To me it looks like an old road that was turned into a walking/biking trail. We have some defunct railroad lines that were converted to pedestrian trails and even though I could easily ride a scoot on them, they've gone to the point of posting signs that say no motorized vehicles. So with that said, you can probably figure out by the surroundings if it is a motor vehicle road or a walking trail.
When I see that sign it suggests to me that a bridge is closed and the only access is to the individuals that happen to live on that road.