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Congestion is inefficient, polluting and dangerous. So why not reduce it? Removing just 5% of traffic at peak times could substantially reduce or even eliminate rush hour congestion from many cities. One approach that is beginning to stoke interest among municipal leaders, even in large metropolitan capitals like Paris and London, is road pricing. The theory seems sound enough: introduce a price on bringing cars into congested areas that incite drivers either not to travel unnecessarily or to vary their times of travel or, indeed, to try public transport, walking or cycling. With the right approach, drivers who incur higher prices during rush hour periods would benefit from reduced congestion and travel time, while non-essential travel would take place at less congested and cheaper times.
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http://oecdobserver.org/news/fullstory.php/aid/647/Road_congestion:_What_s_the_deal_.html