I walked through this tunnel nearly every day, five days a week, for four years during my undergrad stint at NCSU (I rarely made it over to that part of campus when I was getting my MA in English). I once saw a section of wall peel downward in thick sheets of layers of paint that had accumulated for years. It always struck me that if people continued to paint over the top of what already existed, the tunnel would continue to narrow until it became unidirectional.
The Free Expression Tunnel is the longest, widest, and most heavily used pedestrian tunnel under the railroad tracks at North Carolina State University in Raleigh, North Carolina (also goes under Yarbrough Drive). The special aspect of the tunnel is that anyone is permitted to decorate its walls, ceiling, and floor. — Wikipedia