Unfortunately, it is NOT just a SoMD thing. Roadside memorials trash up many parts of this state, as well as other states. Some states specifically outlaw them outright (CO, MA, WI). Some tolerate them with some caveats (WV, DE), but most states have laws - like MD - which makes it illegal to 1) erect private signs on state rights-of-way, and 2) to abandon property on rights-of-way.
Also unfortunately, MD is like a lot of other states in which the DOT looks the other way or has a "policy" which says that they don't clean up this particular form of roadside litter. How a state agency can have a "policy" which has no lawful authority, is beyond me. By law, MD DOT is charged with maintaining (including cleaning) the state's roads. By law, it is illegal to litter on the state's roads. By law, any private property left on the side of the road is considered "abandoned property", and becomes (legally) trash. So, how then can the agency "officially" say they are not going to enforce their lawful mandate and clean up this trash?
Understandably, this is an emotional issue. But people like me, who have lost a family member in a roadside accident, don't neccesarily agree that there is ANY justification to these small-minded people who think they can appropriate a piece of public property for their own personal use just because someone they were related to died there. It's just not an appropriate place to build a private memorial. That's what your own property and graveyards are for. Try setting up a memorial in the parking lot of Wal-Mart where your uncle died of a heart attack, and the Boys from Bensonville will have you in court on trespassing and possibly destruction of private property charges.