Earlier this month, the New York City Department of Transportation announced plans to experiment with 20 mph zones -- replacing the city's default 30 mph speed limit in one pilot neighborhood. Whoever gets the first 20 mph treatment will see benefits that residents of British cities and towns have become increasingly familiar with in recent years.

In the UK, some 3 million people live in areas with 20 mph speed limits. The experience there shows that not only do slower speeds save lives, but lowering the limit to 20 mph improves the way local streets function in more ways than one. According to the 20's Plenty for Us campaign, the change has produced wide-ranging benefits, including less traffic, increased walking and biking, greater independence for children, the elderly and infirm, better health, and calmer driving conditions for motorists.

The mission of 20's Plenty For Us is to establish 20 mph as the default speed limit on all residential roads in the UK. I recently met up with the campaign's founder, Rod King, as well as other advocates in the towns of Warrington and York, to understand how the idea of slowing down traffic has spread so fast throughout the country

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