When officials in Tannersburg released their plan to widen the city's main roads, environmentalists protested that widened roads would attract more traffic and lead to increased air pollution  In response, city officials pointed out that today's pollution-control devices are at their most effective in vehicles traveling at higher speeds and that widening roads would increase the average speed of traffic
However, this effect can hardly be expected to offset the effect pointed out by environmentalists, since
most of the air pollution generated by urban traffic comes from vehicles that do not have functioning pollution-control devices