Safety at railroad crossings evolved from simple warning signs to electrically activated signals along the Camden and Amboy Railroad, which began operating on these tracks in 1835 and became part of the Pennsylvania Railroad after 1871. From the 1830s until the early 1900s, wooden or cast iron signs reading Stop Look and Listen or Look Out for the Locomotive were often the only protection at roadway crossings, though some busy crossings also used flagmen and manually controlled signals, bells, or gates. The X-shaped crossbuck remained the most common warning device, but the introduction of the low voltage track circuit in 1872 allowed railroads to develop audible and visual warning signals at grade crossings. Between 1890 and 1920, hundreds of warning devices were installed at unmanned crossings, many combining flashing lights, ringing bells, and swinging arms to warn of approaching trains. As electrical technology advanced during the mid 20th century, railroads adopted more standardized crossing signals with a crossbuck, red flashing lights, and a bell, operated by switches and relays controlled by the track circuit and supported by storage batteries in concrete battery boxes during power outages. Illuminated No Left Turn and No Right Turn signals, known as blankouts, also warned vehicles away from tracks on streets running parallel to the road, and the Pennsylvania Railroad's extensive use of them helped give the route between Camden and Bordentown a distinctive character. During the 1950s, the Pennsylvania Railroad installed grade crossing equipment with electrically powered groups of 4 or 8 red flashing lights and a bell at most crossings along the former Camden and Amboy Railroad right of way. Though today's signals are larger, brighter, and controlled by advanced electronics, their purpose remains the same: to protect pedestrians and motorists and remind everyone to use caution when crossing the tracks.