Denverite... It's like "Where do I start?"
If CDOT could impose taxes chances are they wouldn't be in such a state of poverty.
Colorado is significantly hampered by TABOR. If you're unfamiliar with the TABOR Amendment, I would recommend asking your BFF Google. One upshot is that essentially ALL tax increases in the state have to be approved by voters. That has led to a leadership vacuum.
Without both political parties getting together on a plan to take to voters to increase revenue for CDOT it won't and hasn't happened. For a current snapshot check out yesterday's
DBJ article
With respect to your concern over a VMT tax - or vehicle miles traveled tax, this has been looked at by most of the states and at the Federal level. To date nobody has actually tried to implement something. CDOT is merely, as the article points out, doing a pilot study using volunteers.