From what I know, those sensor are wire loops. Thus, the best way to trigger them is to position the metal in the center of the loop. It is easy to see the cuts in the asphalt from where the wire is buried.
We had a problem in our garage door. Sometimes, when the car was manoevring, it won't cut the infrared and the door would start closing, and a couple of times it hit the car. So we installed wire loops in the garage, in such a way that when the car is manouevring the door won't go down.
The hard part was to fine tune the sensitivity. Too much sensitivity and the door closing down would be sensed as a car, and the door would start opening again in a never-ending cycle. Too little sensitivity and the car wouldn't be detected. The hysteresys cycle was very narrow. It also depended on the temperature. Finally, we had to cut new grooves in the floor to make a smaller loop, further from the door, and now it works fine.