I sent my friend, who spent ten years in Japan (and who revisited recently) a copy of that link. Here was his perpective on it...
Ah, my mind goes back to those halcyon days of riding in Tokyo. I know
this road - the Chou Expressway. In the video it appears to be the
average weekday traffic with all those trucks, (most are not allowed on
local streets). The traffic appears to be not so heavy since there are
spots where it is actually moving faster than snails, (that, in fact makes
it more dangerous since it would be better to tackle those lanes in a
gridlock - which does often happen). This lane splitting is legal and the
norm in Japan. I myself did allot of it - but not at that speed. Also,
that Virago 250 of mine was way too wide to slip through some of those
fissures. The big difference between doing this in Japan and in the
states is that drivers are quite aware of the "sub-traffic pattern" of
motorcycles that trickle through the cracks. Truck and car drivers are
taught to leave space to bikes. Even as a cyclist, I would be constantly
checking for highballing bikers trying to overtake me - sometimes passing
me while we're both between trucks! I was just on this expressway last
December, taking a bus to a train station - strange - I was watching those
lane splitters thinking how crazy it for me was to even attempt this years
ago. It's much safer to split lanes on the local roads - at least you can
stop to change your underwear if things get too close. By the way, one
time, coming back from a camping trip, Gitti and I were on the Virago
loaded down with camping gear - there was a thirty-mile long gridlock on
the highway in front of usgoing into Tokyo. We ended up following the
stream of motorcycles that were weaving their way through the traffic -
one the wrong side of the highway! I remember being in this strange
state-of-mind that this was perfectly normal - just trust in the hundreds
of motorcycles heading upstream - nothing could go wrong!
...pretty interesting, eh?