Railroad signals took many forms, including position light signals, dwarf position light signals, semaphores, and grade crossing signals. Introduced by the Pennsylvania railroad in 1915, position light signals used rows of high-intensity yellow lights to communicate track conditions by mimicking the positions of semaphore blades, with yellow chosen through trial and error as the easiest color to see in all weather conditions. Smaller dwarf position light signals served as route indicators in slow-speed areas such as busy junctions. The semaphore was the first electrified railroad signaling system; at first the blade's position alone conveyed information, but railroads later added green, yellow, and red lenses rotating over a single white bulb to make the signals easier to read at night. At grade crossings, where railroads intersect with roads, warning signals activate about 30 seconds before a train approaches, using red lights and a bell to warn people to stop and wait for the train to pass.