Recently, some states have begun to operate statewide radio networks with varying levels of participation from police on the county and city levels. Some of them are:

Illinois: StarCom21
Louisiana: LWIN Louisiana Wireless Information Network
Michigan: MPSCS Michigan Public Safety Communications System
Minnesota: ARMER [1]
Montana: Montana Statewide Interoperable Public Safety Radio System [2]
North Carolina: VIPER [3]
Ohio: MARCS - Multi-agency communications system
South Carolina: Palmetto 800
Wisconsin: WISCOM [4]
It is generally legal in the United States to listen to unencrypted police communications. However some states prohibit such listening within vehicles.