Of course the motorcyclists are allowed to "lane split" -- otherwise, they'd be stuck behind my 60 mph trailer! ;)
Seriously, though, it's "white knuckle" time on the LA freeway system whenever I look in my mirrors and see a motorcycle approaching from behind, zooming between stopped or slowed vehicles, weaving around the protruding side mirrors (including my towing mirrors).
Because of the size of the trailer, I can't edge over to the other side of my lane, as I sometimes do when not towing, in order to give the biker a little more room. I just have to drive as straight as possible and hope that the skill of the rider will keep him safe.
Experienced motorcyclists have told me that lane splitting is perfectly safe, that they can tell when (for example) someone is going to suddenly change lanes in front of them without signaling. Not being a biker, I have no way to know if that is true or not.
And now, back on topic: Since most of our towing is on the open road (and not in town), I am guessing that the 55 maximum for trailers (which is more like 60 in the real world) is not a big problem for motorcyclists -- there is usually plenty of room for them to pass safely. 95 percent of the time, I am in the right lane, and they are in the left.