Considering the revenue incentive behind express lanes, this is likely the reason. The most salesworthy point of buying your way into that lane is so you can go

Considering the revenue incentive behind express lanes, this is likely the reason. The most salesworthy point of buying your way into that lane is so you can go faster than the flow of traffic. Capping that at the speed limit is the same as capping the marginal utility. That said, I use the lanes regularly on my motorcycle, and I've witnessed triple-digit speeds (estimated, as I would never violate the law so egregiously) from vehicles in those lanes. At some point, license plate readers and toll tags tell a tale of recklessly high speed.