In this sentence
He went outside. He looked down the road.
What is meant by down? I don't think he's in higher place.
"He looked down the road", or "he looked up the road" can simply mean he looked in a particular direction along the road. People tend to use these even when the ground is flat. Though up and down can have particular geographic meanings to particular people in particular locations.
If the road was at a different elevation to himself, you might say: "He looked down on/onto/at/to/toward(s) the road", or "he looked up at/to/toward(s) the road."
I always thought that "looking down the road" was a way to make a summary of your past: "As he looked down the road he realized he'd make so many mistakes since he broke up with Annie". Is that incorrect?
But literally, looking "down the road" points at the direction car are crusing to; when looking "up the road" would indicate the direction they come from.
In most cases when down is used with roads, as in this case, it means that the road is leading to a place that's probably at an elevation lower than where the person stands.
So if you go from point A to point B, and point B is located at a lower elevation than point A, you can say that you looked down the road or you are going down the road to get to point B. If it's the other way around, you can say you looked up the road.