Cambridge, Mass. — Q. When is an automobile like a box of cereal? A. When it is outfitted with a ''bar code'' that can be read by a laser beam. Most of us have stood in supermarket checkout lines where clerks run items by an electronic ''reader'' that interprets a black-and-white striped patch on the side of a cereal box and understands that a large box of Post Raisin Bran, for example, has just zoomed by.
A variant on this technological theme is already in use for automobiles. Frequent travelers over the Delaware River Port Authority bridges between Philadelphia and Camden, N.J., buy a window sticker for $10 a month that lets them make the crossing for a bargain 25 cents instead of the usual 75 cents. An electronic reader recognizes the bar code and raises the barrier gate despite the reduced toll.
As the need to maintain infrastructure such as roads and bridges becomes more urgent, user fees are becoming a more and more attractive way to finance such maintenance. And with greater reliance on user fees, ''the importance of these more efficient counting mechanisms and toll collecting is critical,'' says Daniel W. Greenbaum, chairman of Vollmer Associates in New York. He was at the Massachusetts Institute of Technology recently addressing a seminar on the subject of new counting technologies.
Another is the use of transponders - a device like a small pocket radio mounted somewhere in one's car. It would emit a ''signature'' signal that would activate a toll gate, for example. Farther down the road, so to speak, might be a system whereby transponder readings would trigger charges on a central computerized billing service. At the end of the month a motorist would get a bill from the turnpike authority, not unlike a telephone bill. Advantages would be labor savings (no more toll collectors would be needed), more efficient collection, less need to restrict access to toll roads, and less slowdown to fling one's coins into the receptacle. Mr. Greenbaum, however, expressed concern that both the expense of the system and the potential for invasion of privacy could be serious drawbacks.