Monitoring Traffic
Some law enforcement agencies design their own internal codes. Hence, you have to monitor radio traffic to ascertain what they are saying. Here in Texas, I have monitored an elusive police department without much success. I have yet to figure it out their codes. Furthermore, I've been listening for quite some time. This LE agency has mixed the expanded APCO ten codes with a version of their own
Some may use one of many sources for their communications. Here is a link to various U.S. codes:
RADIO CODES & SIGNALS - OKLAHOMA
For the most part, I have generally referred to this source for APCO 10 codes across the U.S. since I believe it is the most widely used listing:
Expanded APCO "10" Codes"
At times, it is necessary to make field observations of an officer stopping someone to ascertain what codes are being used. Of course, the most reliable method of figuring these codes out is to monitor radio traffic when you are pulled over for a violation. No, I don't recommend this method for figuring these out, but you might as well take notes if you are stopped by police.
If you get a warning or a ticket, the codes they use might be enlightening.
Good luck.