I was a member of the Los Angeles County Transportation Commission from 1977 to 1985 (appointed to three terms by Mayor Bradley) and a resident of the Valley (Chatsworth). It was my motion that created the Proposition A set-aside for rail construction in 1980. I supported rail at that point because of consultant and staff claims that it would make a material difference in traffic congestion --- indeed, it was that concern that led me to enter transportation public policy in the middle 1970s.
Since that time, it should have become painfully clear that rail has made virtually no difference in traffic congestion in Los Angeles. This should not be surprising, because new urban rail systems have not reduced traffic congestion anywhere in North America or Western Europe. As for the Sepulveda Pass (I-405), it is a classic corridor for which there is no transit solution. The basic problem is simply that not enough people are going to the same place. The destinations of people driving on such a peripheral (as opposed to downtown oriented) corridor makes it impossible to deter anything but a very small percentage of the traffic, and that would be quickly replaced by growth.
Because transit cannot serve door-to-door travel, nearly all trips by transit are much longer than by car --- the national average is double and the data in Europe is not much different. Regrettably, proposals such as monorails, subways and even express buses are a reflection more of rhetoric than reality. They will make little difference and that is especially true in a corridor like Sepulveda Pass.
As politically incorrect as it may be, there is no way to reduce traffic congestion and improve travel times except by expanding roadways. High occupancy toll lanes can be very effective and could make a real difference, especially in light of the fact that Census Bureau estimates indicate a near stagnation of population growth in Los Angeles County (and even Orange County)