At the moment I cannot offer to help testing---I would like to but I can't. However, I'm really interested what your approach is. Please explain in detail!
One guy also tried something similar, he made an extension for MTA with ped and car traffic, but I don't know why it didn't become so popular. Perhaps performance issues. Here is the link: http://crystalmv.net84.net/
I was also making thoughts how to do it (I wrote it in small font below to not distract from your approach).
I'm not working on it due to other pending projects. I call it "dynamic peer-to-peer multiplayer": There is a single worldwide server for all players. The server receives only position data from the clients and dynamically connects them to each other when two players are in the same area, e.g. radius 200. When two players are connected then the whole synchronization (cars, peds, etc) is done between the two clients only (hence, peer-to-peer). This means, only two player can see eachother at a one time, but since players are dynamically reconnected to other players you can meet hundreds of other players when you move around. If a player is disconnected his ped/car becomes a normal AI ped/car. If a p2p connection is established the algorithm tries to "morph" the positions of overlapping cars; half of the peds/cars are controlled by one client and half by the other. It would be 100% single player plus other human players that you encounter as you play. Special modes, like races, gang wars, sport games, etc would have special modes where number of AI peds/cars is reduced allowing synchronization between more clients (e.g. 4 clients). (goodidea82)