NORTHAMPTON — Familiar exit numbers on Interstate 91 will begin being replaced by exit numbers based on mileage starting Wednesday.

The highway corridor that runs between Connecticut and Vermont will be the final highway in the state to get the new numbers, bringing Massachusetts highways into compliance with federal standards.

As part of the changes, for example, northbound and southbound travelers entering Northampton at Route 5 will see Exit 18 become Exit 23.

Northbound travelers exiting I-91 at Route 9, now Exit 19, for Damon Road in Northampton and the University of Massachusetts and Hadley will see that become Exit 25, while southbound travelers exiting I-91 onto North King Street at Exit 20 will see that become Exit 26.

Exit 21 for Routes 5 & 10 and for Hatfield will become Exit 27.

The old exit numbers will continue to be indicated by yellow signs attached below the new exit number signs for at least two years to help the public get acquainted with the new system.

The state announced the project will cost $2.8 million and is being paid for mostly through federal Highway Safety Improvement Program funds.

— SCOTT MERZBACH

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