Moody’s Investors Service downgraded $764 million of Massachusetts Bay Transportation Authority assessment bonds Thursday to Aa1 from Aaa to reflect the credit rating agency’s pool financing methodology.

The outlook is stable. The MBTA is Massachusetts’ largest mass-transit provider, serving more than one million customers each day, on average.

The agency’s assessment bonds are secured by fees that 175 cities and towns in the mass-transit system’s service area pay to the state. The weighted average default probability of the cities and towns is Aa1, according to Moody’s. The rating agency now calculates an average default probability for borrowers within a pooled credit and applies that result in its credit evaluation.

“The downgrade to Aa1 reflects the application of our new pool financing methodology, including the evaluation of the underlying credit quality of the pool participants, the nature of their obligation to pay, and structural attributes of the financing,” according to a Moody’s report.

The assessments are general obligations of the municipalities. The state’s Treasury Department subtracts those fees from local aid payments to the municipalities. The assessments that cities and towns pay have been lower than their state aid allocations, Moody’s said.

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