In Utah, many justice court judges won't convict on that charge, and because of that fact many Wasatch front prosecutors won't prosecute it now if the violator is impeding but over the posted speed limit. I think the one you are thinking of is 41-6a-701, subsection (3). That's the duty to operate on right side of roadway statute, and specifies that vehicles traveling less than the speed of traffic must do so in the right-hand lane then available.

Several years ago when I did traffic enforcement I cited for impeding at 72 mph in a 70 (among a few other violations), with a stack of 5-6 cars clearly behind the offender on dash cam. I thought it was articulable that the "normal speed of traffic" in the left lane is 75-80 mph in most situations, and due to the stack of cars behind the violator, that driver needed to be operating in one of the right lanes. That charge got tossed out. I've cited multiple below the speed limit for impeding in the left lane in the 55-68 mph range, and all of those have stuck.

I actually have asked multiple judges about this outside of open court (citing for failing to operate in the right hand lane when the driver is above the speed limit), and one particular justice court judge made some great points. One, that the "normal speed of traffic" is the aggregate, not just the left lane, and two, if there are any cars at all in any of the right lanes moving at a slower speed than the violator (thus being 'passed'), the statute is not met and that would qualify as an affirmative defense. Some judges have also said that they won't condone speeding, and that all traffic should be operating at or under the posted speed limit, therefore a violator cannot be impeding if they are also speeding (regardless of the language of the statute). Defense attorneys also have made some very good arguments in cases where impeding was cited above the limit, getting officers to either condone speeding, or getting them really jammed up over the justification between 41-6a-701 and 41-6a-601. There are a few judges out there that may convict on a duty to operate in the right lane citation above the speed limit, but the majority I have had experience with will not, and there are several jurisdictions in which the prosecutors won't prosecute on a 41-6a-701 violation if the noted speed is above the posted speed limit.