I asked a question about how congestion density factor works, as I couldnt understand the complicated maths i was reading in the explanations. But with more experimenting I seem to have found what I was looking for. I couldnt figure out why I got no traffic in 128 Britain experimental. It seems that if you maximise the traffic density, maximise the city car life, and put the congestion density factor up to a high number, the city cars eventually appear. Probably obvious to many but I didnt know that. So, sorry to have wasted your time, though maybe this comment is helpful.
Don't worry - glad that you have it worked out now.
Bear in mind also: (1) city cars will not appear during the first month of the game; (2) the number of city cars will be low if playing without a timeline; (3) if playing with a timeline, the number of city cars will increase as car ownership increases; (4) the number of city cars will vary with the passenger factor; (5) the number of city cars will be less the higher that the congestion is, as fewer people will want to travel by car; (6) the congestion density factor affects the calibration of the congestion level, and does not directly affect the number of city cars; and (7) the better that the public transport infrastructure, up to a point, the fewer people will use their car, and therefore the fewer city cars will appear.
Thanks James- veryhelpful