http://www.slate.com/blogs/the_eye/2014/08/27/roman_mars_99_percent_invisible_on_thomassons_architectural_relics_that.html
one connection to roads is that there is a sign on the 26th St. bridge over the Arroyo Seco Parkway in Los Angeles that says "11/163 separation". 11 is now 110; 163 is no longer a state highway. the sign is obsolete as of the early 70s. yet I have photos of the sign covered in graffiti in 2006, and meticulously cleaned off in 2007.
I'll bet there are other examples as well of road infrastructure that does not serve any purpose - certainly not the original intended purpose - but is maintained anyway.
I guess abandoned traffic lights count as Thomassons.
Edit: they probably don't, actually. Unless the city is still maintaining them in case they ever get activated.