This graph shows how the U.S. Environmental Protection Agency (EPA) has regulated heavy-duty truck emission standards, dramatically reducing pollution from new vehicles. Moving from the top right corner of the chart to the lower left-hand corner, the chart shows that in 1988, EPA's emission certification standards were 0.6 g/bhp-hr (grams per brake horsepower-hour) for Particulate Matter (PM) and 10.7 g/bhp-hr for Oxides of Nitrogen (NOx). After 2010, the PM emission standard will be just 0.01 (g/bhp-hr), and NOx emissions standard will be 0.30 (g/bhp-hr) for new vehicles. This is a 98 percent reduction in allowable PM and a 97 percent reduction in allowable NOx emissions for HD on-road trucks over the 30-year time frame.
Source: Analysis by M. Claggett, FHWA, (2005). Emissions impacts are calculated using Mobile 6.2, estimating the total of 6 priority MSATs (diesel particulate matter, benzene, butadiene, acetaldehyde, formaldehyde, and acrolein) for a potential build scenario of adding 2 or 4 lanes to a 4- or 6-lane congested highway versus the no-build scenario.
This graph shows the hypothetical emission impacts for a highway widening project. According to this comparative analysis, mobile source air toxics are expected to decrease substantially over the next 25 years. This is due to the implementation of the United States Environmental Protection Agency's various new mobile source vehicle engine emission standards and fuel programs. Small differences in emissions are shown between the build and no-build scenarios. As illustrated in this graph the highway widening project results in fewer emissions than if the highway was not improved. This is because the highway widening relieves congestion in this example and thus reduces emissions. The emission benefits of reducing congestion are shown to offset the emission increases from the additional vehicle miles of travel (VMT) on the wider highway. Analysis results from other transportation improvements may vary somewhat from these results depending on the actual and anticipated levels of congestion, VMT and other factors. A complete analysis can be found in Mobile Source Air Toxics (MSAT) Emissions Analysis for a Hypothetical Transportation Congestion Mitigation Project.