Automobiles (and, of course, other passenger vehicles such as light-duty trucks and SUVs) are a big factor in Houston's air pollution. There are three basic factors that are fundamental to assessing the impact of automobiles on Houston's air pollution problem.

1. Passenger vehicles are among the most effectively controlled sources of air pollution. Beginning with the 2007 model year, all passenger vehicle manufacturers will sell vehicles that, on average, meet a control standard for nitrogen oxides (NOx) of 0.07 gram per mile. Thus, by 2007, new automobiles will have achieved a 93% reduction emissions rates since 1997. In comparison, industrial sources in the Houston region have cut their NOx emissions by only 80% since 1993.
   
   
2. The phase-in of the 0.07 gram per mile standard began in 2004 (a prior reduction occured between 1997 and 2001). However, it will take until after 2012 (or so) for most of the benefit to be felt, since older vehicles will still be driving for several years. By 2012, regional emissions of NOx from passenger vehicles should decrease by over 50% from estimated 2005 emissions. Short of a massive vehicle scrappage program, there is little that can be done to accelerate this trend.
   
   
3. Although passenger vehicles will account for about one-sixth of all NOx emissions in 2007, the actual impact of passenger vehicles on air quality is far lower. (Trucks also account for about one-sixth of all NOx emissions, so all vehicles together account for about one-third of regional NOx emissions.) Air quality problems in the Houston region are driven by other pollutants as well - especially VOCs - and passenger vehicles are an even less important source of those emissions. Furthermore, emissions from passenger vehicles are distributed throughout the Houston region, while the vast majority of industrial and related emissions are concentrated in east Harris County. If you're experiencing bad air pollution at some location in the Houston region, probably less than 10% of the pollution in the air can be attributed to passenger vehicles.

Because the federal government regulates the emissions coming out of passenger vehicle tailpipes, the two things that can be done at a local level are (a) make sure the emissions controls work and (b) encourage land use patterns and personal behaviors that reduce emissions. We've done the first - every year, passenger vehicles must be tested to make sure their pollution control devices are working (which isn't true for all industrial equipment). We've done a pretty poor job at the second, with government subsidies for sprawl, low-cost parking and other inducements to maximize driving. And by default, and lack of attention, we've minimized walking, biking, carpooling and transit use. Over the past decade, it is estimated that the number of miles travelled on the roads has increased by 50%. In the past few years, there have been some positive changes for the better, but they will take years or even decades to bear fruit.

If we were long-term thinkers and planners in this region, there would be plenty more we could do to reduce pollution caused by private vehicles. But in the context of a state agency that refuses to do anything ahead of the absolute last minute, it is not practical to expect major reductions in air pollution by focusing on emissions from private vehicles.