The plural of Deer is Deer.
The plural of deer is deer, by the way.
They're not stupid, sounds like your father was ether going faster then the speed limit or doing the speed limit, at night time in Australia you're a moron if you do ether because of the amount of kangaroos that bounce across the road, same would go for deers. I narrowly avoided a kangaroo once, doesn't matter where you are they'll jump across the road.
Uh... That's his Dad's car, and his Dad was driving...
This isn't Australia. Driving on the highway at night nobody in my family has ever even had a deer run across the road in front of our cars so he really had no reason to be overly cautious.