There are mods like this, i don't know if they are working or not. Game keeps getting updates and shitting into good old mods. Check their comments to see if they are still working.
I personally use those, one of them makes cop spawn regularly; other mod makes cops fine you for being an asshole without getting a wanted level; with final one you can properly off road and escape cops without thinking if my vehicle is going to explode when it fells from a small distance, like it was in gta IV.
I don't know if they work with story mode missions, i completed story mode and i just love shitting around.