Air Quality Trends Analysis

VMT Growth and Improved Air Quality:
How Long Can Progress Continue?

Travel Growth and Improved Air Quality:
How Long Can Progress Continue?

Residents of the Nation's urban areas are breathing easier these days. Atmospheric levels of all four pollutants to which motor vehicles contribute significantly -- ozone, carbon monoxide, airborne lead, and nitrogen dioxide -- have declined consistently for almost two decades, and violations of the National Ambient Air Quality Standards (NAAQS) for airborne lead, carbon monoxide, and nitrogen dioxide have been virtually eliminated. Controlling ground-level ozone (or "smog") has proven more challenging, but violations of the Federal ozone standard have also been sharply reduced (figure 1). ¹

Motor Vehicles and Cleaner Air

Most of the reduction in atmospheric concentrations of these pollutants can be attributed to reduced emissions by motor vehicles. Tighter emissions standards for cars and trucks have significantly reduced their emissions of carbon monoxide (CO) and volatile organic compounds (VOC, a primary ingredient of ozone) since 1970, while emissions of these pollutants from other sources have fallen only slightly (figures 2 and 3). At the same time, motor vehicle emissions of nitrogen oxides (NOx) -- which contribute to both ozone and nitrogen dioxide -- have been held at about their 1970 level, while those from all other sources have increased significantly (figure 4). Since emissions of these pollutants from other sources have declined only slightly or even increased, cleaner cars and trucks apparently account for most of the improvement in air quality.

The Challenge Ahead

While the recent improvement in air quality is impressive, many urban areas still occasionally violate the federal ozone standard, and airborne particulates are a growing public health concern. Because air quality planners in these areas view continued reductions in motor vehicle emissions as a key strategy for further improving air quality, many are concerned that pollutant emissions from motor vehicles may begin to rise in the near future, as declines in vehicles' per-mile emission rates are ultimately offset by continued growth in the number of vehicle-miles traveled (VMT). Thus, they argue, the recent progress in reducing air pollution may be impossible to sustain without implementing measures to limit future growth in automobile use.

This analysis examines whether reductions in motor vehicles' volatile organic compounds and NOx emissions rates likely to result from recently adopted control strategies could be offset by continuing vehicle miles traveled growth during the foreseeable future. It also investigates how rapidly motor vehicle emissions of various pollutants might resume their long-term decline is reversed, a potential future increase in emissions to their historical decline. Finally, the analysis explores how this potential increase in motor vehicle emissions might be postponed by further tightening of new-car emissions standards or other proposed emissions control strategies that have not yet been adopted. It is intended to provide factual analysis of the relationship between VMT ,growth and air quality, not to serve as a policy statement on the desirability of either continued growth in travel demand or specific emission control strategies.

Analysis Procedures

Emissions of VOCs and NOx from motor vehicles were estimated for the period 1990 to 2015 for two major U.S. urban areas (Baltimore and Milwaukee) classified by the Environmental Protection Agency (EPA) as in "severe" violation of the Federal ozone standard, and a third area (Boston) classified as in "serious" nonattainment. To produce these estimates, projections of annual VMT by various types of motor vehicles in each urban area were multiplied by estimates of the per-mile rate at which vehicles of each type would emit VOCs and NOx during each year from 1990 to 2015. Forecasts of VMT were supplied by transportation planners at each urban area's metropolitan planning organization (MPO), while future emission rates were projected using the current version of the EPA's MOBILE5a vehicle emissions model.

The estimates of future emission rates were developed using exactly the same assumptions on vehicle fleet age, composition usage, and on vehicle operating conditions (such as temperature and average speed) that were employed by air quality analysts in preparing each area's State Implementation for complying with Federal air quality standards. These forecasts also assume that emissions control strategies mandated by Federal legislation or adopted in each area's SIP will be implemented as scheduled, including:

• Continued phase-in of the Tier I emissions standards for light-duty gasoline vehicles through model year 1998;
• Completion of Stage II vapor recovery system installation at retail fuel outlets;
• Sales of Phase I and II Federal reformulated gasoline beginning in 1995 and 1998; and
• Further reductions in NOx emission standards for new heavy-duty diesel engines beginning in 1998.

Because it appears unlikely that Employee Commute Options (ECO) programs will be implemented as originally mandated by the 1990 Clean Air Act Amendments (CAAA), the VMT reductions originally anticipated to result from each area's planned program were assumed not to result within the time horizon for this analysis. Similarly, the emissions reductions originally projected to occur over the 1995 to 1998 period as enhanced inspection and maintenance (I&M) programs became fully operational in the three nonattainment areas were assumed to be delayed by three years and to require approximately a decade to be fully realized. In effect, this assumes that an equivalently performing vehicle inspection and repair program will ultimately be adopted in each of the locations studied, but will require much longer to implement than originally anticipated.

Projected Emissions

Volatile Organic Compounds. Over the decade from 1990 to 2000, aggregate VOC emissions from motor vehicles are projected to decline by approximately two-thirds in the six-county Milwaukee nonattainment area, and by somewhat more than half in the Baltimore and Boston nonattainment areas. (This is shown by the solid lines in figure 5; the figure omits results for the Baltimore nonattainment are they are nearly identical to those shown for Boston.) These declines reflect the combined effectiveness in reducing fleetwide VOC emissions rates of reductions in gasoline volatility and sales of Federal Phase I reformulated gasoline, installation of Stage II vapor recovery devices at gasoline stations, and the phase-in of the Tier I automobile emission standards. By the year 2000, about 35 percent of the Nation's light-duty vehicles will have complied with the Tier I emission standards when new, but because newer vehicles are used more intensively than older models, these vehicles are expected to account for nearly half of light-duty VMT.

Motor vehicles' VOC emissions in the three nonattainment areas appear likely to continue declining after the turn of the century, although at a considerably slower pace than over the 1990 to 2000 decade. The slower decline after the year 2000 reflects the fact that with the exception of continued absorption of new light-duty vehicles meeting the Tier I standard into the fleet, the VOC control strategies implemented during the 1990s will have reached their full effectiveness before 2000, and MOBILE5a does not incorporate the effects of any measures likely to be adopted after that time. Without additional control efforts, VOC emissions from motor vehicles in each of the three nonattainment areas are projected to continue declining through at least 2005 in areas expecting moderate VMT growth (such as Boston), and for as much as a decade longer in areas (such as Milwaukee) that anticipate slower VMT growth. By this time, VOC emissions would have been reduced by 80 to 85 percent from their levels during 1970, when Federal emission standards for motor vehicles first took effect.

If no new control measures are adopted after 2000, VOC emissions could rise slowly after about 2010 in areas such as Baltimore and Boston that project VMT annual increases of more than 2 percent. Even in areas expecting steady VMT growth, however, VOC emissions from motor vehicles are projected to remain well below half of their 1990 levels through at least the year 2015, as figure 5 shows. Tightening emissions standards for light-duty vehicles beyond their current Tier I levels, however, could extend the projected reduction in their VOC emissions considerably (the dashed lines in figure 5). Adopting the National Low-Emission Vehicle (NLEV) standard proposed by the EPA would reduce VOC emissions from cars and trucks through at least the year 2010 in faster growing areas such as Boston, and beyond 2015 in slower growing regions such as Milwaukee.²

Oxides of Nitrogen. Achieving continued reductions in motor vehicle NOx emissions is likely to require broader-based controls, since passenger vehicles and freight trucks each contribute about equally to fleetwide NOx emissions. The combination of the Tier I emissions standard for light-duty vehicles and tighter NOx controls for heavy-duty diesel engines mandated in the CAAA should reduce motor vehicle NOx emissions in the nonattainment areas studied by about 20 to 25 percent over the 1990 to 2000 period (the solid lines in figure 6). These strategies are less effective in reducing motor vehicle NOx emissions than in curtailing VOC emissions because the CAAA holds some light-duty trucks used as personal vehicles to less stringent NOx than passenger cars, and because new heavy-duty trucks meeting the CAAA's tighter standards are absorbed into the fleet more slowly than are new passenger vehicles.

Without additional controls or improvements in engine and catalyst technology, motor vehicle emissions of NOx should remain below 1990 levels over the foreseeable future in forecasting modest VMT growth (such as Milwaukee), but could approach their 1990 levels by the year 2015 with more rapid VMT growth (as projected for Baltimore and Boston). But, as with VOC emissions, further tightening, of NOx emissions standards for light-duty vehicles under the EPA's proposed NLEV standard could extend considerably the reductions in fleetwide NOx emissions (the dashed lines in figure 6). Adopting tighter standards for new cars could prolong reductions in NOx emissions through at least 2010 in areas expecting more rapid growth and beyond 2015 in those areas expecting slower VMT increases, by which time they would be reduced to levels 30 to 40 percent below those during 1990.³

Future Emission Controls

A variety of other emissions control strategies that are not reflected in these results would make it less likely that declines in motor vehicles' VOC and NOx emissions rates could be offset by growth in motor vehicle usage during the foreseeable future. The most important of these is detecting and repairing vehicles with malfunctioning emissions control systems, since their sharply elevated emissions contribute a disproportionate share of the motor vehicle total. Others include further tightening controls on NOx emissions levels for new heavy-duty diesel engines, revising the driving cycle used to certify new vehicles' compliance with emissions standards to more accurately reflect actual driving behavior, and more effective control of evaporative VOC emissions during vehicle operation and storage.

Broadening the current focus of emission control efforts to include tighter standards for large gasoline trucks, off-road vehicles such as recreational boats and railroad locomotives, and gasoline and diesel engines used in lawn, garden, and construction equipment could produce substantial further reductions in both VOC and NOx emissions. While many of these initiatives have already been adopted by the EPA and others are under serious consideration, none of their effects is reflected in the results shown in figures 5 and 6. Including these measures would not only prolong the declines in motor vehicle emissions shown in the figures, but would also ensure that total emissions from all sources would continue to decline even if those from highway vehicles resumed growing.

At the same time, continued technological evolution of motor vehicle emissions control systems could further reduce new vehicles' emissions rates and thus total fleetwide emissions, even without continued tightening of emission standards. As an illustration of how important such progress can be, gradual improvements in engines and emissions control systems reduced new cars' expected lifetime emissions of VOCs, CO, and NOx by 36 percent, 15 percent, and 10 percent over the thirteen-year period (model years 1981 to 1993) when the "Tier 0" emissions standards remained in effect. At the VMT growth rates projected by the nonattainment areas included in this study, reductions of this magnitude could postpone the onset of increases in total emissions by as much as another decade.

How Fast Will VMT Grow?

While the VMT growth rates projected for the three nonattainment areas included in this study (which range from 1.7 percent to 2.6 percent annually) are low by comparison to those experienced during much of the 1980s, there are two major reasons to suspect that future growth in travel will be slower than its recent pace. First, VMT growth during the mid-1980s was unusually rapid by historical U.S. standards, and coincided with the economy's recovery from a severe recession, plummeting fuel prices, and rapid improvement in automobile fuel economy. The resulting combination of robust economic recovery with a dramatic decline in the per-mile cost of driving during those years caused VMT growth to surge temporarily from its historical growth rate, but in retrospect it is clear that these developments were short-lived.

Second, the pace of VMT growth has been declining slowly throughout most of the postwar period (figure 7), as the various demographic forces that propelled VMT growth have slowed, and in some cases even ceased. These include rapid growth in the driving-age population, large-scale entry of women into the U.S. work force declines in the size of American households, and increasing geographic dispersion of the Nation's metropolitan areas. Since these developments are unlikely to continue at their historical pace, travel growth should continue to slow during the next century to rates well below its post-war average. In fact, some slowing of growth in personal travel is already apparent: automobile and light-truck VMT growth has averaged less than 2.5 percent annually since 1988, after increasing at more than 4 percent per year over the 1982 to 1988 period.⁴

Future Air Quality

Air quality in the Nation's urban areas continues to improve rapidly, due largely to reductions in motor vehicles' emissions of air pollutants. Because declines in the per-mile emissions rates of motor vehicles have been so dramatic, their aggregate emissions have fallen despite rapidly increasing vehicle ownership and usage. Emissions control measures already in effect are likely to extend the decline in motor vehicles' VOC emissions for at least another decade, and further tightening of new-car emission standards could prolong this decline by approximately another ten years. In the case of NOx, tighter standards for new vehicles (including trucks as well as automobiles) are likely to be necessary to achieve the same result, although significant emissions reductions from off-road vehicles and equipment should also be possible. The experience of the past twenty-five years demonstrates convincingly that increasing travel demand and improving air quality can coexist, and continued VMT growth -- particularly at the moderate rates likely to prevail in many U.S. urban areas -- need not be an insurmountable barrier to nearly universal attainment of Federal air quality standards.

1. Declines in peak ozone levels can result from less frequent occurrence of weather conditions that are conducive to its formation, but even after adjusting for meteorological trends, a clear downward trend is evident in both violations of the Federal ozone standard and measured ozone levels in most U.S. urban areas.
2. An important advantage of the NLEV standard -- which applies to all 49 states outside California -- over the proposal to adopt the California emission controls within the 13-state northeast Ozone Transport Region (the so-called OTC LEV program) is that ozone nonattainment areas located outside that region would realize continued reductions in VOC emissions under the EPA's proposal, but not under the OTC LEV program. Within the Ozone Transport Region, the two programs would produce virtually identical reductions in VOC emissions under the same assumptions about fuel composition and the effectiveness of vehicle inspection and maintenance programs,
3. Since the OTC LEV program would not tighten NOx standards, reductions in NOx emissions would occur only as a by-product of the program's tighter standards for other pollutants, and would again be limited to nonattainment areas within the 13-state Ozone Transport Region.
4. Computed from information reported in Federal Highway Administration, Highway Statistics, various years.