If you have a transponder for the express toll lanes on E470 you can use it on the I-25 HOV lane (if you want to fork over about $5 bucks during rush hour). If you use the transponder lane without a rfid tag, expect a huge ticket in the mail!
(FYI, I believe those interested in getting a transponder can get one at King Soopers. I got mine a few years back. $20 deposit for the tag w/ no monthly fees. You do have to put another $20 or so on you "tab" that is deducted each time your pass is used. When it reaches $5 or so, your credit card is charged again to raise your balance again. Well worth the investment... )