Modal share studies from the Environmental Protection Agency (2006) show that freight greenhouse gas (GHG) emissions increased by 46% between 1990 and 2003 in the US. The majority of this increase was due to a switch to less sustainable, more energy intensive modes. The GHG emissions from 1990 to 2003 exceeded improvements in the energy efficienty of the modes. GHG emissions by heavy-duty trucks increased by 57%, while those of light-duty trucks increased by 51%. Non-road modes had either a decrease, or a negligible increase in emissons from 1990 to 2003. Environmental Protection Agency (2006) also states that the improvement in the energy efficiency between 1970 and 1980 is primarily due to the replacement of less fuel-efficient vehicles by new vehicles.
Rail consumes less energy per ton-miles of transportation, and utilizes 90% and 80% less energy than truck and ships, respectively (in terms of Btus). According to Freight Rail Works (2014), a member of the Association of American Railroads (AAR) that operates over 140,000 miles of rail network across North America, US transports one-third of its exports by rail, and approximately 40% of the intercity freight volume. Freight rail’s contribution to total GHG emissions caused by the transportation sector, including passenger cars, is 2.3%, while that of the trucking industry is 22.4%. This relation of mode share and emissions in the US is explained in detail in the section below.
The mode share of all commercial freight activity in the US in terms of tons of commodity transported for the years 1993, 1997, and 2002 is displayed in Table 4. The table shows that for those three years, trucking maintained its position as the predominant mode in terms of weight of commodity transported, while air was the most negligible of the modes. In terms of tons, there was not a significant change in the mode share for truck and rail over the period. For truck, there was an increase in mode share by 9% between 1993 and 97, and a decrease by 2% from 1997-2002. The rail mode share decreased by 8% between 1993 and 1997, and increased by 10% between 1997 and 2002. The share of tonnage of commodity transported by water and multimodal combinations decreased by 3% and 11%, respectively, in both 1993-97 and 1997-2002. This information is based on the Commodity Flow Survey (CFS) data plus additional estimates from the Bureau of Transportation Statistics (BTS).
Table 5 shows the mode shares in terms of ton-miles of commodities transported in 1993, 1997, and 2002. In 1993 all predominant modes–truck, rail and waterways–had an almost equal mode share, nearly 25%. Air mode had the least mode share, less than 0.5% from 1993 to 2002. In 1997 the mode share of waterways decreased to 20%, while that of truck and rail increased to 28% and 27%, respectively. Although the air mode share is 0.37%, we can see a 50% increase in the mode share of air between 1993 and 1997. By 2002 the mode share of truck increased to 32%, while the mode share of rail remained constant, close to 27% from 1993 to 2002. From the percentage change in the share of different modes, the rate of increase of truck mode share is increasing; 10.94% from 1993-97 to 13% growth between 1997 and 2002. The growth in rail mode is decreasing, from nearly 3% from 1993-97, to 2% between 1997 and 2002.
Comparing Tables 4 and 5, the higher the mode share for rail in terms of ton-miles, the lower the mode share in terms of weight. This indicates that rail is used to carry less cargo weight longer distances. Water, air, pipeline and multimodal combinations have similar trends as rail; less weight carried larger distances. Truck mode, on the other hand, is used to transport more cargo smaller distances.
Figure 9 depicts the overall percentage change in mode share between 1993 and 2002 in terms of ton weight transported. We can see that air mode has grown by 23%, followed by truck by nearly 7%, while rail grew by just 2%. Waterways mode share by weight decreased by 7%. Mode share of multimodal combinations and other modes decreased by nearly 20% each.
The growth in the truck mode share is compensated for by the drastic decrease in waterways and other modes. Multimodal combinations increased between 1993 and 1997, but decreased between 1997 and 2002. Figure 10 depicts the overall percentage change in mode share between 1993 and 2002 in terms of ton-miles of cargo transported. Air mode has grown by 32%, followed by truck by 26%, while rail and pipeline grew by less than 5%, and waterways and other modes decreased by 33%. The mode share of multimodal combinations increased by 10% from 1993 to 2002.
The comparison of data in Figures 9 and 10 shows that truck mode share increased by 7% by weight, while ton-miles by truck mode increased by 25%; in other words, the mode share by ton-miles increased more than the mode share by weight. This indicates that trucks constitute, by mode, more mileage traveled in 2002 compared to 1993, with less increase in the weights carried. Rail mode also faced less increase in weight, but more travel distances. Waterways and other modes faced decreases in both weight and distance. The airline industry faced increases in both weight and distance, nearly of equal margins. Multimodal combinations and pipeline are exceptional, as weight-wise the mode share decreased for both, while increases in the distance of transportation led to increases in the mode share in the ton-miles traveled. This analysis is also supported by Environmental Protection Agency (2006), which states that the increase in the haulage of heavy-duty trucks led to an increase in the VMT by 48% from 1990 to 2003.
The data for Tables 4, 5, and Figures 9 and 10 are based on 1993, 1997, 2002 Commodity Flow Survey data, plus additional estimates from Bureau of Transportation Statistics. Of note is that the rail mode encountered less increase in both weight and ton-miles compared to truck mode, which indicates a greater expansion of roads than railways. The mode share in ton-miles as per CFS data alone is presented in Table 10. The mode share of truck increased from 1997 to 2007, and decreased from 2007 to 2012. Rail mode share increased between 2007 and 2012. This could be due to the economic downturn, and rising fuel prices during that period (see Figure 23).
The impact of the recession and fuel prices can also be observed in the total Vehicle Miles Traveled (VMT) in the United States. Figure 11 shows the total VMT from 1970 to 2012. The VMT of total vehicles increased at a constant rate of 2.4% from 1970- 2012. We can observe a drop in the VMT from 3,117,292 to 3,063,059 million miles between 2007 and 2008. VMT further declined till 2009 to 2,956,816 million miles, and from 2009 to 2012 the total VMT remained more or less constant.
Figure 12 gives the modal distribution of VMT between 1970 and 2012. Passenger cars are the predominant mode in VMT, which had been decreasing from 80% to 50% between 1970 and 2012. The share of two axle, four-tire trucks increased tremendously from 10% to 40% between 1970 and 2012, while single unit and combination trucks increased by 1-2% over the same period. This analysis is also supported by Environmental Protection Agency (2006), which states that the sales of light-duty vehicles increased from 1988 to 2003, leading to more VMT. In 2002, light-duty vehicle sales surpassed the sales of passenger cars.
It is important to consider the different types of trucks while evaluating the modal distribution of freight. Table 7 gives the eight trucks classes, categorized by the gross vehicle weight rating each vehicle is assigned, and a few examples of vehicles belonging to each class. Class-1 and 2 are called light trucks, while classes 3-8 are called heavy trucks. It can be observed that lightweight vehicles contribute more to emissions than heavy trucks (see Figure 20).
Figure 13 shows the total emissions of carbon dioxide (CO2) from all the sectors from 1990 to 2012. Total emissions increased from 1990 to 2007, then decreased sharply in 2008. From 2007 to 2012 the total emissions showed a huge decline. This could be due to improvements in fuels and vehicle technologies, as well as the economic recession in 2008 followed by a steep rise in fuel prices (Oak Ridge National Laboratory 2014).
Figure 14 shows carbon dioxide (CO2)emissions in million metric tons (MT) from various sectors from 1990 to 2012. The transportation sector is the second major contributor to emissions, after electricity generation. The emissions from the industrial sector had been decreasing from 1990 to 2012, while the emissions from agriculture, commercial and residential sectors remain more or less constant through that period. We can observe a steep decline in CO2 emissions from the transportation sector from 2008 on.
Figure 15 shows the transportation sector’s contribution to the total CO2 emissions from 1990 to 2012. The emission share of the transportation sector has been constantly increasing, except between 2008 and 2010. The transportation sector contributed 25% of total emissions in 1990, while in 2012 the same sector contributed more than 28% of the CO2 emissions. Mode shift from low emission (rail) to high emission modes (truck), in the past twenty years is one of the major reasons behind this growth (Freight Rail Works 2014).
Figure 16 shows the contribution of each mode, including passenger transport, to the overall GHG emissions by transportation sector in 2003. Table 27 and Figure 39 show that air mode, with 0.33% of the modal share in ton-miles, contributes 9% of the GHG emissions, while truck mode, with 32% of the mode share, contributes nearly 50% of the total emissions. It is interesting to see that rail, with 27% of the mode share in ton-miles, contributes just 2% of the emissions, while waterways, with 16% of the mode share, contributes just 3% of the total emissions. This proves how large a role mode shift can play in contributing to, or reducing, GHG emissions by the transportation sector. The studies by Oak Ridge National Laboratory (2014) confirm the importance of mode share in emissions, as explained below.
Figure 17 shows the contribution of transportation and highway vehicles to the total emissions of Carbon Monoxide (CO), Nitrogen Oxides (NOx), Volatile Organic Compounds (VOC), and Particulate Matters of less than 2.5 and 10 microns (PM-2.5 and PM-10 respectively). Clearly the highway sector contributes most of the emissions in transportation. Especially in terms of CO emissions, the transportation sector accounted for nearly 80% of the emissions in 1970, 1980, 1990 and 2000. There is a drop in CO emissions by the transportation sector in 2010 and 2013 to nearly 60%. Almost 50% of NOx and volatile organic compounds, 6-8% of the PM-2.5, and 3-5% of PM-10 emissions are from the transportation sector. In 1995 there was a sudden rise to 10% contribution to PM-10 emissions by the transportation sector. Figure 17 also indicates that the non-highway sector’s contribution to these emissions is less than that of the highway sector. Table 5 and Figure 17 show that highway (trucks) with 28-32% of mode share in ton-miles from 1993 to 2002, contributed nearly 80%, 60% and 50% of the emissions of CO, NOx, and VOC, respectively, during that period.
Despite the increase in mode share and ton-miles traveled by trucks, the total emissions from the transportation sector has declined. Between 2002 and 2013, carbon monoxide emissions declined by 47%, NOx emissions by 49%, volatile compound emissions declined by 41%, and PM-10 emissions declined by 39%. This could be due to technological improvements in vehicles, the recession, and fuel price changes in 2008 (Oak Ridge National Laboratory 2014). In addition to increased emissions, inefficiencies of mode share or the shift towards trucks causes other externalities, including: accidents, congestion, noise, pavement deterioration, and greater consumption of natural resources of energy and fuel by the transportation sector (McAuley 2010).
Figure 18 shows the total energy consumed by various sectors from 1970 to 2012 in trillion Btu. Transportation is the second major consumer of energy after the electric power sector. Total energy consumption in the U.S. increased steadily from 1970 to 2007, and declined slightly from 2008 to 2012. The mode share of transportation in energy consumption was steadily increasing between 24% and 28% from 1970 to 2012. In 2008, there was a decrease in energy consumption from the electric power and transportation sectors. The source of energy explains the sustainability and efficiency of its consumption; the more renewable and pollution-free the energy source, the more sustainable the system would be. So it is important to look into how different sources of energy are distributed across the energy consumption spectrum of the transportation sector.
Figure 19 explains the energy consumed by the transportation sector from various energy sources. The total energy consumed by the transportation sector has been increasing from 1970-2012, with little variation. Recently, between 2007 and 2012, the consumption decreased. The use of biomass and electricity also increased from 2006 to 2012. This could be due to the recession in 2008, followed by the fuel price increase to $90 per barrel from $35 between 2004 and 2008 (Oak Ridge National Laboratory 2014). The important point is that petroleum is the source of energy consumed by the transportation sector; almost 95% of total energy consumed by the transportation sector is from petroleum. Very little energy for transportation is derived from sustainable or environmental friendly sources such as biomass (1%) and electricity (0.1%).
Figure 20 illustrates the energy consumption share of various modes. Highway vehicles, which includes passenger vehicles, consumed 81.4% of the energy consumed by the transportation sector in 2012 (Oak Ridge National Laboratory 2014). The rail mode share includes both freight and passenger transportation. Table 27 shows that the freight mode share of rail is nearly 26-27% of ton-miles from 1993 until 2002. The comparison of this with Figure 20, below, shows that rail mode contributes 26% of the ton-miles of freight transportation, but uses 2-3% of the total energy consumed by the transportation sector, and contributes just 2% of the total GHG emissions (See Figure 18).
In addition, Figure 20 shows that the light truck contribution to energy consumption has been increasing from 10% to 30% over the period of 1970 to 2012. This could also be attributed to the increase in the VMT of light trucks, as explained in Figure 12. Energy consumption depends on the number of miles travelled. So, comparing Figure 20 with Figure 12 we can observe that the VMT share of heavy trucks (sum of single unit and combination trucks) is nearly 10%, which contributes 20% of the energy consumption, whereas light trucks, with nearly 40% of the VMT share, contribute 30% of the energy consumption. This is due to increased fuel efficiency in light-trucks (Environmental Protection Agency (2006). The heavy trucks energy consumption increased from 10% to 20% from 1970 to 2012, primarily due to class 7 and 8 vehicles. The share of emissions by class 3-6 trucks increased from 2% to 5%, while that of class 7-8 increased from 8% to 17%. The share of energy consumption among medium/heavy trucks, class 3-8, is explained below.
Figure 21 shows the mode share (in total number of vehicles) and energy consumption share (in Btus) of classes 3-8 among medium/heavy trucks in 2002. Class 8 vehicles, which constitute 41% of heavy trucks, account for 78% of the total energy use by heavy trucks. According to the Oak Ridge National Laboratory (2014), class 3-6 trucks are economical in fuel consumption, but they are used for shorter distance trips. The fact that these are heavy trucks making shorter trips is also represented by the low VMT values from Figure 12. Single axle, four tire trucks constitute the majority of VMT by all modes. Light trucks caused more GHG emissions (28%) compared to heavy trucks (19%) (Figure 39). Clearly light trucks contribute to more VMT, energy consumption, and emissions than heavy trucks. Among heavy trucks, class 8 trucks contribute the majority of mode share in number of vehicles and energy consumption.
A similar pattern can be observed in the consumption of petroleum by various transportation modes (Figure 22). Highway vehicles consumed 85.9% of the total petroleum consumed by the transportation sector (Oak Ridge National Laboratory 2014), with the most consumption by light trucks, followed by heavy trucks. Class 7-8 trucks are the leading contributors to petroleum consumption among heavy trucks. Rail mode contributes to nearly 2% of total petroleum consumption (Figure 22). Waterways are similarly efficient; with a contribution of 16-20% of ton-miles, waterways consume only 4-5% of the total energy and petroleum consumption of the transportation sector. Figures 20 and 22 show that the passenger car contribution to energy and petroleum consumption has decreased from 60% to 30% over the period of 1970 to 2012.
Overall, transportation in U.S. became more efficient from 1970 to 2009. Between 1970 and 2009, the fuel used by all vehicles per mile declined from nearly 80 gallons to 55 gallons per thousand miles. The production-weighted average annual carbon footprint for light vehicles (including vans, special utility vehicles (SUV), trucks) dropped by 40% from 1975 to 2013 (Oak Ridge National Laboratory 2014). So, transportation overall has improved its functional efficiency. The externalities discussed so far can be attributed to the combined impact of higher economic activities, changes in costs (vehicle and fuel), and more VMT and modal distribution.
Since the national economy and fuel prices influence modal distribution, and by extension such transportation externalities as pollution, GHG emissions, and energy consumption, it is important to look at economic growth and fuel price changes in the US. Figure 23 shows the economic growth rate and fuel price changes in the US from 1970 to 2012. Changes in oil prices affected the markets five times (1973-74, 1979-80, 1990-91, 1999-2000, and 2008) in 30 years. The rise in oil prices can be observed just after the recession. The reduction in the total emissions of CO, NOx, and VOC, reduction in the energy and petroleum consumed by the transportation sector, reduction in the total VMT in the year of 2008 can all be attributed to the economic downturn and rising fuel prices, as described earlier.