Cars are expensive, we all know that. https://www.aaa.com/autorepair/articles/average-annual-cost-of-new-vehicle-ownership according to the American Automobile Association the average annual cost of owning a car in the USA is about $9,000 including the costs of fuel, maintenance, repair, tires, insurance, licensing, registration, taxes and depreciation. Because of this, one of the biggest real life advantages of a car free society would be that people have higher disposable incomes because on a large scale the costs of public transport are way lower than the costs of cars.
However, in Democracy 4 cars are apparently free besides taxes. Which means that the only real reasons to increase bus and train usage are environment, CO2 and traffic congestion. I think that transportation costs should be taken into account for both commuters and motorists where motorists spend more on transportation because cars are more expensive.