in the road generally focuses on the space between the edges of the road;
on the road generally focuses on the surface of the road;
on the road is also an idiom meaning traveling (by car, truck, etc.).
We couldn't get through because there was a herd of sheep in the road / on the road.
It's dangerous for children to play in the road / on the road.
A truck overturned, and gallons of toxic liquid spilled on the road.
After the rain there was a lot of water on the road.
If we got up at 5, we could be on the road by 6.
CJ