aiconfig.cpr just determines the frequency of vehicle type that will appear... you don't seem to change the amount of actual traffic on the roads with it.
In GamePC.cpr you have a range of settings for maximum ai vehicles and the spawn times etc.
some of the code..
GamePC.cpr
// VehicleMaxCount is the hard limit of simultaneous AI vehicles
// But you can set TrafficMaxCount+ParkedMaxCount > VehicleMaxCount
// Example: vehicle=16 traffic=16 parked=8
// -> you can spawn up to 16 traffic if no parked vehicles on the area,
// but only spawn up to 8 traffic if 8 parked are here
// and 0 parked cars if 16 traffic cars are already spawned
//
[b]VehicleMaxCount = 16
TrafficMaxCount = 8
ParkedMaxCount = 6[/b]
// Maximum count of Traffic AI that will be physics enabled at the same time
[b]PhysicsMaxCount = 16[/b]
// 1st radius: Traffic cars inside this radius will switch to physics (for a max. of PhysicsMaxCount cars)
// 2nd radius: Removable cars outside this radius will be uninstaciated
// fov dist.: Visibility coef is ponderated by [1..0] when dist to camera is [0..fov_dist]
// fov magnitude: not used, apart for debug tests
//
// 1st radius 2nd radius fov dist. fov magnitude
CullingConfig 100 200 1000 45
// Ratio Visible MinTimeInterval MaxTimeInterval
[b]SpawnConfig 0.6 0.001 0.002[/b]
// Traffic cars will appear all around the player car, on a circle which radius will be:
// radius = speed * reaction_time
// radius clamped such as: min_spawn_dist < radius < max_spawn_dist
//
// Reaction Time Min Spawn Dist Max Spawn Dist
SpawnDistances 4 200 400