Bogotá did this with TransMilenio, choosing bus routes targeted to the fastest growing areas.<sup>31</sup> Despite the investments in dedicated bus lanes, full service bus stations, and a pay-and-park system, private vehicle use has been increasing 12.3 percent a year.<sup>32</sup> Governments can reduce total emissions by enacting regulations that reduce emissions per mile, and countries the world over focus their regulatory efforts on new vehicles. The United States first implemented emission control regulations in the early 1970s. Such regulations require that manufacturers produce vehicles that meet a given emissions standard and install specific equipment, such as a catalytic converter.

These regulations are easy to enforce but have several consequences. First, in nations with an older vehicle fleet, they might begin to reduce average emissions only years after being implemented. Suppose that a city’s vehicle stock features vehicles that range in age from 0 to 33 years. Suppose also that 3 percent of the fleet is replaced each year with new vehicles. If only new vehicles face more stringent regulation, only years after the regulation is enacted will the fleet’s average emissions decline. An unintended consequence is to encourage households to keep their old cars running longer, delaying the effects of the regulation.<sup>33</sup> The good news is that trends in air pollution across India’s cities suggest that the phase-in of such emission control regulations has reduced urban air pollution. Greenstone and Hanna (2011) exploit the fact that different cities in India phased in catalytic converter regulation at different times. For example, the number of cities with such regulation increased from 4 in 1997 to 22 in 1998. Large reductions in vehicle emissions start roughly five years after regulations are adopted.

Stringent new regulations will not reduce the emissions from the stock of older vehicles. Based on remote-sensing data from the United States, engineer Donald Stedman has argued that a small share of the vehicle stock is the cause of most of the vehicle emission pollution.<sup>34</sup> To address this social externality, economists have advocated that police issue “pollution tickets” similar to speeding tickets. Such policing could sharply reduce used vehicle emissions. Those caught releasing high emissions would be fined a substantial sum, inducing households to invest in more vehicle maintenance. The approach could also regulate diesel trucks. An additional advantage is that it can substitute for a costly inspection and maintenance program for used vehicles.

Another strategy for reducing vehicle emissions is to mandate cleaner gasoline. One major public health success has been to require unleaded gasoline, which richer nations are more likely to do.<sup>35</sup> Urban lead emissions have declined sharply in richer nations, improving public health by reducing lead’s negative impacts on child development. Until recently, leaded gasoline was the key source of widespread exposure to lead in urban Africa. With its elimination from all of SubSaharan Africa in 2007, ambient lead emission levels will continue to decline, offering direct health benefits for children and the exposed population.<sup>36</sup>