased nearly 30-fold from 9,350 vehicles in 2000 to 269,000 in 2011. Escalating fuel prices and government support helped to increase hybrid vehicle sales over this period, but a combination of factors, including the economic recession; competition from fuel-efficient, gasoline-powered vehicles; and a drop in the number of vehicles manufactured has slowed the rate of hybrid sales in recent years, down 24 percent from their peak in 2007 (as shown in figure 5-7).

Environmental Impacts

As the U.S. transportation system moves people and goods, it impacts the environment. In addition to effects on the natural landscape and habitats, other unintended byproducts include air pollution, greenhouse gases, oil spills, and noise.

Air Pollutants

Air pollution from transportation has declined in recent decades, even though travel has increased. Figure 5-8 shows the estimated decline in most of the transportation emissions for six common air pollutants identified in the Clean Air Act and covered by EPA healthrelated standards. The six air pollutants include carbon monoxide (CO), nitrogen oxide (NO), volatile organic compounds (VOC), particulate matters (PM), sulfur dioxide (SO2 ), and ammonia. Such pollutants can harm human health and the environment. Of the six, four (CO, VOC, PM, and SO2 ) decreased by 50 percent or more despite an increase in vehicle use since 1990. Sulfur dioxide decreased by almost 100 percent. Ninety-five percent of U.S. automobiles are equipped with 3-way catalysts for pollution control, which may produce ammonia [NOAA ERSL, p. 5].

Greenhouse Gas Emissions

Transportation is the single largest sector generating greenhouse gas (GHG) emissions in the United States, accounting for about one-third of the U.S. total GHG arising from energy consumption in 2010. This is a result of the use of petroleum-based fuel for transportation, discussed in the previous section. While only about one-fourth of transportation GHG emissions are from the freight modes, their GHG emissions have been growing twice as quickly as GHG emissions from the passenger modes [USDOT FHWA 2011, p. 63].

Carbon dioxide (CO2) emissions from transportation reached highs in 2005 and 2007, and then declined during the economic downturn. The highway sector accounts for over 85 percent of total CO2 emissions from the U.S. transportation sector (table 5-14).

Fuel Spills

The USDOT Pipeline and Hazardous Materials Administration (PHMSA) publishes data on hazardous liquid pipeline spills (table 5-15). PHMSA reported 108,140 net barrels were recovered of the 139,017 gross barrels spilled in 2011. In addition, the U.S. Coast Guard estimates that oil spills from tankers and other sources discharged into U.S. waters were at a record low in 2009, with a nationwide total of 211,600 gallons of spilled in U.S. waters [USDHS USCG 2012].


According to an article published by the National Academy of Engineering, "Transportation noise can be annoying, disrupt sleep, interfere with communication, reduce property values, adversely impact health, and adversely affect academic performance [WAITZ 2007]." The article cites estimates from the Environmental Protection Agency that people in the United States exposed to average sound levels over 65 dB in 1981 included 16.3 million from highway traffic, 4.7 million from aircraft, and 2.5 million from rail traffic. The number of people exposed to aviation noise dropped to 500,000 with the introduction of quieter aircraft, and sound-mitigation efforts around airports have likely reduced the number further. More recent estimates for highways and rail are not available.

While noise barriers have reduced population exposure on major highways, increases in the volume, speed, and mix of traffic affect a wider range of locations. The Federal Highway Administration notes that highway traffic at 65 miles per hour sounds twice as loud as traffic at 30 miles per hour, 2,000 vehicles per hour sound twice as loud as 200 vehicles per hour, and 1 truck at 55 miles per hour sounds as loud as 28 cars at 55 miles per hour [USDOT FHWA 1980].

A current national noise exposure inventory across all modes of transportation does not exist in the United States. The "Noise Mapping England" website maintained by the Department of Environment, Food, and Rural Affair in the United Kingdom provides a capability that could further analysis of noise issues in the United States [UK DEFRA 2012].


System Reliability

American Association of State Transportation and Highway Officials (AASHTO). 2010. Transportation Reboot: Restarting America's Most Essential Operating System. July. Available at as of June 2012.

American Railroad Association (AAR):

  • 2011. Railroad Facts 2011. (Washington, DC: November).
  • 2007. National Rail Freight Infrastructure Capacity and Investment Study. Prepared by Cambridge Systematics, Inc. Washington, DC: September. Available at as of March 2012.


Texas Transportation Institute (TTI). 2011. 2011 Urban Mobility Report. September. Available at as of March 2012.

U.S. Army Corps of Engineers (USACE),

U.S. Department of Transportation (USDOT), Research and Innovative Technology Administration (RITA), Bureau of Transportation
Statistics (BTS):

U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA):

—2011. Freight Facts and Figures 2011. November. Available at as of March 2012.

— 2005a. Traffic Congestion and Reliability; Trends and Advanced Strategies for Congestion Mitigation. Office of Operations. Available at as of July 2012.

—2005b. An Initial Assessment of Freight Bottlenecks on Highways. Prepared by Cambridge Systematics. Available at as of March 2012.

Transportation Safety

AAA Foundation for Traffic Safety. 2010. Asleep at the Wheel: The Prevalence and Impact of Drowsy Driving. Available at as of April 2012.

Governors Highway Safety Association (GHSA).

  • Cell phone and Texting laws (February 2013) Available at as of February 2013.
  • New Study: No Progress in Reducing Motorcyclist Deaths (May 2012). Available at as of June 2012.

U.S. Department of Homeland Security (USDHS), United States Coast Guard (USCG):

U.S. Department of Health and Human Services (USHHS), Center for Disease Control (CDC):

  • 2010: "Cost amounts to nearly $500 for each U.S. licensed driver in one year" Press Release. August 25. Available at as of March 2012.
  • 2012: Deaths: Preliminary Data for 2010 (National Vital Statistics Reports, v. 60, no.4, January 11, 2012). Available at as of March 2012.

U.S. Department of Transportation (USDOT), Federal Aviation Administration (FAA). 2011. "FAA Administrator to Change Air Traffic Controller Scheduling Practices." Press Release. April 11. Available at as of April 2012.

U.S. Department of Transportation (USDOT), Federal Motor Carrier Safety Administration (FMCSA). 2007. The Large Truck Crash Causation Study. Available at as of February 2013.

U.S. Department of Transportation (USDOT). National Highway Traffic Safety Administration (NHTSA):

  • NCSA: National Center for Safety Analysis. Data Resource Website, FARS Data Tables. Available at as of March 2012.
  • 1994: Traffic Safety Facts 1994. Available at as of March 2012.
  • 2009: Traffic Safety Facts 2009. Available at as of March 2012.
  • 2011a: "DOT Estimates Three Percent Drop Beneath 2009 Record Low." April. Press Release. Available at as of July 2012.
  • 2011b: National Phone Survey on Distracted Driving Attitudes and Behaviors. Report No. DOT HS 811 555. Available at as of March 2012.

2012: Traffic Safety Facts: 2010 Overview. DOT HS 811659. June.U.S. Department of Transportation (USDOT).

U.S. Department of Transportation (USDOT). Research and Innovative Technology Administration (RITA):

  • ITS: Intelligent Transportation Systems Joint Programs Office. Available at as of April 2012.
  • BTS: Bureau of Transportation Statistics. 2013. Pocket Guide to Transportation, The American Landscape, available at as of January 2013.

2012. National Transportation Statistics. Online. Available at

2011: Hazardous Materials Highlights—2007 Commodity Flow Survey. Special Report. Available at as of March 2012.

Transportation Energy Use

U.S. Department of Energy (USDOE), Energy Information Administration (EIA):

U.S. Environmental Protection Agency (USEPA), 2011. Light-Duty Automotive Technology, Carbon Dioxide Emissions and Fuel Economy Trends: 1975 through 2010, Table 1 and Figure 1. Available at as of March 2012.

U.S. Department of Transportation (USDOT), National Highway Transportation Safety Administration (NHTSA). 2011. NHTSA and EPA Propose to Extend the National Program to Improve Fuel Economy and Greenhouse Gases for Passenger Cars and Light Trucks. Available at

U.S. Department of Transportation (USDOT), Research and Innovative Technology Administration (RITA), Bureau of Transportation Statistics (BTS):

Environmental Impacts

National Oceanic and Atmospheric Administration (NOAA). Earth System Research Laboratory (ERSL). Aeronomy Laboratory. Atmospheric Ammonia: Sources and Fate, A Review of Ongoing Federal Research and Future Needs (June 2000). Available at as of March 2013.

United Kingdom's Department for Environment, Food and Rural Affairs (UK DEFRA 2012), Noise Mapping England, available at as of August 2012.

U.S. Department of Homeland Security (USDHS), United States Coast Guard (USCG):

U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA):

—1980: Highway Traffic Noise, Report HEV-21/8- 80(20M), September 1980, available as of August 2012.

—2011: Freight Facts and Figures 2011. No. FHWA-HOP-12-002. 20. November. Available


1 A flight is considered delayed if it arrived at or departed from the gate 15 or more minutes after the scheduled arrival/departure time, as reflected in the computerized reservation system of that airline.

2 A National Highway Traffic Safety Administration analysis of the 2008 fatality decline found appreciable declines during prior recessions as well. The study concluded that the 2008 decline in fatalities resulted from large decreases in crashes involving young drivers, multiple-vehicle crashes, and crashes occurring over weekends. Areas where unemployment rates rose the fastest also had greater reductions in fatalities. U.S. Department of Transportation, National Highway Traffic Safety Administration, An Analysis of the Significant Decline in Motor Vehicle Traffic Crashes in 2008 (DOT HS 811 346), June 2010, p. 24.

3 USDOT NHTSA, Traffic Safety Facts (Final Editions), 1997 (table 53); 2008 (table 54), 2009 (table 54), 2010 (table 54), available at as of January

4 Due to a reporting change, the 1990 number for transit counted more kinds of injury circumstances than the 2010 number.

5 The complete report is available at

6 Light-duty vehicle fuel efficiency data for 2007 through 2010 are based on a new Federal Highway Administration methodology and are not comparable to previous years.

7 CAFE is the sales weighted average fuel economy (expressed in mpg) of a manufacturer's fleet of cars or light trucks with a gross vehicle weight rating of 8,500 pounds or less, and manufactured for sale in the United States for any given year. The Energy Policy Conservation Act of 1975 (Public Law 94-163) established the first CAFE standards in response to the 1973-1974 Arab oil embargo.

8 The Energy Independence and Security Act of 2007 (Public Law 110-140).

9 A fuel mixture containing 85 percent ethanol and gasoline or other hydrocarbon.