Have you seen Cessena Fun mission? Not exactly what u are asking, but the point is that there will be necessary to have A LOT of units. That causes lag. A lot of lag. Of course, if you want them all going and doing whatever such people do. Maybe it could be possible to make it just in one city, some Petrovice or smthn. I don't know if it reduces lag, when you deletevehicle, when the player is more than some distance away from it, remember position and stuff, createvehicle, when player come back. Could be the solution to the lag, but needs good scripting capabilities.