By Tim Harford (FT + BBC "UNDERCOVER ECONOMIST")
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My environmentalist friends tend to dislike cars in general, and big, bulky, dangerous cars in particular. I don’t like lorries dressed up as cars either, but I am forced to acknowledge that the costs we drivers impose on society accrue less from what we drive and far more from when and where we drive it. For example, an SUV driven in rural Wales at 10pm on a Sunday imposes far less social cost than a Prius driven into central London at eight o’clock on a Tuesday morning.
What we drive matters far less than one might think, partly because engines are much cleaner than they used to be and, therefore, emissions of important pollutants (with the key exception of carbon dioxide) have fallen precipitously over the past two decades. No, the real problem with cars is that they get in the way of each other, and a super-efficient car gets in the way of other cars just as much as the most ridiculous Chelsea tractor.
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http://www.ft.com/cms/s/2/9635dd08-a476-11e1-a701-00144feabdc0.html