Air quality will be better with the Legacy Highway than without it. A long held myth is that the way to decrease auto related air pollution is to build new freeway lanes, thereby freeing cars from congestion and allowing them to go faster. The EIS, in fact, claims air quality improvement for the build options as opposed to no-build for this very reason. First, being able to drive faster encourages more and longer car trips, adding up to more cars and more total emissions from cars. Then the new lanes fill up and become congested and more new lanes are built, and so on.
Auto dependency becomes further entrenched.
People driving in congestion have other choices than driving faster on new lanes. Reducing the absolute number of vehicle miles traveled through increased use of mass transit, people living closer to jobs, and other factors improves air quality far more than increasing the speed of cars for a little while. Third, car emissions actually increase at higher speeds. Nitrogen dioxide emissions begin increasing at 45 miles per hour, while VOC and carbon monoxide emissions start to increase at 55 miles per hour. We need to seriously reduce air pollution from transportation, not to stick with a myth.