RB- the study was for California, I assume that meant congested California cities.
The study was for California, meaning a place where they've been doing it for years and, from what I've seen while driving and riding there and have been told, personally, by riders who live there, what's "safe" and effective there may not be anything of the sort elsewhere.
I've also read, on other fora, that Cali folk who lane-split there daily wouldn't even think of trying it in Dallas after having ridden "here." (Note that I do not live in Dallas and to me Dallas is a walk-in-the-park compared to Houston, nor do I recall any of the Cali riders specifically mentioning TX cities other than Dallas.)
I have ridden in California, most recently in December '11 when I had the unanticipated (due to un-planning) pleasure of evening traffic leaving Anaheim, headed to Phoenix. Traffic was bumper-to-bumper, stop-and-go, well past Riverside and I just gave up and stopped for about an hour at a gas station/burger joint exit (maybe around Beaumont or Banning - maybe Cabazon?); my clutch hand was
fried!
Full disclosure: I was running with a friend in a P/U, hauling a bike, and so filtering through was out of the question anyway. That notwithstanding, the Connie 14 I was riding, with bags, was too wide to think about doing it anyway except for the "filtering lane" between the fast/innermost-lane and the HOV lane, where it existed.
Observation from that trip and from another one in February '11 (that one in the p/u, picking up bikes). The pros who lane-split everyday do not run willy-nilly through whatever traffic is in front of them. Most of them run along the line between the innermost-lane and the HOV lane and most of them are not blasting through, just carefully working the least densely populated part of the traffic. Most of them get as far away from the outermost lane (the exit-entrance lane, if you will - I've never understood their lane "numbering" scheme, never bothered to since it was of no concern to me) as quickly as they can. And the cars/trucks do not try to cut them off or squeeze them. I found the whole experience most enlightening but I sure as heck wouldn't try it in Dallas evening traffic! (I'm not picking on Dallas here; I just have a lot more experience with driving/riding in or through Dallas than I do in Houston {which scares the beejebus outta me - too fast!!!}, San Antonio, or Austin (in which I've not yet had the pleasure(?) of riding.)
One thing I remember clearly is that on the P/U run (ETX to Colfax, CA, {non-stop to ~Fresno where I M-6'd it for a few hours} then to Colfax, then a flip back down "The 5" [catching
all the LA evening traffic] to Long Beach], I saw exactly
one accident, an almost-not-worth-mentioning-rear-ender-fender-bender with the two cars pulled over on the shoulder, owners calmly exchanging information, no police cruiser in sight. Summary: nowhere near as bad as I'd imagined, not having driven there since 1963.
Didn't mean to be so wordy and hope I didn't miss too many typos.