What construction shows us is that many lanes or even whole roads are unnecessary. Now the 405 is necessary considering its location and the nearest major roads but perhaps the widening or the number of lanes isn't.
Google Earth shows the 405 to have 10 lanes; 5 in each direction. What if 6 lanes were set aside for cars, and 4 lanes for HOV and improved bus service. WAIT, you say, that would spell disaster for everyone! We would only have 6 lanes when we had 10 before!
The doomsday traffic congestion would only happen for about a month or two, as people get used to carpooling and new bus service. The end result might be that the general traffic lanes flow smoother than they did before.
The cost there would be striping modifications, new signs, and say, 100 new buses. The cost there is about 45 million for the buses (assuming no bulk discount) and whatever it costs to stripe and change signs. Savings of about a BILLION (1,000,000,000.00) dollars that can be used elsewhere.
Air pollution reduced. Traffic reduced. Commutes improved for most people. One BILLION dollars saved.