I think you need to open up the traffic.ini and manually add car entries right now.
It goes like this:
Code:
cars
{
car0
{
car=car_folder
waypoint=1
}
}
They start in the track's starting grid rather than at waypoints though, so you need a starting grid with enough entries for the number of traffic you want.
Once the game's started, you can warp them to specific waypoints with console command "traffic warp <car> <waypoint>", like "traffic warp 0 38". By default they'll be on random-roam, and picking up any waypoints they run over, so you have to be careful that if two routes overlap there are no waypoints near the intersection.
Each waypoint does include a velocity in m/s that the car will try to reach, every time it hits a waypoint it change the velocity target.