|Length||97 mi (156 km)|
|Length||53.392 mi (85.926 km)|
|South end||Route 3 in Plymouth|
| US 44 in Plymouth|
Route 53 in Kingston
Route 53 in Quincy
|North end||I‑93 / US 1 / Route 3 / Route 203 in Boston|
|Length||22.504 mi (36.217 km)|
|South end||I‑95 / US 3 / Route 128 in Burlington|
|Lowell Connector in Lowell|
Route 110 in Lowell
Route 113 in Tyngsborough
|North end||NH 3A in Hudson, NH|
|Counties||Plymouth, Norfolk, Suffolk, Middlesex|
Route 3A has two major posted segments, separated by a lengthy concurrency with Route 3 and US 3. Its southern portion parallels Route 3 from Cedarville in southern Plymouth to Neponset in the Dorchester area of Boston. Towns and cities that Route 3A traverse along its path include Plymouth, Kingston, Duxbury, Marshfield, Scituate, Cohasset, Hingham, Weymouth and Quincy.
North of Neponset, Route 3A runs, unsigned, concurrently with Route 3 and U.S. Route 3 to Burlington, before separating again (MassHighway counts the mileage along MA 3 between the two sections as part of MA 3A mileage).
The northern portion of which parallels U.S. Route 3 in northwestern Middlesex County. It stretches from Interstate 95 (Route 128) in Burlington to the New Hampshire state line, where it continues as Route 3A.
Plymouth to Boston
This segment parallels Route 3 from Cedarville in southern Plymouth to Neponset in the Dorchester area of Boston. Towns and cities that Route 3A traverse along its path include Plymouth, Kingston, Duxbury, Marshfield, Scituate, Cohasset, Hingham, Weymouth and Quincy.
Boston to Burlington
Burlington to Tyngsborough
This segment parallels US 3 from Interstate 95 (Route 128) in Burlington to the New Hampshire state line, where it continues as New Hampshire Route 3A. The part south of Route 113 in Tyngsborough is former US 3.
The section from Burlington to Billerica is currently being widened from 2 to 4 lanes.
In the 1930s Route 3A's route was shifted onto new alignments built to reduce traffic problems in several communities through which the highway traversed. This included a bypass of Hingham Square along Broad Cove Road and Otis Street along Hingham Harbor replacing the original route that took it down Lincoln Street to downtown Hingham then along North Street to Summer Street. Chief Justice Cushing Highway was built around the same time and Route 3A was put on this highway from south of the Hingham Harbor rotary through Cohasset to Scituate. This replaced a routing along long-existing roadways such as Country Way in Scituate, South and North Main Streets in Cohasset and East Street to Summer Street in Hingham. Along East Street from Hull Street to Summer, Route 3A shared the highway with Route 128 in the late 1920s and early 1930s.
In the 1950s and early 1960s Route 3A was extended to take over the original path of Route 3 south of Kingston to Plymouth and north of downtown Quincy to Neponset when Route 3 assumed its current freeway route. In between those locations, old Route 3 was designated Route 53. The section of old Route 3 from Cedarville south to the Sagamore Rotary near the Cape Cod Canal in Bourne is unnumbered.
Much of Route 3A is sometimes referred to as the "Cape Way" due to its history as the only major road to Cape Cod from Boston prior to the opening of Route 3. The "Cape Way" name is reflected in numerous business names along 3A's length.
The section of Route 3A south of the intersection with Route 113 in Tyngsborough (all but the northern few miles of the route) was formerly U.S. Route 3, prior to the construction of the Northwest Expressway, a freeway connection from I-95 in Burlington to the Everett Turnpike in Nashua, New Hampshire, which was given the U.S. Route 3 designation.
This section contains a table that is missing mileposts for one or more junctions.
|Plymouth||Plymouth||0.00||0.00||Route 3 – Boston, Cape Cod||Southern terminus; Exit 2 on Route 3|
|State Road south – Scusset Beach||Route 3A joins/leaves State Road|
|11.40||18.35||Plimoth Plantation Highway to Route 3 – Plymouth, Boston||Southern terminus of Plimouth Plantation Highway|
|15.00||24.14||US 44 west – Middleborough, Providence||Eastern terminus of US 44|
|Kingston||18.30||29.45||Route 3 – Plymouth, Cape Cod, Braintree, Boston||Exit 9 on Route 3|
|18.70||30.09||Route 80 west to US 44||Eastern terminus of Route 80|
|18.90||30.42||Route 106 west to Route 27 – Halifax||Eastern terminus of Route 106|
|20.30||32.67||Route 53 north – Hanover||Southern terminus of Route 53|
|Duxbury||20.60||33.15||Route 3 – Plymouth, Cape Cod, Quincy, Boston||Exit 10 on Route 3|
|24.20||38.95||Route 14 west – Pembroke, Boston||Eastern terminus of Route 14|
|25.00||40.23||Route 139 – Pembroke, Green Harbor|
|Marshfield||27.70||44.58||Route 139 east – Brant Rock||Southern end of concurrency with Route 139|
|28.10||45.22||Route 139 west – Hanover||Northern end of concurrency with Route 139|
|Scituate||34.50||55.52||Route 123 west – Norwell, Brockton||Rotary; Eastern terminus of Route 123|
|Hingham||42.20||67.91||Route 228 – Hull, Rockland|
|Norfolk||Quincy||49.00||78.86||Route 53 south – Weymouth||Northern terminus of Route 53|
|Quincy Shore Drive south||Interchange; Southbound exit and northbound entrance|
|Suffolk||Boston||53.60||86.26||Route 203 west (Gallivan Boulevard) / Morrisey Boulevard – Ashmont||Eastern terminus of Route 203|
|53.61||86.28||I‑93 south / US 1 south / Route 3 south||Southern terminus of silent concurrency with Route 3 / US 3|
|See Route 3/US 3|
|Middlesex||Burlington||74.8||120.4||I‑95 / US 3 / Route 128 – Waltham, Lowell, Peabody, Portsmouth, NH||Northern terminus of silent concurrency with Route 3 / US 3; Exit 33 on I-95 / Route 128|
|77.20||124.24||Route 62 east – Wilmington, North Reading||Southern terminus of concurrency with Route 62|
|77.80||125.21||Route 62 west – Bedford, Concord||Northern terminus of concurrency with Route 62|
|Billerica||82.40||132.61||Route 129 east – Reading, Wakefield||Southern terminus of concurrency with Route 129|
|83.60||134.54||Route 129 west to US 3 – Chelmsford, Lowell||Northern terminus of concurrency with Route 129|
|Lowell||87.30||140.50||Lowell Connector south to I-495 / US 3||Exits 5A-B on the Lowell Connector|
|87.70||141.14||Route 110 east / Thorndike Street – Haverhill, Lawrence||Partial interchange; Southern terminus of concurrency with Route 110|
|87.90||141.46||Route 110 west – Chelmsford, Worcester||Northern terminus of concurrency with Route 110|
|Chelmsford||91.10||146.61||Route 4 south to US 3 – Chelmsford, Lexington||Northern terminus of Route 4|
|91.80||147.74||Route 40 west – Groton||Eastern terminus of Route 40|
|Tyngsborough||95.10||153.05||Route 113 west – Dunstable||Southern terminus of concurrency with Route 113|
|Tyngsborough Bridge over the Merrimack River|
|95.30||153.37||Route 113 east – Lowell||Northern terminus of concurrency with Route 113|
|97.00||156.11||NH 3A north – Hudson||Continuation into New Hampshire|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
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