The traffic light.
The Traffic light was invented in 1868, but numerous years passed before it was perfected. JP Knight created a signal based on red and green gas lights. It had to be operated manually. About one month later it exploded on an officer.
In 1912 Lester Wire invented the electric traffic light. In 1914 James Hoge improved upon the system by adding a buzzer to notify drivers that the light would soon be changing colors. In 1920 William Potts made a four way signal. William replaced the buzzer with a yellow light. Finally in November 23, 1923 Garret Morgan added a mechanical linkage to allow officers to operate the signal from a distance.