This is an extremely old trick I know, and no doubt every fanboy and his mum knows.. But after searching I can't see anyone put it into the Cheats & Tricks area.
The reason im putting this up now, is because ive just spotted a few threads moaning about cars.. mainly the underground tunnel from island to island.
So heres how you do it.
First off, it's gonna involve cheating.. so simply don't save after it if you're against cheats..
1. Find an open area, preferably the docks.. as its off-road... 2. Do the spawn Rhino cheat around 9 times, every two times you do it.. move a few feet so they don't land ontop of each other causing an explosion (then they tip upside down) 3. Liberty City is car-less... Vehicle-less.. Empty.. call it what you will.
Why does this happen? Easy.. the game can only handle a certain amount of cars at one giving time.. Now with the tank its always recognized, and kept at its last location.. therefore no more cars can be found as the computer/console is remembering the tanks.
Im not sure if Police come.. If I recall I dont think they do.. but who knows. This is a great tip for speeding around.. however, if does also mean if the car you're in blows up..or you lose it.. you will do a lot of running to find a new one.. which has to be parked. Enjoy.