TLDR: On roads with multiple lanes in same direction where the speedlimit is 45mph or under, when cars are stopped you may pass them between the lanes riding at 15mph or under.
For my fellow Utah riders, effective 2019 May 14, lane filtering will be legal, defined as the following:
383 (5) An individual may engage in lane filtering only when the following conditions
385 (a) the individual is operating a motorcycle;
386 (b) the individual is on a roadway divided into two or more adjacent traffic lanes in the
387 same direction of travel;
388 (c) the individual is on a roadway with a speed limit of 45 miles per hour or less;
389 (d) the vehicle being overtaken in the same lane is stopped;
390 (e) the motorcycle is traveling at a speed of 15 miles per hour or less; and
391 (f) the movement may be made safely.
Lane filtering will be legal from 2019 May 14 until 2022 July 01 unless the provisions are renewed or established permanently.