Both the technological and the land use approaches to improving air quality share one very important thing in common: they require significant changes in the habits and preferences of Americans; either what we drive or how we live (in terms of internal and external space). Although what we drive has been moving in the "wrong" direction in recent years (given the passion for SUVs, trucks and minivans), that could easily be turned around with a sharp enough spike in gas prices. Of course, whether an increase in gasoline taxes large enough to change automobile and driving preferences would be politically palatable is another question (as pointed out above, and especially since the protests throughout Europe on this very issue in September 2000). But certainly changing driving habits on a nationwide scale is much easier and more feasible than increasing densities. Even if more Americans were willing to put up with higher-density living, it could only affect the increment to the housing stock. In other words, the vehicle fleet turns over much faster than the housing stock.21
Furthermore, we know that changes in driving will, especially if buttressed by higher driving costs and other incentives22/disincentives, reduce air pollution by significant amounts, whereas it remains unclear whether densification would (bearing in mind that pollution damage consists primarily of human health impacts, and higher densities mean more people exposed for a given level of pollutants).23
Another important point is that the technological solutions are now (after early technology-forcing mandates) being driven by market competition whereas the land use approaches would require continuous and more invasive government intervention at all levels of government. Inducing people to drive differently via a combination of attractive products and policy incentives (e.g. emission fees on SUVs and trucks, tax credits for buying/leasing hybrids), while not easy, has much more prospects for success than forcing them to live differently via planning regulations. As an example, a new law in California (AB 71) that took effect on July 1, 2000, allows solo drivers of EVs, SULEVs and ULEVS to use the carpool lanes; this could be a powerful incentive on some routes, reducing trip times in half. Similarly, in April 2000, the City of Los Angeles adopted an ordinance allowing electric vehicles (and some natural gas vehicles) to park free at parking meters. Like the carpool lane exemption, measures of this kind are an incentive to new car buyers/lessees to shift preferences. The Bush Administration's energy plan includes $4 billion of research for hybrid vehicles, and offers the prospect of a $4,000 subsidy towards the purchase of hybrid vehicles. The combination of a tax credit, gasoline savings, and access to carpool lanes might easily tip the balance for many against the SUV/truck and in favor of the hybrids. Another option, currently being explored by several major auto manufacturers, is to develop hybrid SUVs (e.g. a proposed Dodge Durango, a Ford Explorer, and a protype GM Suburban), thereby achieving a balance between fuel economy (and emissions) and existing consumer preferences for the larger, more spacious vehicles. One reason among many for doubt about the land use approaches is that unless we are willing to adopt a national system of uniform land use controls, the "exit" option (i.e. moving to a less regulated jurisdiction) will still be there and may become increasingly attractive.