This piece is part of the Sidewalk Talk series "15 Innovations That Shaped the Modern City."
For nearly a century, the traffic signal has changed a lot more about our cities than what happens when cars meet at intersections. They brought much-needed safety and order to bustling urban streets, enabling more efficient movement of residents and workers and more vehicle-driven commerce. Then again, safety and growth had consequences: Cities quickly came to embrace auto-centric cultures in which pedestrians became outcast "jaywalkers," bicyclists had to fend for themselves, and public spaces were shifted to the periphery. As Henry Petroski, professor of history at Duke University and author of The Road Taken: The History and Future of America's Infrastructure explains: "Had it not been for a way to control traffic, cities never would have grown the way they did."
Traffic in cities had become a growing public safety issue in London in the 1860s, when horse-drawn carriages, pedestrians, and early-generation "boneshaker" bicycles all shared the roadways. In 1866---a year that saw 1,102 people killed on roads in London---British train engineer J.P. Knight proposed using semaphore signals to regulate city traffic the way the railways system did. A school dropout at age 12, Knight went to work for the London railways before he was a teenager and rose to become superintendent of the South Eastern Railway by age 25.
On December 9, 1868, operators lit the gas lamps of Knight's first-known traffic signal at a crowded intersection near Westminster Bridge and the House of Parliament. Its 22-foot, cast-iron pillar supported two arms that could be raised or lowered by crank to halt and slow traffic, each topped by green and red lamps. Police officers worked in shifts to operate it. While they became an instant hit with the city, disaster struck after just one month of operation, when a leaky gas main caused one of the lights to explode in the face of a policeman, badly injuring him. The project was deemed a public hazard and immediately scrapped---but other inventors soon took up the cause.
By the early 1900s, as automobiles began to flood city streets, traffic control often overwhelmed beat cops stationed at intersections to maintain order. In 1912, Salt Lake City police officer Lester Wire created the first electric traffic light. Aiming to relieve patrolmen from standing long hours directing traffic in every kind of weather, Wire built a plywood box with red- and green-painted light bulbs in each of its four sides. Wire's "birdhouse" signal was at first ridiculed by locals---and was still operated manually---but it caught on.
A few years later in San Francisco, inventor William Ghiglieri came up with a key feature---the addition of a timing mechanism that automatically changed the light from green to red at preset intervals. The timer ran like a clock using a weight suspended from a cord, which had to be wound by hand regularly. In Detroit, police officer William Potts expanded on the idea by building a three-light system, adding a yellow light for "caution" for the first time. The signal proved so effective that Detroit had installed 15 of them within a year.
In 1923, Cleveland inventor and newspaperman Garrett Morgan patented another automated signal. In addition to two arms that signaled Stop and Go, it featured a third "all stop" arm that halted traffic in all directions. The beauty of this tool was that it could be mass-produced at low cost. This advance caught the attention of General Electric, which bought Morgan's patent for $40,000 (or about $560,000 today) and used it to build an early monopoly in the marketplace for traffic-light manufacturing.
Traffic signals arrived just in time. By 1925, car-related accidents accounted for two-thirds of all fatalities in cities with populations of 25,000 or more, many of them pedestrians and children. Traffic lights gave cities a simple, cost-effective means of controlling the flow of cars and reducing risk of collisions. Ultimately, they contributed to the steady decline in car fatality rates in the 1930s and '40s.
Signals also quickened the pace of urban life---particularly commerce. Before the installation of traffic lights on New York City's Fifth Avenue in the 1920s, a 1.25-mile trip between 57th and 34th streets took more than 45 minutes. With signals, it took less than nine minutes. Before signals, Petroski says, "it cost a lot of money to a business to have its trucks stuck in traffic."
The flipside of that evolution is that cars and streets began to dominate and shape a city's physical and cultural makeup to the detriment of pedestrians, who could no longer inhabit public spaces with the freedom of earlier eras. In his book Fighting Traffic: The Dawn of the Motor Age in American Cities, historian Peter Norton writes that cities were "physically destroyed and rebuilt to accommodate automobiles." By the 1970s, software took over the operation of signals, but cars still ruled cities, as they still largely do today.
Recent advances in technology are helping traffic lights become efficient and responsive to all street users, not just cars. Smart traffic signals can react to live traffic conditions using artificial intelligence software. Some are equipped with cameras and other sensors to detect walkers and cyclists, and adjust signal timing as needed. Vehicle-to-infrastructure technology lets cars communicate wirelessly with traffic lights to improve traffic flow and safety.
Once more fully autonomous vehicles emerge, the days of the traffic light may be numbered. Researchers at MIT's SENSEable City Lab envision a signal-free city, with connected cars using sensors to maintain a safe distance from each other through four-way intersections. But the concept poses the same question to local leaders raised by traffic lights a century ago: Will cities prioritize cars at the intersection, or will they prioritize people?