The script mod adds traffic cameras at some traffic lights, as well as a few traffic patrols at any locations you set. If you drive too fast, ignore a red traffic light, go against traffic, drive on the sidewalk - the police will send you a ticket to your account.
Also, the police will try to detain you if they see a violation or if you have too many unpaid fines. All your fines can be paid at the police station (displayed on the map) or by phone by typing "Pay fines" in your phone book.
If you crash into some other vehicle, the victim will call the police and make you pay for the broken vehicle. If you have old unpaid fines, they will also charge them.