People did make that mistake quite often in the start. But in valve maps people get quickly used to the maps as most servers overplay them. Many servers where you join in half of the players know the way and the other half just follows them. On the server where i played that map came only on once every 3 hours for just 20 minutes. There those direction problems showed up the first 2 days and after that people finaly got used to it.I've always kind of wondered what makes people go left out of spawn in Badwater, when RED initially spawns and go to the first point. As far as I know there are no signs telling you to go left rather than right, but I have never seen anyone go right and wonder where the hell they just ended up.
The mistake he made is to have just 1 sign. instead have 2 signs at least and have 1 a bit more to the left so if they turn left they can still spot it and 1 to the right to let people confirm they walk the right way. And both should be visible on leaving the area in front of it. maybe even have some part of the wall sticking out that forces them to move right.
Blocking the way isnt needed but its the easiest thing to do.