I have an EZPass (the east coast automated tag thing on my car). I go to school in Washington, DC, and I live in South Jersey. When I drive, I don't notice the total cost of the tolls, and I think it's because of tag. I asked a friend to pick me up at a train station near the end of the MARC (Maryland's train system) line, and I ended up costing him $13 that he paid out of his wallet, and that's just on the way back from the train. I paid him back, but I didn't realize how many tolls there are on I95.
As far as the idea of turning roads into toll systems, I'd prefer the tags. They're a bit more anonymous than a license plate, which could easily be traced by DMV records. I know many people who own one or two tags and just put it in whatever car they're taking for a long trip. My big worry is the day that the politicians realize you can use these things to track how fast people are driving by comparing the time into the system to the time out with the distance driven. I did read an interesting article the other day about an economist proposing we solve our traffic congestion and road funding problems by implementing a dynamic tolls system on all the major highways. A busy road would have a higher toll than a less crowded road, encouraging people to take the cheaper route, and at the same time, providing funds for the highway system. Usually in my travels between DC and home, or between home and my friends in Delaware, I take either 295, a bit longer of a drive, but less crowded, or the New Jersey Turnpike, which has an exit that is a few miles closer to home, which is more direct but seems to be more crowded when I drive it. With some well placed electronic signs, I could tell which route will have more traffic, and the state would make money on both routes, not just the turnpike.