I think that the Spare the Air day is a great idea.  The only problem is that the transit authorities don't use it as a tool to market their mode of transportation.  For instance:

• Why not make public transportation to and from San Francisco FREE all day or until at least 6:00 PM so that all employees that live outside San Francisco can make it home after working a regular shift?
• Why don't all of the transportation systems comply with the idea, (Hello Vallejo Ferry)

Also, it is not easy for everyone that commutes to randomly pick up a stranger and give him/her a ride to work, and vice-versa, would you just get into anyone's car for a ride?

Well that's my two cents.