From Ann Ball:
In many parts of the American Southwest and in Mexico, travelers pass numerous reminders of journeys not completed. Solitary crosses by the wayside stand as sentinels of love and death, and the promise of the resurrection. “Be careful,” they silently plead to the passerby, “finish your journey on this earth in safety before you go on.”
The descansos (resting places) are a death-related aspect of folk art which stems from old traditions in Mexico and the Spanish – settled areas of the Southwest. In the old days, when the body was carried from the church to the cemetery, the pallbearers often had to stop and rest, as the distance was far and the burden heavy. The places where the procession stopped to rest became known as descansos. As they entered the graveyard (camposanto), there was a ritual stop at the entrance and each of the four corners of the cemetery. Here, decades of the rosary or requium prayers were said. Many cemeteries were built with special shelters to mark these stops, and shelters were also sometimes built at the stopping points on the way from the church to the cemetery. Although the early Spanish settlers preferred to bury their dead in a camposanto, this was not always possible and many were buried where they died. Crosses were placed to mark the gravesites. Later, the custom extended to marking the place of death, even when the body was carried to a cemetery. The descansos on the way to the cemetery and, of course, the crosses marking the graves in the open, were hallowed by custom, and soon the crosses and small shrines erected along the roadside at the site of a death in an automobile or other accident acquired the same hallowed ambiance.
Created out of love in a time of pain, today’s roadside descansos are truly folk art. They are made of many materials, sometimes even including the parts of the cars involved in the accidents. Each is unique, and yet similar in the expression of love and bewilderment on the part of the families who erect them. Generally, the families who choose to erect a descanso in memory of a lost loved one, place a temporary wooden cross at the site soon after the accident. This cross remains until the permanent memorial is built. Sometimes the wooden cross is left after the memorial is built, or incorporated into it in some way.
Also. here is an informative short video about this tradition: