Building new roads and increasing road capacity do not help. Sane people will try to schedule their car trips doctors appointments, shopping, work, if possible to avoid the hassle of rush hour. But if you reduce rush hour headaches, people will be more tempted to schedule their trips during that time. It s as if there is an equilibrium level of commuting frustration. Try to reduce the frustration level, and more people will hop on the road, bringing it back up to where it was before.
Alternatives reduce the number of cars on the road, and therefore ease congestion.
This causes an easier commute, and less people schedule their trips during other times, and so the net effect on congestion is minimal. Well, not quite. We have in fact not made a dent in congestion, but we have moved more people during the same time. Thus, if we were to keep increasing the number of people taking alternatives, we would see a constant number of other cars on the road, but more people could be moved in the same time period. This is not to advocate shoving everyone in a bus and carting them downtown.