I thought the HOT (express) lanes were based on the level of congestion in the non-express lanes. The trouble with I-66 is there are no non-express lanes to measure.
I thought so as well. However, I got caught one time on a Saturday when there was an accident right at the terminus of I-95 South express lanes. It cost me $5 for 25 miles and $20 for the last two miles because the express lanes were blocked due tot the accident, causing that two mile stretch to skyrocket in price.
(BTW, the question is: why use the express lanes on the weekend? Answer: because I-95 in VA can get clogged up 24x7)