Express lane nor in the left lane of I-15. (note Utah has an express lane which is similar but different from a carpool lane or an HOV lane).
I don't think there is good data to support a speed limit in those two left most lanes. It's just something that "feels safe". If you are a good driver and have a competent well cared for car ... then go 100 mph for all I care. Germany's roads are just fine. The Autobahn actually has fewer deaths per mile driven than a typical US freeway.