Roadside memorials have a long history and in the Southwestern US they are usually marked with crosses as the main feature originally due to the Hispanic heritage of the region. It does not mark the place where someone is buried but where they were last seen alive and not necessarily where they died. These memorials are created by family and friends to commemorate the person and the cause of death is due to some sort of traffic mishap.
These markers are also meant to serve as a warning to travelers cautioning them to drive safely. The Arizona Highway Patrol started placing white crosses in the nineteen-forties and fifties to mark the site of fatal car crashes and the tradition was carried on by family members of fatal collisions. In Arizona and New Mexico Department of Transportation workers are not required to protect them but generally they are left unaltered or moved if necessary out of respect for the caretakers.