It is a highway shield, usually with a cardinal direction sign, that repeats the name or number of the current route.
On larger roads, reassurance markers are sometimes posted on a sign that is elevated on a gantry.
In the United States, reassurance shields are defined in Section 2D.31 of the Manual on Uniform Traffic Control Devices (MUTCD).
[1] The MUTCD recommends that reassurance assemblies be placed: The MUTCD requires a cardinal directional sign to be posted with the route shield to further reassure travelers that they are traveling the correct direction on their route.
The Vienna Convention on Road Signs and Signals specifies that "road identification signs" consist of the route number framed in a rectangle, a shield, or the relevant state's route classification symbol (if one exists).