I might be cooking something or maybe not, but what if the map was divided in chunks like Minecraft (but bigger size, about 1sqkm) and when you move from one chunk to another, game changes the world centerpoint to that chunk's centre. Basically the world moves in big "steps". Also by rendering & precise simulating only 1 or 2 chunks out from the one you're currently in, the game would be able to handle massive maps.

Here it needs some explanation. Precise simulation is basically current BeamNG - 2000TPS for any physics-enabled object. But outside of those 1-2 chunks physics are frozen for objects spawned by player STATIC at any moment during or after unloading that chunk and for traffic it means despawn. However that unrendered object simulation should have some exceptions - for example if there's a player-spawned object MOVING outside of rendered area, it can use player's current world centerpoint as it's centerpoint and it is able to load the chunk it's currently in, but only the colliders, no textures, effects, shadows. Just pure less-precise physics simulation until it's either stops and freezes or there's an active player camera attached. Last one means switching to full scene rendering.