"In 1894 the Times of London estimated that by 1950 every street in the city would be buried in nine feet of horse manure." (http://nofrakkingconsensus.com/2011/03/29/the-horse-manure-problem/)
Each horse produced between 15 and 35 pounds of manure per day. With each year, the manure pile grew higher and higher. It emanated a wretched odour. It attracted flies, insects sometimes infected with typhus. "The streets of London were beginning to poison its people."
In the early 1800's, the horse manure could be sold to farmers who used it to fertilize their fields. However, by the latter part of the century, there was such a surplus that stable owners had to find ways to get rid of it. Much of the manure rotted in the London streets. "Wet weather turned the streets into swamps and rivers of muck." On dry weather days, when the wind whipped up, the buildings became coated in a layer of manure and pedestrians choked on the toxic dust.
Four years after the Great Horse Manure Crisis, an international conference was held to solve the problem. The ten day conference was shortened to three days as no one was able to come up with a solution.
The answer came in the car for the masses, produced by Henry Ford, who launched his car company in 1903. Ford vowed that he would put a chicken in every pot and a car in every driveway in America (http://alinefromlinda.blogspot.ca/2012/01/cars-in-driveways-chickens-in-pots.html). By 1913, he perfected the assembly line, and with it, the availability of an inexpensive car. Bit by bit, city streets would fill up with automobiles, electric trams and motor busses.
London, the world's biggest city at the turn of the last century, was manure free for the first time in its history.
A hansom cab in London courtesy http://www.historic-uk.com/HistoryUK/HistoryofBritain/Great-Horse-Manure-Crisis-of-1894/.