PS: Most, if not all, rental car companies will screw you over on the toll card. They add it into the fees and charge you a PER DAY use even on days when you do not use it. The per day fee is on top of the toll fees and is often charged via AMERICAN TRAFFIC SOLUTIONS. IMO, that is one evil company. Go ahead and Google it...
There's a highway heading north out of Denver that is usually empty and, therefore, quick. There are tolls on it. I have never seen a way to pay a human so the only way is with the toll card or by them photographing your license plate. Either way, you will be screwed re how much you pay b/c they will charge you more than just the toll fee that the locals would pay (if any locals actually used that road, which it seems they do not).
To avoid that highway, you need to be certain to remove toll roads as an option from your GPS/mapping tool and see if you like the route your map tool gives you.
I had to fight a rental car company for charging me 1) for the use of a toll card I did not want to use 2) for charging me for tolls via the toll card when I paid in cash.
It took a long time but I got the $$ back. Hoping to save you the trouble...or at least let you know what you are getting into when you rent a car.