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In a different thread on this same topic, I posted some technical information relevant to AM and FM radio, trying to bring a bit of technical nuance to the discussion, from my background as an amateur radio hobbyist.

The short version is: People talk about AM having longer range both during the day and especially at night, when really it has more to do with the wavelength than the modulation scheme. The commerical AM broadcast band is transmitted at a wavelength of ~200 meters, which has both groundwave and skywave propagation properties, where commercial FM broadcasts at a wavelength of ~3 meters which is strictly line of sight. The real relevant detail is MF vs VHF, not AM vs FM.

The modulation scheme matters with the construction of the radio itself. AM radio predates WWI, and a functioning receiver can be improvised out of literal junk you find lying around. Consumer AM radio receivers have been in production for a hundred years, and the AM broadcast band is kept deliberately primitive to maintain compatibility with those ancient radios. VHF FM radio is about 50 years old, and though it is a bit more technically advanced also goes through some pains to remain compatible with radios made during the Ford administration. Both technologies are older than the majority of Americans today and have functionally "always" been here.

In the other thread, I didn't get around to this part: Is MF AM broadcast radio a good technology for "emergency communication?" Maybe. Depends on the emergency, depends on the information, depends on the audience.

If we're talking about information of interest to motorists, AM radio isn't a terrible choice because the vast majority of cars built in the last 70 years have AM radio receivers. Same could be said of FM radio; you can reasonably expect the average car on American roads to have an FM radio. A lot of people tout the superior range of MF over VHF. For traffic, road closure and weather advisory, how much range do you really need? If the message is something like "Interstate 40 Westbound between mile marker 76 and 81 is out of service due to road construction. Follow detour marked by signs from Exit 75. Delays of up to 45 minutes expected during peak traffic times." You might want to deliberately limit the range of that signal because it's a waste of spectrum to broadcast that message from Tennessee all the way up to West Virginia, a state that has no I-40 in it at all. Mind you, not all motorists are able to receive either one. Some cars, and nearly all motorcycles, have no AM or FM radios. Things like road closures, detours and such should be 100% navigable by signage alone.

If we're talking about weather alerts, the NWS maintains a network of VHF FM transmitters called the National Weather Radio or NWR that transmit on one of seven channels around 162 MHz with weather observations, forecasts and warnings. They have a technology that transmits a code that can automatically turn on radios for severe weather warnings. These transmit with a power of 5 to 1,000 watts, and there's about 1,000 of them dotted around the country. I've been spending the last day saying to myself "Yeah, why don't all car stereos have NWR receivers in them?"

General emergency things, like widespread natural disasters, civil emergencies, national security emergencies etc. are perhaps a different story. If there's a hurricane or something, the ability for one station in Tennessee to broadcast to the entire Eastern seaboard could come in handy. Temper this with the idea that they might only reach the Outer Banks or Savanna Georgia or Rhode Island at night.

Something to consider though is do 21st century people "have" AM radios? Sure, every alarm clock, boom box and walkman have AM radio tuners...but how many people these days actually have those devices? Search on Amazon for "boom box." They've gone out of fashion. Especially for younger folks tend to use their cell phones as alarm clocks and music players. I bet there's a lot of households nowadays that don't have a working AM radio, in the same way that they probably don't have cassette players or VCRs. I imagine a large swath of the population whose only functioning AM radio they own is in their car stereo, and I imagine they never use it. Outside of Rush Limbaughs audience, who listens to AM radio these days?

It is a bit of a problem, actually. Up until the 90's, you had AM, FM and TV. You didn't have to worry about congestion on these platforms, so the normal stations people listened to or watched could be redirected to emergency broadcasts. That has changed. As noted, who listens to the radio? We all have Spotify or Apple Music. Who watches "television?" We all cut the cable for streaming services delivered via the internet, who has a set of rabbit ears hooked up to their TVs anymore? We're all used to getting information from our cell phones, which are nearly useless during a widespread power outage because everyone suddenly hits the cell towers slowing everything to a useless crawl.

Hell, winter before last there was a weather related power outage. Normal stuff, ice took down some power lines. My local FM radio station stayed up under generator power, and they were trying to broadcast a press conference with the governor...but couldn't because their downlink from the state capitol was through Facebook and the internet wasn't working correctly. They had no microwave relay or satellite uplink or whatever, they were trying to stream audio from the state capitol, across the internet via Facebook, then over the air via FM. This is the state of emergency communications in 21st century America.

Is mandating car stereos have AM receivers the solution to that problem? I'm not so sure; is it the Republicans just want to keep their rage bait shit on the air? Prolly. As I said it would be much wiser to mandate NWR receivers in new vehicles.