Only a single state, New Jersey, has a law requiring (some) drivers with a learner’s permit mark their vehicles. This small decal is affixed to the license plate and only identifies teen drivers, not the older or foreign driver that has finally decided they need to learn.
Other drivers need to be able to identify inexperienced drivers at a glance. Foreign L-Plates (a square decal with a large “L” for learner) are often colorful window decals located in both the front and rear windows. This identification is for BOTH the safety and comfort of the new driver and the vehicles around them.
The vast network of roadways in the US provides a logical and valid reason that uniform L-Plates be introduced. As states control licensing, rather than federal, a recommendation is a good place to start.