An automobile could quickly deliver several officers to a trouble spot, but the biggest advance in police work came in 1920 when the New York City Police began using a fleet of Radio Motor Patrol Cars. The increased mobility took such a bite out of crime that cities across the country, but few areas were as well suited to Radio Cars than sprawling Los Angeles.
Phillips H. Lord's Gang Busters, which premiered as G Men in July 1935, is considered one of the first programs to use actual law enforcement cases as script inspiration, using material from the files of the FBI until Lord's relationship with J. Edgar Hoover disintegrated and he began looking at state and local police files. On the West coast, however, Calling All Cars began tearing up the airwaves two years earlier.
Sponsored by the Rio Grande Oil Company, which had service stations in California, Arizona, and Nevada, Calling All Cars used cases from the files of the Los Angeles Police Department. Like Dragnet two decades later, the program explored cases from all facets of police work, not just the high-profile cases of the homicide division. Robbery, missing persons, fraud, even traffic enforcement all got their time in the sun. Rio Grande did an admirable job of pushing "police car performance" for cars which filled up at their stations as well as some very attractive premiums. While Dad was getting the tank filled and the oil checked, the kids could fill out their application for the Junior Detectives in the backseat. They would receive a Detective Kit and instructions for neighborhood watch type duties. Stations also handed out a free publication called Calling All Cars News, which mostly featured previews of upcoming episodes.
Another feature shared by Gang Busters and Calling All Cars was using actual cops as announcers for part of the program. For all their talent and dedication as crime stoppers, policemen are not trained radio actors, and their halting delivery read directly from the script add authenticity to the production. Presaging the "Just the Facts, Ma'am" format of Dragnet nearly two decades later, Calling All Cars was willing to display some of the tedium of police work. Several episodes would not hear a shot fired in anger, but the cops always managed to get the bad guy, thanks to the virtues of hard work, tenacity, and having the support of law-abiding citizens on their side.