(Anyone else here who is an emergency responder, jump in)

Be careful when you use a scanner to get information about emergencies. Not because of the legalities of it (check local laws, especially for mobile use) but because over the air (OTA) communications are increasingly a small part of the overall information flow, and OTA communications are still highly fragmented in most jurisdictions - EMS and Fire and Police and Roads Department and everyone else operate on different bands, with different radios and so on. At the dispatch center, they have an array of communications gear that lets them hear and speak with multiple agencies, unless you're ready to drop a lot of money on a lot of scanning gear, you'll never actually get the big picture.

Then there is the matter of how what seems important on the scanner is masking what's important to people not in the area of the emergency.

If you were listening to a scanner when the bomb scare happened in NYC a few weeks ago, you would not have heard a thing of importance about anything other than the immediate closure zone - and there was much other stuff going on that day, in particular incredible traffic at the tunnels.

Just remember that scanners are handy - but not at all the "big picture" information that you need.

Depends upon the complexity of the environment.
NYC and LA are probably the most complex scanner environments around so they'd be difficult to deal with. Some cities have encrypted so they are right out.

Other areas are nowhere near that complicated. Colorado is pretty advanced in terms of using digital trunked systems but this doesn't make it impossible to understand the situation.

2 of the 3 counties, the city, and the state agencies I listen for on my daily commute are on digital and are (so far) straightforward to listen to.

1 county and 1 city I listen to are still on VHF analog systems and even easier to listen to.

Even as a longtime scanner fan and power-user I'd probably not attempt to try to listen to NYC unless I moved there. And then I wouldn't expect easy success.

I think Wyoming still uses smoke signals in some areas :-)

I think even the NSA has trouble figuring out what's being said in radio traffic in NYC and LA.

Orange County, California is one of those areas so that's a large, densely populated metropolitan area that is pretty inaccessible for scanners. I had been thinking of getting into scanning when I first moved here but then I learned that we have a countywide digital, trunked, encrypted (at least all the LE traffic) comm system, so scanning is mostly useless here.

Orange County Fire Authority and the various wildland firefighting groups, like US Forest Service and CalFire, probably are not encrypted yet or on this countywide system so scanning might be useful during wildfires. But around here, there is so much realtime news coverage from the ground and air of wildfires (and high speed car chases) that you don't even need a scanner to know what's going on with your own eyes!

The MVT (Motor Vehicle Terminal) is an increasingly important form of emergency communications, regardless of what the voice network is doing. When I was an officer in the fire company, we also had direct-to-dispatch phone numbers where we could bypass the radios entirely. For really important stuff (like, "tell the coroner the guy is dead already") we would never go over the air. We also used text messages quite a bit for dispatch details.

And, of course, as comms go digital, they get much harder to deal with and the costs go way, way up.

How messed up is NYC Emergency radio traffic? Let's see.

First of all, the radio systems used by FDNY and NYPD are 100% incompatible.
Secondly they use obsolete and absurd 10-codes. Not the same 10 codes for cops and fire - no of course not! There's separate 10-codes that overlap sometimes but not often.
Oh, and Transit uses "12" codes!
There are two separate police agencies at Penn station - NYPD Transit Division and Amtrak, they are on separate radio systems and Amtrak and NYPD don't share codes. Then there's the 800 Mhz trunked city system, used sometimes by some emergency responders in some situations. Oh, did I mention that EMS - part of the fire department- runs separate radio systems too? (with 1.2 Million calls a year, I guess they need it).

It goes on and on and on...LA is much easier, but still complex to scan.

Our local agencies page info they don't want to air such as combinations, where to find the door key and so forth.

NYC wasn't anyplace I was going to visit anyway.....

Isn't NYC one of the scanners are illegal places. KY is another, one more reason to never visit the SIL.

Get a ham license, bypasses all the scanner laws.

OCFA does have encryped, or at least multiple, channels. I used to work EMS in the OC, and our scanners had them programmed; unfortunately we didn't get all their side channels.