In the rain.
Here's what LA Municipal Code says about abandoned vehicles:
Los Angeles Municipal Code 80.73.2 requires that a vehicle parked on a public street be driven at least one mile every seventy-two hours. Additionally, California Vehicle Code 22669 allows traffic officers to tow any vehicle which is inoperable and parked on City streets, constituting a public hazard. Traffic officers cannot simply tow a vehicle because a complaint has been made. The traffic officer must first observe the vehicle over a 72 hour period to determine that the vehicle has not been driven. Additionally, as a courtesy, the traffic officer will generally leave a green warning card on the vehicle, notifying the owner that the vehicle must be moved. If the vehicle has not been moved at least one mile after seventy-two hours, the vehicle can be impounded.
You know what's really handy? An online form to report abandoned vehicles!