There is a service called Avego that is doing a car-sharing pilot in Seattle right now (see more here: http://go520.avego.com/st-pilot/ and here: http://www.youtube.com/watch?v=P58Ug6JpdRU&feature=relmfu). It allows commuters in the heavily travelled 520 corridor to use a smart phone app to instantly find rides, and then securely transfers money from rider to driver to pay for gas, based on the miles travelled.

LA could identify some major traffic corridors (the 405!)and enlist Avego to help drivers find each other, thus reducing congestion.