THE STATE’S DEPARTMENT of Transportation is planning to convert the Tobin Bridge to all-electronic toll collection next year, but that effort may run afoul of some laws on the books.

The state is planning to require drivers to get E-ZPass transponders to reduce back-ups at the tolls and cut staffing costs, but the change would seem to violate Massachusetts and federal laws requiring anyone who accepts payments for goods or services to accept cash for those transactions. All denominations of bills have carried the phrase “for all debts, public and private,” since 1862.

The Tobin conversion, which is going through a series of public hearings, would eliminate the annual 4.1 million cash transactions that currently take place on the bridge. Transportation Secretary Rich­ard Davey says if someone goes through the toll without a transponder, a camera will snap a picture of their license plate and they will be sent a bill. He said drivers can pay the bill with cash, though he was unsure where. He also says E-ZPass accounts, which require an initial $20 deposit and then are replenished with a credit or debit card on file, can be funded with cash. But the state’s customer centers that accept cash payments are in Natick, Auburn, and East Boston—a trip through a toll for most.

One good thing about electronic toll collection on the Tobin is that E-ZPass holders get a 50-cent discount off the current cash fee of $3 for passenger vehicles. But what about the state law that prohibits “discrimination against cash buyers”?

Jack Sullivan is now retired. A veteran of the Boston newspaper scene for nearly three decades. Prior to joining CommonWealth, he was editorial page editor of The Patriot Ledger in Quincy, a part of the...