Typically, there is some part of vehicle law that requires the slow-moving vehicle to pull over as soon as practical when some blocking condition is met. In Indiana it's when 3 or more vehicles line up behind you.
"9-21-5-7. Motor vehicles driven at slow speed impeding or
blocking traffic.--A person may not drive a motor vehicle at a
slow speed that impedes or blocks the normal and reasonable
movement of traffic, except when reduced speed is necessary for
safe operation or in compliance with the law. A person who is
driving at a slow speed so that three (3) or more other vehicles
are blocked and cannot pass on the left around the vehicle shall
give right-of-way to the other vehicles by pulling off to the
right of the right lane at the earliest opportunity and allowing
the blocked vehicles to pass."
In this case, the interesting question also arises . . .does this section apply exclusively to motor vehicles given the way it is written, and thus HPVs are not required to unblock the lane?