When Hi and Ed are speeding in their car to find Hi, Jr. you can hear a police siren coming from it. However, the car they are driving is a sedan, not a patrol car because Ed has resigned from the force. The scene switches back and forth between a police car and their sedan.
Actually... your comment is wrong. There is a sedan and a police car, and it switches shots back and forth between the two cars to build suspense. They are both going towards the same place... Hi and Ed are going to the baby in the road and so is the biker guy. The police car is pursuing the biker. If you will notice, in the upcoming scene, they are driving (in the sedan) and they hear the police car siren and then the police car gets blown up by the biker.
Angie is correct, except the police cruiser is almost certainly not chasing the biker, it is responding to the bank robbery. The officer just gets in Leonard Small's way, and pays the ultimate price.