A lack of public transport, or poor public transport options, will also cause problems. If there isn’t enough buses, trams, or local trains people are forced to take their cars to work. The ratio of passengers to vehicles decreases, whereas if they were able to take the bus people would feel less of a need to drive their cars.
In many places, commuters are forced away from public transport by the private companies which run them. Increasing fare prices, especially on the trains, make driving a car with its associated high fuel costs cheaper than public transport. By pushing people back to their cars again they only exasperate the congestion problem.