It seems that Adam didn't notice hitting Delia that is why he drive off. He needs to tell them what happen before they figure out it was him. I know not going happen, especially since it's common sense to do.
This horrible tragedy. I truly believe Adam thought he had Delia he would had stop. But he believed only living thing out there was the dog. I would steered away from the dog and if I didn't think I hit anything I would have drove off.