A factor that might reduce road vehicle energy consumption is shifting to other road or to non-road transport modes. According to the Federal Transit Agency of the U.S. Department of Transportation, the CO, intensity of passenger transport modes ranges from 0.22 lbs CO,/passenger-mile for both heavy rail transit and van pool to 0.96 lbs CO,/passenger-mile for a single occupant vehicle (SOV), with bus transit having an intensity of 0.64 lbs CO,/passenger-mile. The GHG emissions intensity of public transport modes is highly sensitive to passenger load. For example, bus transit with full seats would have GHG emissions 72% lower per passenger mile compared to average occupancy (Hodges, 2010).
However, such shifts appear likely to be modest. For example, based on policies already in place or soon to be adopted, the share of light duty VMT that might shift to buses by 2030 is estimated to be 0.2 percent in Canada, 2.7 percent in the U.S., 5.5 percent in China, and 9.5 percent in Mexico. Shifts fr om road transport to non-road modes, such as rail, are likely to be even more modest, ranging from 0% in the U.S. to 2.6% in China. If further policies are adopted to more aggressively promote mode shifts from LDVs to buses, the percentages increase to 0.6% for Canada, 8.2% in the USA, 16.5% percent in China, and 19.0% in Mexico (Facauha et al., 2012). Modal shifts from LDVs to buses would typically reduce gasoline consumption and increase consumption of diesel or CNG. Passenger mode shifts to rail would reduce gasoline consumption and typically increase electricity consumption. Although inter-modal shifts to non-road transport would reduce road transport energy consumption and emissions, they would lead to increases for other transport modes.
Light Duty Vehicle Fuels and Technology
In this section, the current status and recent developments in light duty vehicle fuels and technology are reviewed. While most of this review is based on the United States, trends in other parts of the world and internationally are also summarized.
U.S. registered on road vehicles grew from 8.000 in 1900 to 268 million in 2015, as shown in Figure 4 (FHWA, 1997; BTS, 2016). By 1950, more than half of the population of the USA lived in metropolitan areas, and by 2000 over 80% of the population were in urban areas. Most of this growth w'as in the suburbs. By 1960, a larger share of metropolitan populations was in suburban areas rather than in central cities. Concurrently, the population density of metropolitan areas has decreased (Hobbs and Stoops, 2002). The number of people who commute by walking or public transportation has remained at or well below' 10 million each between 1960 and 2010, whereas the number of people commuting by private vehicle increased fr om just over 40 million in 1960 to nearly 120 million as of 2010 (AASHTO, 2015; McKenzie and Rapiuo, 2011).
The share of fuel consumed by U.S. LDVs in 2015 w'as 97% for gasoline and 3 percent for diesel. The fraction of fuel type consumed for buses w'as 84% diesel. 11 percent CNG, and 4 percent gasoline (Davis et al., 2017).
As showm in Figure 5, from 2011 to 2018, USA light duty advanced vehicle technology sales totaled 4 million vehicles, including 2.95 million hybrid electric vehicles (HEVs), 0.57 million battery electric vehicles (BEVs), 0.48 million plug-in hybrid electric vehicles (PHEVs), and 4,800 fuel cell electric vehicles (FCEVs). Annual sales reached a record high of 653,765 vehicles in 2018. However, the 2018 U.S. sales of electric drive vehicles w'ere only 3.9 percent of all light duty car and truck sales, which totaled over 17 million vehicles. The three top-selling vehicles in 2018 were pickup trucks, including
Figure 4. Growth in registered vehicles in the United States from 1900 to 2015. Sources: Federal Highway Administration (FHWA) and Bureau of Transportation Statistics of the U.S. Department of Transportation (BTS/USDOT).
Figure 5. U.S. Annual sales from 2011 to 2018 of Hybrid Electric Vehicles (HEVs). Plug-m Hybrid Electric Vehicles (PHEVs), Battery Electric Vehicles (BEYs), and Fuel Cell Electric Vehicles (FCEVs) Source: Auto Alliance Advanced
Technology Vehicle Sales Dashboard.
the Dodge Ram at 536,980, Chevrolet Silverado at 585,581, and Ford F-series at 909,330. However, consumer choices of electric drive vehicles appear to be continuously expanding. In 2019, consumers in the USA could choose from 58 plug-in vehicle models, including 24 batteiy electric and 34 plug-in hybrid, compared to only a few in 2012.
In the U.S., the average CO, emission rate of light duty vehicles has decreased by almost 50% from 1975 to 2017, from an average of 681 g/'mi to 357 g CO,/mile (EPA, 2019). Over 98% of U.S. light duty vehicles are fueled with gasoline (Frey, 2018). Light duty vehicles include passenger cars and passenger trucks. Passenger cars include sedans, couples, and wagons. Passenger trucks include pickup trucks, vans, and minivans. Sport utility vehicles (SUVs), depending on their weight and drivetrain (i.e., two- wheel, four-wheel), can be classified as either passenger cars or trucks. In 1975, 81% of new LDVs were passenger cars. However, in 2017, passenger cars accounted for only 53% of new LDVs, with the rest being passenger tracks. For some manufacturers, particularly FCA, Subaru, GM, and Ford, passenger tracks accounted for over half of new vehicle distribution in the 2017 model year (EPA, 2019).
In the U.S., the average new vehicle weight among all LDVs reached a minimum of about 3,200 lbs in 1978 and has increased to over 4,000 lbs in 2017. On average, a 2017 model year vehicle has 70% more horsepower than a 1975 model year vehicle. The average USA LDV in 2017 had 233 hp. Although vehicle CO, emission rates increase with vehicle weight, the increase in CO, emission rate in g/mile for an incremental increase in vehicle weight or vehicle horsepower is much smaller in 2018 than in 1978, largely as a result of new vehicle technologies (EPA, 2019).
A growing share of the USA new production LDV fleet incorporates emerging technologies, including but not limited to turbo-charging, gas direct injection, continuously variable transmissions or transmissions with seven or more gears, and cylinder deactivation. There is a small but growing share of HEVs, PHEYs, and BEVs, with a veiy small number of FCEYs available in California. For gasoline LDVs, which comprise the vast majority of USA LDVs, multi-value engines with variable valve timing are becoming more common and are found in well over 90% of new vehicles. Almost 30% of LDV gasoline engines are expected to be turbo-charged in the 2018 model year, with 80% being 4-cylinder down-sized engines. With better designed automatic transmissions, the fliel economy advantage of manual versus automatic transmissions has not only been erased but automatic transmission vehicles are, on average, more fuel efficient than manual transmission vehicles, based on the 2016 and 2017 model years (EPA, 2019).
In Europe, diesel vehicles have been promoted over gasoline for some time because of perception that then- inherently more energy efficient engines, which operate at higher compression ratios than gasoline engines, would lead to lower CO, emissions per vehicle mile traveled. Per unit of energy, CO, emissions are about 5% higher for diesel fuel compared to gasoline. In the past, diesel engines have been about 30% more efficient than a comparable gasoline engine. However, with the recent advances in technologies adopted into production light duty gasoline vehicles, gasoline vehicles can have CO, emissions that are the same or lower than a comparable diesel version of the same vehicle, even though diesel engines retain some efficiency advantage, as has been demonstrated for a YW Golf based on chassis dynamometer and on-road measurements (Mock, 2019).
Increasing the octane rating of retail fuels, such as by increasing the blend ratio of ethanol, provides greater engine knock resistance and. thus, enables better engine performance under high load conditions. Long-term availability of higher-octane fuels would enable design of engines tailored to such fuels that would have higher compression ratios than typical of the current market. Engine efficiency can increase with increasing compression ratio. However, such reductions would likely be modest, at 9% or less for fuel octane increases of 10 Research Octane Number (RON) coupled with an increase in compression ratio of 3 (Leone et al.. 2015). The life cycle implications of higher ethanol blend fuels would har e to be assessed.
The emergence of autonomous vehicles could potentially disrupt light duty vehicle technology and operation. However, the implications of autonomous vehicles on energy use and GHG emissions is highly uncertain, with some studies predicting large opportunities for efficiency and emission reduction, and others identifying sources of latent demand and operational practices that could substantially increase energy consumption and emissions (Frey, 2018). For example, a shared autonomous vehicle (SAY) could provide the same travel sendee as ten self-owned personal vehicles, but w'ould have 11% more travel associate with reaching the next traveler. Results are sensitive to population density, congestion, and vehicle relocation strategies (Fagnant and Kockelman, 2014).
Life Cycle Perspective
The impact of LDYs includes “well to wheel” (WTW) processes that include production, distribution, storage, and in-vehicle use of transportation fuels. In addition to WTW processes, there is the separate impact of vehicle manufacturing, maintenance and end-of-life processes. Thus, the total life cycle impact of LDYs includes WTW and vehicle manufacturing, maintenance and end-of-life processes. For heavy duty trucks, rail, aircraft, and ocean-going vessels, WTW comprises 93% to nearly 100% of the total GHG emissions related to the vehicle and its operation. However, for LDYs, WTW is 67% to 83% of the total GHG impacts. While in both cases WTW accounts for the majority of GHG emissions, there is more to be gained from policies aimed at lov'ering the GHG intensity of manufacturing, maintenance, and end-of-life for light duty on road vehicles compared to other types of vehicles (Taptich et al., 2015). For all modes, reductions in intensity of GHG emissions for WTW aspects of vehicle usage can have significant impact.
An additional consideration in life cycle assessment of GHG emissions are the emission embodied in transportation infrastructure, such as roads, railroads, airports, marine ports, and so on. For example, the GHG emissions per passenger mile can be mostly from infrastructure for some light rail systems but, in terms of total life cycle GHG emissions, passenger rail is typically much lower emitting per passenger- mile than on road passenger transport. Vehicle operation contributes the majority of the GHG emissions impact for passenger road transport (Hodges. 2010).