They hung up trees with ropes about 10 kilometers from the Russian border to hide the road from airplanes.
It looks like it's not enough to cover the road but, when the road was viewed from an angle (not directly above) and a high altitude, the trees concealed enough of the road to make it difficult to identify as a road.
Source for original picture is The Finnish Wartime Photograph Archive
Very interesting! I'd imagine too that even if some of the road was still visible at different angles it still wouldn't really seem like a major roadway with the lines broken up by trees its much more likely to be dismissed as significant from the air.
It's an attempt to reduce the ability to visually navigate. Major or not, you lose the road you lose where you're going.