The number of automobiles and trains increased significantly in the early twentieth century, and so did collisions at railroad grade crossings. Many drivers did not stop or even slow down at crossings. Automobiles presented a more serious problem than horse-drawn vehicles because of their greater weight and speed; collisions endangered not only motorists but trains and their passengers.
Railroad companies placed warning signs on roads leading up to grade crossings and installed bells at the tracks. But many drivers ignored the signs, and automobile engine noise drowned out the sounds of bells and trains. In 1921, Charles Adler, Jr., a Baltimore inventor who devoted his career to improving road, railroad, and aircraft safety, designed and installed a train-actuated crossing signal that turned a double STOP sign toward motorists when a train was approaching. The movement of the sign provided a visible warning that driving conditions had changed.
Railroads then tried flashing lights to get drivers' attention. In 1922, the Association of American Railroads standardized crossing signals, adopting as a uniform design a pair of alternating flashing red lights that were actuated when a train was approaching. Charles Adler, Jr. patented an electrical relay that created an alternating or wig-wag light pattern. Manufactured by SACO (Signal Accessories Corporation), the Adler Alternate Flashing Relay was used by more than 40 railroads in the 1920s and 1930s.
Increased automobile traffic also brought a need for more effective traffic control devices and systems on city streets. Hand-operated semaphore signals at intersections were replaced by electrically lighted signals beginning in 1912. Automated signals were introduced by 1914, interconnected traffic signals by 1917, and four-way signals by 1920.
Refinements to traffic signals helped to control driver behavior and reduce accidents and fatalities. In 1923, Garrett Morgan, an African American inventor in Cleveland, Ohio, patented a signal that stopped vehicles in both directions before changing the direction of traffic flow. This brief pause reduced the possibility of a collision caused by a vehicle continuing in motion after the STOP signal was displayed. The safety interval was standardized in a different traffic signal that superseded Morgan's design: the three-position signal with red, amber and green lenses.
Some inventors thought that signals mounted on automobiles would help prevent accidents by improving communication among drivers. Several individuals patented tail-mounted turn signals that indicated an impending left turn or right turn. Jonathan Cass Stimson invented a reflector that revealed the outline of a vehicle in the dark. Stimson's "central triple reflector" had angled, cube-shaped cells that reflected headlight rays back to their source regardless of direction.
Some inventions provided information about position and speed. Charles Adler, Jr. developed the Spaceometer, a gauge that showed how many car lengths should be maintained ahead of a car at various speeds. John Voevodsky invented the Cyberlite, a rear-mounted flashing light that showed how rapidly the car ahead was decelerating. Thousands of Cyberlites were sold as accessories, but the Center High Mounted Stop Light, a steady light, became standard.