Jim - I know what you mean. I myself have wanted to do a project where I take a section of highway and shoot every single one of those memorials. Sort of a snapshot of that area.
Living in the Southwest, those memorials are VERY common. I have seen them in other states, and they are not just a hispanic or catholic phenomenon, but they seem to have origonated as such. In fact there are a couple of spanish words for them, although I can not remember them right now.
In Tucson, we have one that is very old. In the late 19th century (IIRC), a woman had an affair with local man whom she met at the railroad tracks. The enraged husband killed him and placed him on the tracks so his body was scattered along them for miles.
Locals at some point turned an old adobe wall with a hearth in it into a shrine for the man and his lover, whom legend says can still be seen wandering the tracks looking for him. To this day people leave prayers in small niches cut into the adobe wall and light candles there.