Of all the options in the current public transit mix, for most cities, light rail is probably the worst possible choice. It requires its own special track, so it lacks the flexibility of buses which can be run over existing city streets. Yet, its carrying capacity is far less than that of heavy rail.
…studies have shown that about 80% of new light rail passengers were former bus passengers. Taking this into account, the real impact on [vehicular] traffic is for light rail to remove less than one car in 1,000 from traffic.