14.1: Introduction and Background

Over the last century, the transport of both people and goods has grown dramatically, affecting our economy, society, and environment. The ability to move people and commodities between different locations has allowed our society to prosper, expanded our economic opportunities, and improved our overall quality of life. Our total vehicle miles traveled (VMT) per capita increased dramatically, even as the use of transportation modes narrowed. Our use of most other modes shrank, while our dependence on cars increased.

Unfortunately, with the benefits of greater automotive use has come many negative consequences, including (1) safety and health effects from pollution, crashes, and less walking; (2) reduced productivity and financial loss from congested traffic; and (3) environmental impacts, including local air quality and climate change. In this chapter we first provide background information on mobility and congestion, impacts of transportation on emissions, and the relationship between vehicle emissions and traffic. We then describe a suite of sustainable transportation solutions related to vehicle technology, low-carbon fuels, travel demand management, and intelligent transportation systems (ITS), including connected and automated vehicle technology. The US, the focus of this chapter, was the leader in creating car-centric lifestyles and cities but now is the home of many mobility innovations.

Mobility and congestion

Personal mobility—the ability to access work, school, health services, and other activities—is key to the successful functioning of a modern society. Greater mobility has greatly enhanced our productivity and quality of life. And the greater ease of shipping goods has greatly enhanced our economy.

Motor vehicles—cars, trucks, and buses, as well as motorcycles and scooters—are at the heart of personal mobility and goods movement.

Figure 14.1.1 Rapid increase in vehicles globally, historical and projected. Adapted from Sperling and Gordon 2010.

In the US and many other countries, the number of motor vehicles has rapidly increased, as shown in Figure 14.1.1. The US has the highest per capita motor vehicle ownership of any major nation, as shown in Figure 14.1.2. Indeed, US dependence on the motor vehicle has led to a vulnerable “transportation monoculture,” as outlined in the timeline shown in Table 14.1.1. Over time, the use of public transportation has steadily dwindled in the US to about 2% of trips. Most other nations are following this same path, becoming increasingly dependent on motor vehicles, though not to the same extent as the US.

During the twentieth century, rapid growth in motor vehicle use was accompanied by massive investments in roadway infrastructure, boosted in the US from the late 1950s into the early 1970s by the building of the Interstate Highway System. After the 1970s, new roadway construction was slowed by urban opposition and the high cost of building in urban areas. With more vehicle travel and slowed investment in road capacity, traffic congestion worsened.

Figure 14.1.2 Motor vehicle ownership, various countries. Source: compiled by authors.

Table 14.1.1 Evolution of transportation monoculture in the US

1859	First US oil well discovered
1908	Model T (with ICE) debuts
1926	US transit ridership reaches highest peacetime levels
1930	Car ownership reaches 200 for every 1,000 Americans
1947	Suburban building boom begins following World War II
1956	US Interstate Highway System launched
1973	Arab oil embargo constricts supply
1979	Iran-Iraq war doubles oil prices
2000	First hybrid electric cars sold in US
2003	Car ownership reaches 1.15 vehicles per American driver
2005	Motor vehicle population worldwide exceeds 1 billion
2008	Crude hits $140/barrel
2016	Crude drops below $30/barrel
2018	1 million EVs sold in US

Source: Adapted from Sperling, D., and Gordon, D. 2010. Two Billion Cars, Driving Towards Sustainability. Oxford University Press, New York, NY.

Increased roadway congestion has grown everywhere, from small cities to large megacities (see, for example, the Urban Mobility Scorecard at http://mobility.tamu.edu/ums). Roadway congestion is occurring during longer portions of the day and is delaying travelers and goods more than ever. The costs of congestion are large. These costs include the following (in the US):

• More than 3 billion gallons of fuel wasted annually.
• More than 2.6 million extra metric tons of CO₂ per year being emitted into the atmosphere.
• Nearly 7 billion extra hours in travel time, valued at roughly $160 billion—equivalent to 42 hours and $960 per rush-hour commuter.

Various solutions to roadway congestion exist. We could build more lanes to increase roadway capacity, though this is quite costly and induces more people to drive. Another possibility is to improve traffic system “operations” through traffic management techniques such as responding quickly to traffic incidents (more details on this in Section 14.2). In terms of managing “demand,” we can implement pricing mechanisms to limit the use of current roadway capacity. We could also promote shared mobility programs, such as car sharing and multi-passenger app-based ride sharing, increase the capacity of public transit, construct more walking and bicycling paths, provide greater incentives for use of alternatives to single-occupant private cars, and enact alternative work locations and schedules (for example, telecommuting). We could also implement urban design and land use planning that lead to less VMT, including (1) mixing residential and nonresidential land uses within a neighborhood, (2) increasing housing and industrial density, (3) allowing for innovative planning and zoning, and (4) implementing some type of growth management. These land use measures tend to reduce the distance that people travel, as destinations become closer together, and reduce the share of trips made by the private car.

Some policies that would alleviate roadway congestion would increase emissions if they created more capacity that encouraged people to drive more. And some policies would enhance mobility of physically and economically disadvantaged travelers. If policies are implemented well, though, they will continue to improve mobility and accessibility, while reducing congestion, greenhouse gas emissions, and local air pollutants.

Impacts of transportation on emissions

Figure 14.1.3 Contributors of GHG emissions in the USA in 2017. Adapted from US Environmental Protection Agency 2019.

As described in the previous section, today’s transportation systems depend on motor vehicles. As shown in Figure 14.1.3, by 2017 transportation as a whole had become the largest emitter of greenhouse gases in the United States. In fact, in 2017 the US Environmental Protection Agency (US EPA) reported transportation emissions to be 1,866 million metric tons of CO2 equivalent (MMT CO2e), approximately 29% of total US emissions. If one also considers the refining of fuels and the energy used to build roads, the emissions attributable to transportation are even higher. Where electricity generation has low greenhouse gas (GHG) emissions, transportation’s share of the total is much higher. For example, in California in 2017, transportation-related emissions accounted for 41% of California’s GHG emissions, or about 48% with refining of transportation fuels.

Transportation is also the largest source of directly emitted air pollutants that cause local air pollution, including carbon monoxide (CO), oxides of nitrogen (NOx), and particulate matter (PM). Motor vehicles emit other gases as well, including hydrocarbons (HC) that lead to the formation of ozone (O₃) and secondary PM. And air-conditioning units in vehicles increase fuel use and emit hydrofluorocarbon (HFC) refrigerant emissions that are short-lived climate super pollutants (Chapter 15). Vehicle-related pollution causes about 15,000 premature deaths annually in the US and between 184,000 and 242,000 globally.

Figure 14.1.4 Air pollution emissions yearly trends. Adapted from US Environmental Protection Agency 2019.

Fortunately, air quality has dramatically improved in US cities since the 1970s. Figure 14.1.4 shows that air pollutants from cars and light trucks were reduced by 73% from 1970 to 2016, even though VMT almost doubled.

Figure 14.1.5 California tailpipe HC performance standards for new lightduty vehicles. Source: Authors.

This huge reduction in air pollutants was due to technological advances in vehicle emission control technology and the reformulation of gasoline and diesel fuels. These technology and refining improvements are the result of increasingly stringent performance standards adopted by the US EPA and the California Air Resources Board (CARB). Hydrocarbon and carbon monoxide exhaust emissions from new light-duty vehicles have decreased by over 99% in the US, as shown in Figure 14.1.5. Massive improvements are also being achieved with trucks, ships, locomotives, and other transportation modes, but they are lagging improvements in cars. Chapter 9 discusses California’s air quality efforts in more detail.

Vehicle activity and emissions

In order to understand the breadth of potential transportation-related emissions reductions, it is useful to understand the relationship between vehicle activity and the corresponding emissions. There are several factors that play a role in how much a vehicle emits from the tailpipe. A typical driving trip will consist of idling, accelerating, cruising, and decelerating. The proportion of a trip spent in these different stages will depend on the driver’s behavior (for example, aggressive versus mild driving habits), the roadway type (for example, freeway versus arterial roadway), and the level of traffic congestion.

Figure 14.1.6 CO₂ emissions, in grams per mile, for a representative database of passenger vehicle trips in Southern California. Reproduced with permission from Barth and Boriboonsomsin 2009.

We can create histograms of emissions for large regional areas. Data collected from passenger vehicles in Southern California are presented in Figure 14.1.6. As indicated, most trips produce about 330 grams of CO₂ emissions per mile, corresponding to approximately 26 miles per gallon of fuel economy. Other trips, however, produce far less or far more CO₂ emissions per mile, depending on the specific driving pattern. This variation comes from the driver’s behavior, the roadway type, and the level of traffic congestion. Other vehicle types will have quite different emissions depending on their weight, power, and other vehicle factors.

Electric vehicles have zero tailpipe emissions, but their energy efficiency (and upstream power plant emissions) is affected by these same factors.

Figure 14.1.7 Emission-speed plot of individual trips or trip segments. Reproduced with permission from Barth and Boriboonsomsin 2009.

Figure 14.1.8 Possible use of traffic operation strategies in reducing on-road CO₂ emissions. Reproduced with permission from Barth and Boriboonsomsin 2009.

If one plots emissions against speeds, one observes a U-shaped pattern as shown in Figure 14.1.7. The resulting emissions-speed curve can be generalized for different types of vehicles, different driving behaviors, and different types of trips, as shown in Figure 14.1.8. This generalized curve can then be used as a tool for evaluating different carbon reduction schemes for transportation management. The upper line in Figure 14.1.8 shows a representative emissions-speed curve for typical traffic. We can use this curve to examine how different traffic management techniques can affect vehicle emissions such as CO₂. The lower line represents the approximate lower bound of CO₂ emissions for typical internal combustion vehicles traveling at a constant steadystate speed. Several important results can be derived from this figure:

• If congestion reduces the average vehicle speed below 45 mph (for this particular freeway scenario), emissions increase. At these lower speeds, vehicles operate less efficiently and spend more time on the road, resulting in higher emissions. In this scenario, congestion mitigation programs will directly reduce emissions.*
• If moderate congestion reduces average speeds from a free-flow speed over 70 mph to a slower speed of 45 to 55 mph, this moderate congestion can reduce emissions (because emissions are higher and energy efficiency is lower at very high speeds). With no congestion, average traffic speeds can increase to over 65 mph, increasing emissions.
• Smoothing stop-and-go traffic will reduce emissions.
• Electric vehicles powered by renewable energy will have near-zero life cycle emissions; if electric vehicles are powered by fossil fuels, emissions from power plants will be lower at lower speeds, for the same reason as for combustion engine vehicles but even more so because regenerative braking captures energy in stop-and-go traffic.

*This analysis assumes that the travel demand won’t change when congestion is reduced. However, experience has shown that if congestion is reduced on our roadways, there is often “latent” demand that will increase traffic on those particular roadways. This “induced demand” or “rebound effect” is described in further detail later in this chapter.