Start out by spawning a rhino (this works for any police and other military vehicle, only player spawned tanks don't despawn). Get a wanted level and wait for a cop, fbi agent, swat,etc to come to the passenger side of the tank, as they open the door exit the tank (take too long your busted) and take away your wanted level. when you take away the wanted level the officer will enter the tank and drive away and even chase you when you attain wanted levels and they do not despawn. I spawned them in Portland drove to shoreside vale and got a wanted level and they found me from across the city. Once you have several tanks spawned they will be no traffic. enjoy.
Note: Avoid spawning a high number of tanks since they do not despawn it will exceed the amount of vehicles allowed on the map and freeze the game.