The cost of the Sepulveda Pass project was supposed to be $1 billion. It has now reached $1.6 billion, after transit officials approved $300 million in new expenses last week.
Peak afternoon traffic time has indeed decreased to five hours from seven hours’ duration (yes, you read that right) and overall traffic capacity has increased. But congestion is as bad — even worse — during the busiest rush hours of 4:30 to 6:30 p.m., according to a study by the county Metropolitan Transportation Authority.
This is precisely what a very standard transportation economics model would predict. Eerily precise, so score one for economists! Capacity improvements to a congested highway (even holding the total number of cars constant) will shift driving patterns so that there's a higher share of cars at peak time - making congestion worse then but the high congestion period shorter.
I even made a quick ugly graph, because that's how much I love you, dear readers.
The basic model says ideally everyone would like to get to work at 9AM, but that's unpossible because of congestion. Some people shift their travel times to arrive before and after 9 AM in exchange for lower overall travel times because there's less congestion then. The highway improvements do improve things somewhat if there is no induced demand, as more people show up to work closer to the ideal (9AM) time, but these people actually face more congestion and higher travel times. All this is because congestion is an externality. When you get on the road you make things just a bit worse for everybody else, but you don't take into account to the cost to others when you do so.
tl;dr most of the overall benefits of highway improvements at peak time are eaten up by people choosing to travel closer to peak time, and congestion is actually worse right at peak times. You aren't stuck in traffic, you are traffic.