James-
Currently (in ST Experimental 2.7) private cars are generated without regard to era, e.g. private cars and congestion occur in 1830, although they aren't presented graphically. Are you considering private cars to solely be automobiles, which would mean setting a start date for private car generation (such as 1905)?
Of course, we might consider horses in the 1800s as a type of private car too. :) (Sounds like a joke, but I suppose that's a possiblity)
Edit: fixed in v3.1
Tick Tock,
thank you for your feedback :-) There is indeed provision in existing versions of Simutrans-Experimental for private cars to depend on the era. If you download the configuration files (http://simutrans-germany.com/files/upload/Simutrans-experimental-config.zip), and put privatecar.tab in your /simutrans/[PAKSET]/config directory, private car ownership will be based on the time. Before 1906 or so, there will be no private cars at all. You can customise when the private cars appear by editing the privatecar.tab file.
Using v3.0 and new config files:
Car ownership and congestion are indeed zero in pre-auto era, however Traffic is still being calculated and displayed on the city graph.
Tick Tock,
thank you for spotting that - I have now fixed that in version 3.1.