Roadside memorials are markers that usually commemorate a place where a person died unexpectedly and suddenly away from home. It's not a grave site as there is no body there but marks the last place on Earth where that person was alive. before being killed in a car crash (even if they died in the hospital after the crash. The memorial is usually created and maintained by family and friends and can be made up of anything from flowers (plastic or real), messages, stuffed toys, crosses, mementos to anything else that reflects the deceased.
Roadside memorials have been around for centuries with the origin of crosses in the U.S. being placed by Hispanic settlers in the southwest. In funeral processions when a group was carrying a coffin from the church to the graveyard, a cross was placed where the coffin was set down so that the bearers could get a rest.
At the Delaware Highway Memorial Garden at the Smyrna Rest Area, mourners can sit in a garden lined with bricks inscribed with the names of nearly 600 crash victims. Many people prefer this type of memorial to the roadside ones with crosses and flowers. It opened in 2007 with 268 bricks and since the has grown. This garden is used as a model for other states.
Many states have laws prohibiting roadside memorials but the law is often ignored because states are unwilling to enforce it.
Source: USA Today