I'd say 90/10. The positioning of the light in relation to the road is usually going to be a very good heuristic. However if that light was a freaking arrow I'

I'd say 90/10. The positioning of the light in relation to the road is usually going to be a very good heuristic. However if that light was a freaking arrow I'm going to think "oh, it must be one of those 10% of cases, it's a good thing that indicator was designed so well" shortly before I'm killed.