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Chapter IX

Energy & Environment

Western Uniformity Scenario Analysis graphic depicting western states and illustration of longer combination vehicle


Changes in truck sizes and weights will impact energy consumption, air quality, global warming, and noise emissions. The magnitude of each of the four areas is influenced by the extent of truck travel, vehicle weight, speed, and other truck operational parameters. This chapter discusses how estimated changes in truck travel resulting from the Western Uniformity Scenario might affect each of these four areas. The overall reduction in VMT is expected to result in an overall reduction in energy consumption and emissions.

Noise emissions are very localized. They can be measured in terms of the impact of the noise on residential property values. To be affected, residences must be immediately adjacent to a high volume roadway; the denser the residential development, the greater the total impact. The cost of noise is estimated based on the estimated residential density adjacent to freeway sections, as reported in the Highway Performance Monitoring System (HPMS) database and on changes in noise levels caused by changes in truck VMT resulting from truck size and weight (TS&W) policy changes.

Air pollution impacts are highly dependent on meteorological conditions and to a lesser extent on geographic features that cause air stagnation. Air pollution tends to be regional with some long distance conveyance in the lower levels of the atmosphere. Air pollutant emissions are related to VMT, but the transformation of those emissions into secondary pollutants involves complex chemical processes that may vary considerably from area to area depending on other sources of pollution in the area, climatic factors, and other variables.

Estimating total nationwide economic costs of air pollution attributable to motor vehicles is complex. The Department collaborated with the Environmental Protection Agency (EPA) to develop a nationwide cost estimate in connection with the 1997 Highway Cost Allocation (HCA) Study. Resource constraints prohibited development of such estimates for the illustrative scenarios in the CTS&W Study. In general, the reduction in truck VMT under the Western Uniformity Scenario would reduce air pollution costs, but changes are not proportional to changes in VMT, particularly at specific locations. However, changes in truck emissions would be largely proportional to changes in VMT.

Analytical Approach

Energy Consumption

Table IX-1 illustrates how fuel consumption varies with truck configuration and weight. It shows that a longer configuration at the same weight does not necessarily have a higher rate of fuel use. Inherent for each truck configuration is the selection of the most efficient engine for that configuration and use. Fuel use information developed for the 1997 HCA Study provided the basis for the analysis of annual energy consumption associated with the introduction or elimination of particular vehicle configurations and weights. Although the fuel efficiency values used here do not reflect the more stringent 2004 EPA emissions regulations, it is expected that the differences in miles-per-gallon from one configuration to another and from one weight to another for engines meeting those regulations will be similar to the differences shown here.

A configuration's impact on diesel fuel use depends on its miles of operation at its given weight, speed, and roadway grade. For this study, each configuration is assumed to operate at the same speed under the same conditions. It is important to note that fuel use does not increase on a one-to-one relationship with vehicle weight.

Base Case VMT for the Year 2010 by truck type and operating weight was multiplied by gallons-per-vehicle-mile-of-travel estimates to estimate total truck fuel consumption. The same was done for the Scenario's VMT estimates. The difference measures the fuel consumption impact of the Western Uniformity Scenario for the 13 analyzed States.

Table IX-1
Miles per Gallon by Truck Configuration and Weight
Configurations Gross Vehicle Weight(pounds)
60,000 80,000 100,000 120,000 140,000
Five-Axle Semitrailer 5.44 4.81 4.31    
Six-Axle Semitrailer 5.39 4.76 4.27    
Five-Axle STAA Double 5.95 5.29 4.79    
Seven-Axle Rocky Mountain Double   5.08 4.58 4.36 4.16
Eight-Axle (or more) Double   5.08 4.82 4.58 4.36
Triple-Trailer Combination   5.29 5.01 4.76 4.54

Air Quality

As noted above, relating changes in truck travel to changes in nationwide economic costs of air pollution is complex and resource intensive. Furthermore, effects in any specific location could be very different from effects estimated for the Nation as a whole. As indicated earlier, DOT is working with EPA to develop an air quality impact methodology based on the best and most current information available.

Important factors in estimating changes in air quality costs are the dollar values assigned to mortality (death), morbidity (illness), visibility impairment, soiling, materials damage, effects on plants and wildlife, and other impacts caused by air pollutants. These are extremely difficult to quantify in terms of their effects and wide ranges of costs have been estimated in previous studies. Furthermore, our understanding of the health effects of various pollutants continues to evolve, and thus estimates of motor vehicle related air pollution costs must be periodically updated to reflect the latest scientific knowledge. A key issue that will be the subject of future research is the relationship between vehicle weight and emissions. The EPA's models currently do not differentiate among the vehicle classes of interest in TS&W policy options.

Noise Emissions

Truck noise comes from three sources-the engine (as a function of engine revolutions per minute), the exhaust pipe (particularly from the use of engine compression brakes), and tires (tire noise increases significantly with speed and begins to dominate other truck noise sources above 30 miles-per-hour). Truck noise begins to dominate noise from other traffic once trucks account for more than 3 percent of the traffic. For example, to produce a noticeable difference in highway noise, such as a decrease of 2.5 decibels, the percentage of trucks in the traffic stream would have to drop from 20 percent to 5 percent of all traffic. The cost per noise equivalent was estimated for each vehicle class based on a synthesis of research findings from other studies.

The DOT has developed models for evaluating impacts of traffic-related changes in noise levels. These models served as the basis for the noise emission cost calculations for the HCA Study and CT&W Study.39 Using passenger cars as the base, noise equivalency factors were determined under differing operating circumstances for each vehicle class and weight group. Noise equivalency factors for trucks relative to passenger cars are shown in Table IX 2. These cost per noise equivalent were estimated for each vehicle class based on a synthesis of research findings from other studies.

Table IX-2
Noise Passenger Car Equivalents for Trucks
Vehicle Type Speed
20 30 40 50 60
Passenger 1.00 1.00 1.00 1.00 1.00
Truck 84.85 43.82 27.42 19.06 14.16

Noise-related costs are only estimated for freeway travel. There are several reasons why the analysis was limited to freeway travel including: (1) virtually all studies used as background for the cost estimates were limited to freeway locations, and (2) except in commercial areas where there are many other sources of noise, truck volumes in urban areas are relatively low.

Exhaust Emissions

There has been little past research on relationships between vehicle size and weight and emissions. Changes in overall truck volumes under the scenario are not likely to cause significant changes in speeds or other traffic characteristics that affect emissions rates. The primary factor that would cause emissions to change is the change in total truck volumes and the change in traffic composition with more LCVs and fewer conventional trucks. Since other environmental, technological, and geographical factors that might affect emissions are assumed to be the same for the base case and the scenario, it is assumed for purposes of this study that total emissions vary directly with changes in fuel consumption. This is consistent with methods used by the Environmental Protection Agency to estimate heavy truck emissions in its Mobile 6 model.

Scenario Impacts

Table IX-3 shows the impact of the scenario, both high- and low-cube cases, for energy consumption, emissions and noise costs. As mentioned previously, air pollution costs for the scenario could not be estimated within the scope of this study, therefore the impact table shows that these costs are not available (NA).

Table IX-3
Energy and Environment Impacts for 13 Analyzed States
Impact Base Case Low Cube - Change from Base Case High Cube - Change from Base Case
Energy Consumption (million gallons) 5,084 4,921 - 3.20% 4,471 - 12.06%
Emissions     -3.20%   -12.06%
Air Pollution Costs NA NA NA NA NA
Noise Cost ($ millions) $539 $532 - 1.43% $487 - 9.67%

39 See those studies for further information and documentation of the noise emission model.

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