My take, if traffic can get around you then you are not impeding it. But definitely stay in the right lane.
from
http://www.nhtsa.dot.gov/people/inju...oc/wispeed.pdf
Code:
Minimum Speed Limit: I. No person shall drive a motor vehicle at a speed so slow as to impede
the normal and reasonable movement of traffic. '346.59(1)
II. A person, driving at less than the normal speed of traffic, shall drive in
the right-hand lane then available for traffic or as close as practicable to the
right-hand curb or edge of the roadway. '346.05(3)
Posted (Minimum) Speed Limit: Based on engineering and traffic investigations, the State may establish a
minimum speed limit for State trunk highways. '349.11(1)(b)
...
Violating the Minimum Speed Law (Impeding Traffic)-1st offense-$20 to $40 Subsequent offense-$50 to $100 '346.60(1)
Failing to Maintain Speed Except in Right Lane-$30 to $300 '346.17(2)