There is a reason that blocked-off HOV lanes are used: research has shown that slugging[1] will form spontaneously- and achieve the goal of actually reducing total number of car trips- it seems to need a bunch of things[2], and one of them is the HOV lanes be blocked off rather than easily accessible. Barrier separation discourages cheating, which in turn rewards drivers who carry slugs rather than just cheat, and rewards slugs with faster, better commutes. But the general consensus of road designers is that you need barriers on your HOV lanes to get slugging, and that without those barriers you don't really see as much of a reduction in car-trips- you will see some increase in planned car-pooling, but the slugging is where much of the reduction comes from, because its so much more convenient for everyone to do it anonymously at your own pace rather than having to tether you schedule to 1-2 specific other people. (One of you has a doctor's appointment? One of you has a sick kid? One of you oversleeps by 15 minutes? Now you are all screwed, one way or another. One of you can't get to work, and the rest of you can't take the HOV lanes. Planned car-pooling just doesn't seem to work long-term.)

Note that all of the research I'm aware of is US based, and I have no idea whether it generalizes to other cultures.

1: http://www.slug-lines.com/slugging/about_slugging.asp

2: Houston's Katy Freeway went from HOV-3 to HOT-2 (and from 1 lane to 2) and seems to have gutted its slugging culture. People weren't willing to get into a strangers car anonymously by themselves (or have a single stranger in their car), it looks like 3 people, all strangers, is culturally different from 2. Also, HOT lanes seem to weaken slugging, even in DC where it was most strongly entrenched, the switch to HOT lanes seems to have weakened the culture, because people say 'I'll just pay today' rather than pick someone up. So it can spontaneously form, and when it does can be quite powerful, but is very fragile.