Before cars filled city streets, London tried to keep horse-drawn traffic moving with a gas-lit signal.
In December 1868, a signal appeared outside the Houses of Parliament. Railway engineer J. P. Knight proposed it to help police manage carriages. By day, an officer moved semaphore arms; at night, red and green gas lamps showed drivers when to stop or proceed. It looked more like a railway signal turned upright than the familiar electric lights of today.
The system was manually operated, and the gas lamps created a serious danger. Washington State University's history of traffic signals reports that the apparatus sometimes exploded and injured police officers. The experiment was removed after a short time. Electric signals came decades later, as automobiles changed the scale and pace of street traffic.
That first trial was not a direct ancestor in every technical detail, but it shows cities already wrestling with the same question: how can strangers share a crowded road safely?