[dragonwriter]

> It's probably because in California, 18-wheelers are required to stick to the right lane and have a lower speed limit (55 instead of 65).

Autos with trailers and trucks (vehicles subject to the 55mph limit where other vehicles have a 65 or 70mph limit) are not generally restricted to the right lane in California.

They only are restricted to the rightmost lane on roads where: (1) where there is no other designated (by sign) set of lanes for such vehicles, and (2) The road is not a divided highway with four or more lanes in the direction of travel (where the default, without a designation, is that they are restricted to the two rightmost lanes for normal traffic, plus the next lane out for passing.)

[pzxc]

Okay, but the vast majority of roads do not have 4+ lanes in one direction, and the vast majority of roads do not have a sign indicating a different lane designation.

[dragonwriter]

> Okay, but the vast majority of roads do not have 4+ lanes in one direction

On a road with 1 lane per direction, trucks being restricted to the right lane except for passing has no negative impact on your preferred practice regarding use of the left lane in multilane roads.

On a road with 2 lanes per direction, trucks being restricted to the right lane except for passing exactly matches your preferred practice, though the lower truck speed limit (independent of any lane restrictions) might arguably encourage other drivers to stick in the far left lane (though, except in areas with high density of trucks, doesn't really provide any strong incentive not to follow the use the right lane for traffic, except for passing, it just increases the likelihood that non-truck traffic will want to pass trucks, which is pretty high in any case.)

The lane discipline which doesn't apply on roads with 4+ lanes in one direction thus is really even potentially even a factor in what you are concerned with on roads with exactly 3 lanes per direction.