While I don't see a need to reduce the gas tax, I would have no problem with turning the entire interstate system into toll roads. These are already quite prevalent in the Northeast, and have very little downsides that I can see. If people are unwilling/unable to pay the tolls (which are generally quite small, and based on distance traveled), they can take highways, thereby reducing interstate traffic. If they are willing to pay the tolls, then additional revenue is collected directly from the users of those roadways.
The catch is that on some low volume toll roads the toll revenue barely exceeded the cost of collecting it, and for some did not even get to it. Also, the unbalance between automobile and truck payments as a relationship to cost of maintenance atrributable to each is usually made worse. Frequently toll is per axle or even a simple cars / trucks, so that a truck is paying between 1.5 and 2.5 times the car toll which is an even worse ratio than you get from the fuel tax. To be equitable, the big truck toll should be somewhere between 10 and 20 times the automobile toll.
Everything about this person says she will be the secretary of highways.
The collection problem can be solved by implementing a radio tag system, similar to EZ-Pass. The tags are used throughout the atlantic coast (and aparently now in Illinois too). A single tag gets you through all tolls. The tag is linked to your credit card, and a computer sorts out which highway agency gets what money. The tags even work at high speeds. I went through a booth in Delaware a few weeks ago at ~50mph. The toll showed up on my bill just fine.
I'm not sure how EZ-Pass deals with trucks, but I have seen them using it. I assume that the toll is set to be proportional to the size of the vehicle that the tag is assigned to.
Providing an example of the size of the tolls from my recent bill:
Aug 4: Mass Pike; I entered at plaza 15 and exited at plaza 9. Total toll: $2.65
Aug 18: NJ Turnpike; I entered at plaza 16E and exited at plaza 1 (Delaware Bridge). Total toll: $4.35
Aug 18: Delaware Bridge: $3.00
Google maps says that my travel on the NJ Turnpike was ~115 miles. My car gets 30MPG on the highway, so I used ~ 4 gallons of gas. Tax on gas purchased in NJ is $0.184 federal + $0.145 state = $0.329. Four gallons cost me $1.31 in tax. That is considerably less than the toll. Even if I do the calculation with NY gas taxes (the gas was purchased in NY), I end up with $2.01 in tax.
Truckers (and others) may try to avoid toll roads at first. However, out here the toll roads are packed, even though there are alternate, toll free routes. The toll roads end up being the quickest (unless stuck in traffic), so that is what people take. I'm not sure how to keep trucks off of highways, except to vigerously enforce local "not a trucking road" laws. A ticket is going to cost the trucker much more than the toll.
Tolls can be effective. They just have to be sufficiently large, and well enforced.
Besides, anything to get more people on trains, right?