Background: I live in Phoenix, and the traffic seems to
"coincidentally" improve at the precise location where the HOV lane
opens up to become a general-use lane.  I've seen county/state
websites that claim they are efficient, but offer no real evidence to
back up their claim.  My suspicion is that carpool lanes are effective
only at transporting high occupancy vehicles at a faster rate, but do
not improve overall traffic flow for the betterment of all citizens
within a given city.

My question: Are carpool lanes effective at improving traffic flow for
all citizens travelling on a given highway?  Specifically, if carpool
lanes were to become available for all cars to use, would speed
improve for everyone, or would the improvement be too minimal to
notice?

Incidentally, it would be nice to know (but not a requirement in
addressing my question) if carpool lanes are effective at changing
people's behavior.  In today's busy society, is it feasible to carpool
with a co-worker?  People have to (1) work at the same company, (2)
have the exact same work schedule, (3) never have to work overtime
(hourly employee), (4) live in the same general vicinity, and (5)
never have to run errands before or after work!  Is the perceived (or
actual??) time benefit really sufficient to motivate people to alter
their lifestyle in such a manner?

Here in DC (the traffic capital of the world), HOV lanes are atleast
moderately used.  I doubt they actually reduce traffic in the other
lanes because that would require half as many cars with 2 people (or
1/3 as many with 3 people) in the HOV lane as pass by in the normal
lane.

Without the HOV lanes, driving from some locations to DC during rush
hour (ok, "rush hours" since it's about 3 hours in the morning and 4
hours in the afternoon) is undoable in my book, however some people
don't seem to mind spending 2+ hours each way driving to/from work.

Many larger work locations have commuter boards or forums where people
can match up with other commuters.  These employers are also typically
flexible with work hours to make it easier to avoid traffic or to
change your schedule to make it compatable to another worker's.

One other thing to consider with the HOV lanes is that if they didn't
exist, traffic might be slightly lighter in the short term... but more
people would then decide to drive further to work (saving money on
housing costs and living in better areas) and that would clog the
roads again.

I've always been a fan of much better public transportation to/from
cities and also for the extremely unpopular raising of gas tax.  The
current gas tax doesn't even pay for the roads/polution control that
the govt pays for now.  So non-drivers are sharing those burdens. 
Higher gas tax would encourage the use of more fuel efficient cars and
public transportation.  The roads would be less clogged, people would
live closer to work, there would be less pollution, the govt wouldn't
be as poor...

Anyways, I'm a bit off topic so I'll stop now.