It's in the driver handbook in most states. The problem is it's not anywhere else. In the mainstream media almost invariably when they talk about bikes they throw in misinformation like they should keep as far right as possible.
The Maryland Driver's Handbook warns motorists that bicyclists are
required to ride as far to the right as practicable "only when the
lane can be safely shared by a car and a bicycle, side by side."
Furthermore, "There are certain conditions that allow a bicyclist
to 'take the lane,' such as . . . the lane is narrow in width making
it unsafe for a motor vehicle and bicycle to share the lane side by
side." Finally, the handbook instructs motorists "Always allow at
least three feet to the left of the bicycle when you are
passing. . . . DO NOT attempt to share the lane with the bicycle
when passing. Reduce your speed and move into the next or oncoming
lane to pass."
From the brief for the defense from State of
Maryland vs. Robert A. McCutcheon, available online at
http://crankmail.com/fredoswald/court_ca.pdf. McCutcheon was
charged with not keeping to the right, and acquitted.