Bus rapid transit systems and "complete streets" are great. But to design urban transportation systems that are truly sustainable, we have to think much further ahead.

By the end of the 19th century, cities throughout Europe faced a crisis: They were literally drowning in horse manure. Thought leaders of the day knew it to be a forgone certainty that dealing with the waste of horses was going to be the most pressing concern for urban planners of the 20th century.

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