Around here the police have frecuencies ower the FM band (88-108Mhz) on ~120mhz.
The police in the us also use some phones on the 450mhz band.
But my knowledge on us police bands is limitted. Now days scanners are nearly useless here as the pigs are using a encrypted digital network, like the gsm cellular one.
But in the us a scanner radio would be a good investment.
Also, don't forgret http://www.scanamerica.us/index.php
They stream trafic from police scanners to the net! "ScanAmerica: Live Police, Fire, and EMS Scanners"