Maryland’s policy of charging some out-of-state drivers an extra fee for using the E-ZPass system may be vulnerable to a legal challenge similar to the one that succeeded Monday in the U.S. Supreme Court.

State transportation officials voted this month to eliminate an $18-per-year charge for 675,000 Maryland residents who have E-ZPass accounts, but they left the charge in place for out-of-state members who use the system less than three times a month. The fee is described as an “account maintenance charge.”

In March, Maryland General Assembly Kathryn M. Rowe said in an advisory letter to Del. Neil C. Parrott (R-Washington) two months ago that the policy could violate the U.S. Constitution’s Commerce Clause, which prohibits states from discouraging business across state lines.

On Monday, the Supreme Court ruled 5 to 4 that Maryland ran afoul of the same clause by denying tax credits to residents who pay income tax to other states, with the majority finding that the policy might restrict interstate commerce.

Rowe cautioned in her letter that courts might reach the same conclusion about Maryland’s E-ZPass fee for out-of-state drivers, saying that “the proposed amendment raises significant constitutional issues under the Commerce Clause of the United States Constitution and could possible by found invalid.”

The amendment she was referring to came from Parrot, who proposed legislation that would have imposed a fee structure similar to the one that state officials approved.

Rowe said in an e-mail on Monday that she doesn’t know of any lawsuits challenging the state’s account-maintenance fee.

The Daily Record of Maryland first reported the advisory letter in an article last week.

Rowe said in the letter that the courts have upheld differential tolls as legal in several recent cases, but she noted that “none of them, however, have involved systems under which all state residents are favored and all non-residents are disfavored.”

She also said that there is no added difficulty in collecting charges from nonresident E-ZPass holders, suggesting that courts might find the account-maintenance fee to be unjustified.

“While the current fee can be presumed not to be ‘excessive in comparison to the benefits received,’ it looks less reasonable when residents are not charged at all,” Rowe said.

Maryland Gov. Larry J. Hogan (R) and Transportation Secretary Pete K. Rahn have described the account-maintenance fee for residents as one of the most despised charges for state motorists.

“Instituting those monthly fees was a mistake that caused tens of thousands of people to drop their Maryland E-ZPass or switch to other states and discouraged others from signing up,” Hogan said at a news conference this month.