Smaller counties in Iowa usually use the county number and badge number. County numbers are assigned alphabetically. The sheriff of the county is typically badge #1. Cities that are dispatched by the county may or may not use the county number, but usually still use their badge number unless it's a larger city (Polk County cities, Sioux City, Council Bluffs, Davenport, Cedar Rapids, etc.) For example, one of the towns near me in Boone County has badge number 500-50x, so they would be sent as 8-500, 8-501, 8-502, etc.
Des Moines, our largest agency, does it this way (courtesy of the Des Moines Scanner Squad Facebook):
***Watches and car/officer numbers--
First number is watch:
100 series = 1st watch (9 pm - 7 am)
200 series = 2nd watch (6 am - 4 pm)
300 series = 3rd watch (2 pm - 11 pm)
2nd number is assigned side of town:
0 & 1 = west side (ex.: 210, 106)
2 = east side (ex.: 321, 123)
3 = south side (ex.: 234, 331)
4 = downtown (ex.: 142, 246)
The north side is covered by either west side or east side, depending on what side of the river it’s on. They don’t have their own middle number.
3rd number is beat.
123 is 1st watch, east side, beat 3, and so on.
Wagon is 61, so 161, 261, and 361 depending on watch.
CSI/Ident are 99 and 99 Adam.
500 = watch captain
510 = watch lieutenant
501 = west side sergeant
502 = east side sergeant
503 = south side sergeant
504 = downtown sergeant
400 series are traffic
600 series are neighborhood and school officers
700 series are the detective bureau
Adam is used if there is more than one car on duty on the same beat at the same time.
Baker, Charlie, and David are the supervisory (Sgt, Lt, or Capt) cars for each shift. Baker is first watch, Charlie, is second watch, and David is third watch.
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If you're talking specifically about unit numbers on their cars, I'm not sure how any of the cities do that. Some have numbers, some don't. The county sheriff's offices that have take-home cars (most if not all), the license plate is usually their badge number. State agencies also generally use badge numbers for their cars (I'm not sure what DNR does with their conservation officer's trucks).