How do you free roam on la noire

You can free roam in LA Noire at any time, but if you do so during cases, it will affect your rating if you cause chaos and damage. If you want to truly free roam, complete one of the ranks and you will unlock a cost free roam. For example, complete Traffic Desk and you will get to free roam in that rank. Complete Homicide Desk and you will unlock free roam in that rank, etc.