Here is my answer for the National Journal Transportation Blog.
"User pays" was the foundational concept and an interesting one to reflect on. The question notes that current gas taxes inadequately cover even simple maintenance requirements on existing roads, yet the phrase resonates strongly with drivers. They sincerely believe that they have paid for all that is required with their gas taxes at the pump.
If the road user really paid what driving costs to maintain, what driving costs to widen and build new, what driving costs in police forces, emergency personnel and equipment, lifetime effects of accident road deaths and injuries, watershed destruction, groundwater and run-off pollution, excess asthma rates, higher incidence of heart disease and negative effects for those living near highways, congestion, and CO2 emissions (etc, my list is truncated), we wouldn't be in the unfunded situation we are in today.
Also, if "user pays" included all those "externalities" (so many things in quotes), it would seem perfectly appropriate for the gas tax to include pedestrian and sidewalk improvements, mass transit, electric charging stations, and environmental remediation efforts because all of those things are attempts to mitigate the real and costly negative impacts caused by the car-driving users.
At the end of the day, if we take political realities into account, the one thing I ask for is for drivers to truly understand what their fuel tax is actually paying for, and what is quietly and covertly being subsidized by their other taxes. Because we haven't included these costs in the gas tax, we are using local, state, and government money brought in from other sources to cover the difference. When we say we don't have enough money for education, or welfare, or parks, or elderly programs, we need to recognize that this shortfall is in part because we are paying for all sorts unfunded car-related expenses with non-gas-tax dollars.
To read how other experts weigh in on this, go to the National Journal Transportation Blog.