"Split speed limits" are the bane of commercial drivers and the direct result of politicians who latch onto stupid ideas and refuse to give them up no matter how much evidence stacks up to prove its stupidity.
What ultimately lies at the heart of the matter is stopping distance. Nearly all stock POVs (privately owned vehicles, mainly cars, pickups, and the like) can come to a complete stop from a given speed in roughly the same amount of time and total stopping distance. Old studies done on old POVs, towing trailers without any actual towing features, concluded that a vehicle with a trailer traveling 55 MPH requires roughly the same stopping distance as the same vehicle sans trailer travelling at 70 MPH.
But the fact remains that actual tow vehicles - particularly those using fifth-wheel couplers - have specific features and capabilities which enable them to stop with a fully-loaded trailer in roughly the same distance as any other vehicle. (In the case of tractor-trailers, a bobtail [truck without a trailer] takes nearly twice as long to stop as one with a fully-loaded trailer.)
Meanwhile, every study done by transportation departments, individual researchers, State and US Governments, and pretty much anyone else have all come to the same unanimous conclusion concerning highway traffic - EVERYONE is safer when ALL traffic, regardless of classification, sharing travel lanes is going the exact same speed. A motorcycle and semi can run bumper-to-bumper at 80 MPH and be safer than the motorcycle going 70 MPH and the 40-ton rolling roadblock choked back to 55 MPH.
(In fact, Federal DOT regs specifically define any vehicle going 15 MPH slower than prevailing traffic to be a "slow-moving vehicle" and, as such, require the big red SMV placards and to have all its signal flashers going while it's on the road. I've personally pissed off a number of California troopers doing precisely that, as my way of protesting how stupid it is.)
But California's traffic law stupidity doesn't stop there - it's just getting started!
Countless other studies have consistently determined that 80%-95% of all freeway accidents occur at onramps and offramps - which, of course, makes perfect sense because those are places where two traffic flows at differing speeds are coming together or branching off. Additionally, around 90% of all onramps and offramps in the US merge with the rightmost lane.
Truckers know this well and, elsewhere in the United States, will move over a lane or two when approaching congested onramps/offramps as a matter of courtesy, to clear room for other traffic to merge more readily, to avoid any significant slowdown themselves, and to present a wide berth.
California, on the other hand, demands that trucks stay in the rightmost lanes - no matter what - to keep those 40-ton freight-trains-on-asphalt pointed directly where they know that most accidents are going to be.