國內這樣解釋 那來參考看看美國怎麼解釋吧
http://www.mit.edu/~jfc/right.html
All states allow drivers to use the left lane
(when there is more than one in the same direction) to pass.
Most states restrict use of the left lane by slow-moving traffic
that is not passing. The table below describes the law in effect
in each state.
A few states permit use of the left lane only for passing
or turning left. These have "yes" in the "keep right" column.
Some states require drivers to move right if they are blocking
traffic in the left lane. These have "yield" in the "keep right" column.
Most states follow the Uniform Vehicle Code and require drivers
to keep right if they are going slower than the normal speed of traffic
(regardless of the speed limit; see below). These are listed as "slower".
A few states either do not require vehicles to keep right ("no"),
or permit vehicles moving at the speed limit to drive in the
left lane regardless of traffic conditions ("< SL").