OTOH, if traffic engineers really believed in the power of signs, they wouldn't bother locking up their offices at the end of the day. They could just hang "No Trespassing" signs on the doorknobs.
I was visiting my mom recently; her neighborhood is full of roads designed to kill people and the verges are full of signs uselessly exhorting drivers to slow down. Drove me crazy. The delusion that (other) people will just do exactly what they are told is sooo pervasive.