I did something similar. However instead of changing the private cars vehicle speed, or the overall traffic, I changed the amount of passengers each of the player vehicles can carry.
For me, the main problem is not so much congestions (that can be reduced by going off road, and seems to be somewhat realistic (if a bit over the top for the top roads)). However I wouldn't mind that if I wouldn't go bancrupt on every map, unless I rush to metro, a one way tactic I do not deem fun. I play on easy, and I want to rake up insane amounts of cash to create an insane public transport network. I can't do that if the ROI of my whole network is either 2'000 in the negative, or barely profitable, often 100 or less, even when I pay minimum wages, and neglect all the infrastructure, while keeping the prices in dark red. All that, and i'm still not able to transport all the customers, it's insane.
To do a similar thing as I did, you need to use the gs-unpack tool (mentioned in the first post) but instead of the metro.gs file you need the object.gs file (btw. what _is_ this gs archive format? I was unable to find any information online), then go to the "vehicles" subfolder and change all the *.script files entry on the "capacity" line. I used a quick and dirty script that added half to what already was there, for example the streetcar used to carry 10 but now carries 15.
This increases the income a bit much, but as I said, I wanted it easy. Obviously I also now need less busses, so the whole network isn't overworked by 20 buses serving less then 10 stations in addition to the private cars. Even better, one can now actually achieve the scenario help demands, before they come back and say "you helped a lot the first time now do another thing for me". I might reduce the multiplier or actually subtract from the preset for metros, one of the desingers must hate all other forms of public transport
Caveat: at the end you need to put the resulting "objects" folder
into a "metro" folder in the application directory, otherwise your changes will be ignored by CiM.
For example (note objects in metro, not directly in CiM folder):
/Applications(x86)/Paradox/Cities in Motion/metro/objects/