15 years ago when I started wearing a medic alert bracelet, police and emergency personnel were required to notice a bracelet, but not a necklace. This came from two cases with diabetics in one year, one in California and one in NM - in both cases, diabetics were thrown in drunk tanks and (at least one of them) died. The courts decided it wasn't the emergency personnel’s fault for not noticing necklaces.
Don't know if this is still the case, and don't know how it would apply to road ID, just a thought.