People generally spend an hour a day traveling:
So any road improvement simply means that people will journey further to their destinations. This means there will be the same number of cars on the road as before but they are spread over a longer road mileage. But immediately other drivers will work out that they can spend the same amount of time as previously, ( about an hour) and driver from their house to a better shop / job / school further away. This means the road improvement will be rapidly eaten up and you will be back to the same congestion as before very quickly.
This phenomena of suppressed or latent demand has been known about since 1930 (1).
Conversely, if a rapid transit system which car drivers will actually used is offered, then people will be prepared to walk to the tram stop, take the tram, and walk to work, providing the journey time is still only about an hour.
(1) Report of the Transportation Survey Commission of the City of St. Louis (1930), p.109, cited in Fogelson, Robert M. (2001) Downtown: Its Rise and Fall, 1880-1950 New Haven, Connecticut: Yale University Press. p.66. ISBN 0-300-09062-5
