In a number of areas, there are de facto (and possibly legal, though by reg or statute I don't know) truck routes. Around the SF Bay Area, highways 101 and 880 are for trucks, 280 and 580 are the alternate "commute" routes. To the point that traffic alterts tell trucks when they can hit the otherwise prohibited routes.

Makes quite the difference for driving in a passenger vehicle as well -- I generally prefer not having a scad of trucks around.

Now, the worst truck route I've encountered is I-80/90/94 at the Indiana/Illinois state line. <bold>Uhuhgly</bold>.

There's also a route in SoCal near the Arizona line that's rumored to have 18 wheelers rolling at way high speeds -- 90+. So the speed split seems somewhat unlikely to me.