Simple achievement to get but just in case:(possible spoilers)
After the car crash, you need to break through the window, the walk around the car and upto the cop (its all pretty linear)
then you need to get the keys for the hand cuffs, undo the hand cuffs. The cop wakes up as a walker, you need to crawl back to the car, look right, grab the shotgun shell, look left and grab the shotgun. then he drops the bullet trying to load it, pick it back up and shot the zombie in the head. IMPORTANT! the target must be on the zombies head as you only get one shot at this so take your time.