Rampant speculation and riling ahead:
(note: this is not directed at anyone specifically)

Perhaps these people who've been cooped up all week feel the need to get out and go somewhere on Saturday. If the workweek were reduced they may not feel the need to do so. So not only would you get the benefit of them not driving the one day of the workweek, but they may not feel as compelled to go anywhere else otherwise, so you'd get a reduction on Saturdays as well.

More likely, whatever driving they would be doing on Saturday may get simply get shifted. So the workweek benefit may still be there, but they may simply do their Saturday driving on a different day.

Here's stirring the pot: I'd say that fuel burned during recreation is a worthy use. Fuel burned during commuting is completely wasted. I'll make a side note that fuel used in creating the items we need for survival - food, shelter, clothing, etc. is also a worthy use. I'll go even further to say that even if they drive on the "off" day, that is a much better use of the resource. For the reason, I'll answer with a question...what's the purpose/meaning of life? I'll let you decide.

Now, to be a little less controversial, one of the things this WOULD do...is to make a *mandatory* driving day into an *optional* driving day. If you must drive to get to work and you work 5 days a week, you need to drive 5 days a week - though you could drive 7. If you've got a 4 day workweek, you need only drive 4 days a week - though you could still drive 7. The other day becomes optional. So if gas is $10/gal - perhaps on that 5th day that you would have *had* to drive to work, you'll hop on your bicycle and cruise downtown instead (or sit on your fat, lazy ass and ogle the one-eyed god...but that's another story).