14.2: Sustainable Transportation Solutions

Pollutant and GHG emissions can be reduced in many ways. See Figure 14.2.1 for a simplified framework. GHG emissions may be treated as primary energy carbon intensity multiplied by vehicle and transportation efficiency multiplied by total travel demand.

Primary energy carbon intensity can be reduced by using lower-carbon fuels or low-carbon electrification, which is described later in this section. The energy needed to drive a specific distance can be reduced by improving both (1) vehicle efficiency and (2) transportation system efficiency, again assuming no induced demand (note that induced demand can be mitigated using the methods outlined in Section 14.1). This analytical construct—separating the determinants of emissions into carbon intensity, efficiency, and demand—can be used as a policy framework. A large carbon tax would address all three strategies, though it must be very large to be effective. In practice, an environmentally sustainable transportation solution will depend on a mix of policies and strategies.

Figure 14.2.1 General approach for calculating GHG emissions from transportation.

Vehicle technology

Figure 14.2.2 Average passenger car GHG emissions normalized by distance traveled for different regions. Reproduced with permission from the International Council on Clean Transportation 2019.

There has been considerable effort over the years to make vehicles more energy efficient, thereby reducing pollutant and GHG emissions. Many of these vehicle-based technologies are described in Chapter 13. In recent years, vehicles have benefited from lighter materials and more-efficient combustion engines and powertrains. In just the past few years, the greater use of electric powertrains, including gasoline-electric, plug-in hybrid, battery electric, and fuel cell electric technologies, has provided the promise of even much greater efficiency improvements. Overall vehicle efficiency improvements are illustrated in Figure 14.2.2 for different areas of the world. The improvements are due to a combination of aggressive policies and large technology investments by automobile manufacturers.

Figure 14.2.3 The number of electric vehicles (EVs) being introduced for different regions. Reproduced with permission from BloombergNEF 2018.

The increasing use of electric powertrains provides the promise for continued improvements in energy efficiency. The continuing drop in battery costs assures that this trend will continue into the foreseeable future. Figure 14.2.3 illustrates the number of electric vehicles (EVs) that are being introduced in different parts of the world.

Low-carbon fuels

Figure 14.2.4 Fuel utilization for the US transportation market, 2018. Data from Energy Information Administration 2019.

Another key strategy for reducing GHG emissions is to utilize low-carbon fuels. Today’s dominant fuel for transportation is gasoline, followed by diesel fuel and then jet fuel (Figure 14.2.4). All of these fuels are petroleum based and contribute significantly to CO₂ emissions. A number of other fuels are being introduced that are less carbon intensive, including bio-based fuels, electricity, and hydrogen. Their market share is currently quite small when compared with petroleum-based fuels. As described in Chapter 13, both electricity and hydrogen (as well as biofuels) can be utilized as effective energy carriers for transportation. Liquid biofuels have the advantage of being easily portable and having high energy density, like petroleum fuels. When made from crop and food wastes, liquid and gaseous biofuels have very low life cycle greenhouse gas emissions, sometimes even less than zero because waste disposal and methane leakage are avoided. With steady improvements in processing and farming, even biofuels made from crops, such as corn and sugarcane, tend to be significantly superior to petroleum fuels. As processes for converting grasses, trees, and other cellulosic material into liquids are improved (Chapter 18), resulting in even lower life cycle greenhouse gas emissions, biofuels will likely prove the superior alternative fuel for aviation and perhaps long-haul trucking, where portability and high energy density are valued most highly.

Life cycle analysis is necessary for comparing emissions of different fuels. A life cycle analysis includes all emissions from extraction through combustion, including, for example, the energy from farm machinery and carbon released from soils when growing biofuels, emissions from the operation of refineries, and the transport of fuels in tankers, pipelines, and trucks.

Table 14.2.1 provides rough estimates of life cycle emissions of different vehicle-fuel combinations, compared with gasoline-powered internal combustion engine vehicles. Note that these life cycle emission comparisons (per kilometer) could vary considerably since they rely on a large number of assumptions. For example, GHG emissions for an electric vehicle depend on the carbon intensity of the electricity used to charge the vehicle. This varies widely across space and time, from close to zero carbon in regions powered predominately by nuclear and low-carbon renewable sources, to carbon emissions exceeding those from internal combustion engines in places where electricity is generated from coal.

Table 14.2.1 Greenhouse gas emissions per kilometer, relative to gasoline-powered internal combustion engines, full energy cycle

Fuel/Feedstock	Percent Change
Fuel cells, using hydrogen from solar	−90 to −85
Cellulosic ethanol	−90 to −40
Battery electric vehicles, electricity from low-carbon sources	−60 to −25
Hybrid electric vehicles	−40 to −30
Battery electric vehicles, current US power mix	−40 to −20
Diesel	−25 to −15
CNG from NG	−20 to 0
Gasoline	—
Battery electric vehicles, new coal plant	0 to +10

Source: Adapted from Iteris. 2016. Connected Vehicle Reference Implementation Architecture. http://www.iteris.com/cvria/html/app...lications.html.

In general, petroleum-based fuels are convenient fuels for vehicles, since they have high energy density (per unit of volume), are easily portable and refuel vehicles quickly (because they are liquid), and have energy infrastructure already in place. However, petroleum-based fuels have high GHG emissions and emit large quantities of conventional pollutants. As a society, we have grown dependent on petroleum and have become quite cost-efficient at extracting and refining fossil fuels, resulting in low prices.

In some cases, though, alternative fuels are demonstrably cheaper than petroleum, even in the US, where petroleum products tend to have lower prices than elsewhere. For example, as of 2019, a kilowatt hour costs about $0.12 in the US on average—equivalent to about 3–4 cents per mile, an energy cost about one-third that of gasoline-powered cars. In areas with low-carbon electricity, these electric vehicles also offer significant GHG emission savings.

VMT reduction methods

As described earlier, total VMT in the US continues to grow at a steady pace (for example, see Figure 14.1.4). VMT was flat from 2008 to 2012, primarily because of the economic recession, but has been increasing since then.

In terms of potentially reducing VMT, we can refer back to a variety of mobility measures outlined in Section 14.1. In general, these include the following:

• Use pricing mechanisms to encourage users to reduce the number and distance of their trips and increase the number of passengers per vehicle. Several regions across the US are already increasing the number of toll and commuter lanes on their roadway networks, while cities such as Singapore, London, and Stockholm have implemented congestion pricing schemes that charge drivers to enter the city center.
• Provide incentives for using alternative modes such as transit and biking, as well as shifting work locations and schedules, for instance by telecommuting.
• Reduce urban sprawl, increase land use densities, and improve the mix of jobs and housing.

Transportation efficiency

Another important strategy for reducing emissions from transportation is to improve the efficiency of transportation system operations. As described above, today’s transportation systems are often congested, which wastes time, money, and fuel. This wasted fuel translates to increased pollutant and GHG emissions. Over the last several decades, a number of intelligent transportation system (ITS) techniques have emerged that are squarely aimed at reducing these environmental impacts. Referring back to Figure 14.1.8, ITS techniques and applications target three general areas: (1) congestion mitigation (for example, advanced signal control, predictive ramp metering, incident management), whereby congestion is reduced and speeds increased; (2) better management of speeds (on the right side of Figure 14.1.8) for different roadway types, using techniques such as variable speed limits and intelligent speed adaptation; and (3) smoothing of traffic by using techniques such as cooperative adaptive cruise control and speed harmonization. These “eco-friendly” intelligent transportation system technologies are typically categorized into three areas: vehicle systems, traffic management systems, and travel information systems.

Vehicle systems represent vehicle features and functions that allow a vehicle to “see,” respond to, and communicate with its surroundings. Sensors such as on-board radar and computer vision technologies enable a vehicle to monitor the distance to the vehicle in front and to detect when a vehicle is leaving a lane, and they support adaptive cruise control systems that allow a driver to select a desired speed and set a following distance. In addition, communication devices (for example, dedicated short-range communications, cellular) will likely be deployed to enable vehicle-to-vehicle, vehicle-to-infrastructure, and infrastructure-to-vehicle applications that are primarily focused on improving safety. It is important to point out that improved anti-collision systems may have a significant indirect energy and emissions savings: fewer crashes result in less congestion, allowing for higher average traffic speeds with less stop and go (Figure 14.1.8). In addition to safety applications, a variety of mobility and environmental applications have also emerged, as illustrated in Table 14.2.2. These applications take advantage of connected vehicle technology such as cooperative adaptive cruise control where vehicles communicate with each other to cooperatively manage following distance, braking, accelerating, and more. These technologies are allowing vehicles to become increasingly automated, with the goal of full vehicle automation coming in the next decade.

Table 14.2.2 Intelligent transportation system applications utilizing connected and automated vehicle technology

V2I Safety Red light violation warning Curve speed warning Stop sign gap assist Spot weather impact warning Reduced speed/work zone warning Pedestrian in signalized crosswalk warning (transit) V2V Safety Emergency electronic brake lights Forward collision warning Intersection movement assist Left turn assist Blind spot/lane change warning Do not pass warning Vehicle turning right in front of bus warning (transit) Agency Data Probe-based pavement maintenance Probe-enabled traffic monitoring Vehicle classification-based traffic studies CV-enabled turning movement & intersection analysis CV-enabled origin-destination studies Work zone traveler information Environment Eco-approach and departure at signalized intersections Eco-traffic signal timing Eco-traffic signal priority Connected eco-driving Wireless inductive/resonance charging Eco-lanes management Eco-speed harmonization Eco-cooperative adaptive cruise control Eco-traveler information Eco-ramp metering Low emissions zone managment AFV charging/fueling information Eco-smart parking Dynamic eco-routing (light vehicle, transit, freight) Eco-ICM decision support system

Road Weather Motorist advisories and warnings (MAW) Enhanced MDSS Vehicle data translator Weather response traffic information (WxTINFO) Mobility Advanced traveler information system Intelligent traffic signal system (I-SIG) Signal priority (transit, freight) Mobile accessible pedestrian signal system (PED-SIG) Emergency vehicle preemption (PREEMPT) Dynamic speed harmonization (SPD-HARM) Queue warning (Q-WARN) Cooperative adaptive cruise control (CACC) Incident scene pre-arrival staging guidance for emergency responders (RESP-STG) Incident scene work zone alerts for drivers and workers (INC-ZONE) Emergency communications and evacuation (EVAC) Connection protection (T-CONNECT) Dynamic transit operations (T-DISP) Dynamic ridesharing (D-RIDE) Freight-specific dynamic travel planning and performance Drayage optimization Smart Roadside Wireless inspection Smart truck parking

Traffic management systems have become more sophisticated with the advent of better sensor technology, more reliable communication channels, and advanced information processing. Transportation managers are better equipped to estimate traffic conditions, detect and remove traffic incidents, and craft better travel demand management strategies (that is, manage the number of vehicles on a congested roadway). The overarching goal of traffic management is to take full advantage of the existing roadway capacity, thus keeping traffic flowing smoothly at moderate speeds. In doing so, it will have a large impact in reducing energy consumption and GHG emissions from each vehicle. In addition, traffic management system strategies go even further by reducing the number of vehicles and VMT in the transportation network without compromising overall travel needs, thereby reducing the total contributions of energy consumption and emissions from the transportation sector.

Travel information systems provide information to drivers, such as route guidance systems, geolocation systems, and electronic payment systems. All of these systems add convenience to the traveler while reducing energy consumption and emissions. For example, a route guidance system will cut back on unnecessary travel that may occur when a driver gets lost or chooses a long, out-of-the-way path. En route driver information can reduce energy and emissions associated with driving around in search of a specific location or parking. Electronic payment systems also eliminate the need for a driver to decelerate the vehicle, idle while a manual transaction takes place, and then accelerate the vehicle back to a desired speed. If this payment can occur without slowing down, energy consumption and emissions are greatly reduced.

In general, environmentally friendly ITS applications (that is, specific ITS applications that reduce energy and emissions) have slowly been emerging over the last decade, as have safety and mobility programs mentioned in Table 14.2.2. Pioneering research programs in the US, the European Union, and other regions have made significant progress in developing and testing these ITS applications and technologies with a focus on environmental benefits. From these research programs, it is clear that specific environmental benefits can be maximized when different ITS applications are “tuned” so that emissions and energy consumption are reduced. The actual energy and emissions savings vary, but they are typically on the order of 5% to 20%. It is important to point out that there is not a single ITS technology solution that has demonstrated a large reduction in energy consumption and emissions. But since most of these applications are additive, greater benefits may be achieved when a combination of environmentally friendly ITS programs is put into place.