R and R

Last week, on May Day, I found this cryptic symbol sprayed on the pavement next to a bus shelter on Hill Lane in Southampton:

I photographed it mainly because of the shadows, of course.  Then, at the weekend, without realising what I was looking at, I found something very similar in a tunnel under the railway line near Hockley Viaduct outside Winchester:

Is it an "R"? Or a "12"? Or is it the common symbol for some infrastructural object  -- pipes or wires, maybe?  It seems too simple, discreet and utilitarian to be someone's grafitti "tag".  I'm intrigued.