During World War I, police often warned civilians using signs hung around their necks:
This picture from July 1917 shows exactly how different wartime warnings to civilians used to be. Forget about text messages, public service announcements, or sirens — in the past, one of the best options was a guy walking around with a sign.
Air attacks were a new experience in WWI, and Britain's warning systems were primitive
As Tanya Britton writes in her history of World War I, air raids were a new threat during the Great War, largely because of unexpected Zeppelin attacks on London that began in 1915. (Britain eventually developed military countermeasures that stopped most of them by 1918.)
It was a substantial test for a government that had no experience with air raid threats and no effective way to communicate that threat to citizens.
That led them to devise countermeasures that seem primitive today. Londoners hid in subway stations around the city and darkened their windows to avoid detection, and the government silenced public clocks.
Police also needed a way to warn civilians about an impending attack. Though sound rockets were occasionally used, sometimes the simplest way was the best: sending police and cars around the city with warning signs. Following an attack, they might patrol again with a sign reading "all clear," occasionally accompanied by bugle calls.
The air raid siren became a necessity in later conflicts
Air raid sirens — also known as civil defense sirens — were adopted once the technology was more widely available and it became clear that attacks against homes would continue to be a threat. By World War II, the sirens became ubiquitous in Europe.
They also appeared in the United States. In Oregon, for example, where Japanese air attacks were a possibility, air raid sirens and some siren-equipped cars were available. Occasionally, these air raid sirens even led to panic, as in the 1942 false alarm that resulted in the "Battle" of Los Angeles. That alarm, triggered during the tense months after the attack on Pearl Harbor, sent the city into a panic.
In the '40s and '50s, the increasing use of radio, television, and other forms of mass communication augmented the existing infrastructure. During the Cold War, the British government developed a warning that included air raid sirens, new home sirens, and emergency broadcasts on radio and television.
In 1951, the United States established CONELRAD, a program that broke into radio broadcasts. That was followed by the National Warning System in 1957, which included the National Emergency Alarm Repeater (NEAR), a device designed to plug into a standard electrical outlet, often at home, and provide audio warnings sent through radio signals.
In large, the Emergency Broadcast System removed the need for devices like NEAR, and today the Emergency Alert System is the official United States warning system. The Emergency Alert System gives the president the ability to make a national broadcast in 10 minutes, and also allows for quick notification about weather emergencies.
Cellphones are the next frontier in public warnings
In the US, the FCC and FEMA currently use Wireless Emergency Alerts to reach people, as the agencies did after the Boston Marathon bombing in 2013 and during Hurricane Sandy in 2012.
In addition, companies like Google, Facebook, and Twitter have developed their own emergency alert systems. Along with these corporate initiatives, there have been new high-tech emergency warnings in areas with more active threats — some reports say Israel has text-messaged Palestinians before bombing.
In the future, cellphones — and other cloud-based identity services — will probably continue to be the best way to reach people wherever they are. However civilian warnings are broadcast, one thing is clear: we've come a long way from a policeman with a sign hanging from his neck.