That's the thing, if they just drive somewhere else on Friday (or whenever the extra day off is), then the energy savings pretty much evaporate. At my employer we do have our Public Works guys on a 4 day, 10/hr per day schedule. They all love it. Why? Because that gives them an extra day to go hunt or fish -- activities they must drive some distance to. I might also point out that all those people with an extra day off are going to want to go shopping or to some entertainment venue, so the people staffing THOSE places can't have that day off.

I'm pretty sure employers would be able to work their schedules so that there was coverage on all days. This excellent suggestion didn't say that it is mandatory for every person in the country to take Friday off.

Personally, I would rather have Monday off. Nothing better than coming to work on Tuesday and thinking 'wow, part of the week is gone already'.

As far as what people might do on their 3rd day off is not really an issue, considering all the other mentioned benefits of a 4 day work week. Of course it would have to be 4 10 hour days. There is no way this would happen if the number of work hours in a week were reduced.