Basically, about a quarter of the lobby act as 'cops', whilst the other players act as 'cruisers'. We play on Le Mans and have set speed limits for each stretch of road. When a cop sees a cruiser acting in an illegal manner, they follow the cruiser until he/she pulls over. If they don't pull over, a pursuit is engaged.
When you are pulled over you need to go to the party chat with the other cops and discover your punishment.