I agree, and it seems like they're working on it. There are already some roadsigns in the vehicle/prop selector, but I know that's not what you mean. Searching a bit through the Developer Section (https://www.beamng.com/forums/developer-section.170/) I've found some appear like they are going to be in at least one of the new maps: Hopefully they will update all of the maps to have them once they decide on a way of implementing them. I'm really hoping they find a way of having them be physics objects.
Since there are road signs in the vehicle selector, someone could try adding them to a current map as vehicles and see how it performed compared to how it does without any, that's probably the best way to go about this I would think.
There aren't many road signs because right now you only have 2 options for objects: collision, or no collision at all. Neither of which is realistic for road signs, unless you have the sign behind a guardrail or in front of a rock. Hopefully though, in a future update, environmental jbeam objects will be introduced.
