Tyler Cowen notes studies which suggest that switching electronic toll-collection leads to higher tolls. Mark Thoma's suggestion seems plausible -- electronic tolls reduce congestion and inconvenience, allowing authorities to raise the monetary price while keeping the overall hassle level the same.
From where I sit, though, a low toll is a scandal. Somewhere between Portland and Brooklin driving up I found myself paying something like a 50 cent toll -- and, of course, dealing with the various delays associated with this toll booth. Better fewer, but higher (as Lenin might say). If your tollbooth's only going to raise a little bit of revenue, just get rid of it, let the traffic flow freely and make taxes a little higher. If you are going to put a toll somewhere, make it higher and really get into people's pockets. The point is that it's not worth causing all that inconvenience as a way to raise money unless the money is going to be a large fraction of the total costs to drivers.
Photo by Flickr user Redjar used under a Creative Commons license