U.S. Route 20 in Massachusetts

U.S. Route 20 begins its eastward trek crossing the New York state border into Hancock, Massachusetts, passing through the southern edge of the Pittsfield State Forest before entering Pittsfield and meeting the northern end of Route 41. The highway enters town and meets U.S. Route 7, near the western end of the cross-state Route 9. US-20 turns south along US-7, and the two routes form a concurrency into the town of Lenox. In Lenox, Route 7A splits off to serve the center of town, and US-20 splits from US-7 shortly thereafter to serve the town of Lee to the southeast. The highway passes through town and interchanges with the Mass Pike at Exit 2 south of town, near the eastern end of Route 102. US-20 turns to the east at this point, and closely parallels the Mass Pike for several miles, before entering the town of Becket and meeting Route 8. Route 8 northbound joins US-20 east to directly serve the town, while the Mass Pike passes south of Becket. Upon crossing through the town center, Route 8 splits from US-20, and the federal highway continues eastbound, passing into the town of Chester in Hampden County.

US-20 passes through the small town of Chester, and clips the southern portion of Huntington in Hampshire County before re-entering Hampden County and the town of Russell. US-20 passes through Russell, passes under the Mass Pike and meets the eastern end of Route 23, and continues into Westfield. US-20 crosses the U.S. Route 202/Route 10 concurrency in the center of the town, and intersects the northern end of Route 187 just to the east. US-20 begins to enter urban landscape as it crosses into West Springfield, making its way through the streets, intersecting U.S. Route 5, and crossing the Connecticut River into the city of Springfield. Immediately upon entering the city, US-20 interchanges with Interstate 91 at Exit 8, where it leaves the surface street (which is picked up by Route 20A, to join Interstate 291, which begins just to the south. (Note: Route 20A is NOT a former alignment of US-20. Before the construction of I-291, US-20 followed Main, State Streets, and Boston Road). US-20 is overlapped over most of I-291, running on the Interstate for about 5 miles (8.0 km), and leaving via Exit 5,[2] where it meets the eastern end of Route 20A and resumes its surface alignment on Page Boulevard in Springfield, rejoining its original alignment in Springfield on Boston Road. US-20, as Boston Road, crosses Route 21 and proceeds east into the town of Wilbraham, where it goes back to paralleling the Mass Pike for several more miles, until entering Palmer. In Palmer, US-20 meets the southern end of Route 181 and then intersects Route 32, which provides close access to the Mass Pike at Exit 8. US-20 and Route 32 share a short concurrency in Palmer, before US-20 splits east. Before leaving Palmer, Route 67 meets its southern end at US-20. Brimfield is the last town US-20 passes through in Hampden County; it enters from the west, crosses Route 19 in the center of town, and proceeds out of town to the east.

US-20 crosses into Worcester County and the town of Sturbridge, immediately meeting the southern end of Route 148. The route continues east where it meets the western end of Route 131 before interchanging with Interstate 84 at Exit 3. I-84 is another quick route to the Mass Pike, as it ends there less than a mile to the northeast. Route 49, a short connector to Route 9 in Spencer, has its southern terminus at US-20 east of the I-84 interchange. US-20 enters the town of Charlton at this point, passing through Charlton City where it meets the northern end of Route 169 and crosses over Route 31. Continuing east, US-20 enters the town of Auburn, south of Worcester, where it briefly overlaps Route 12. Route 12 provides direct access to the Mass Pike at Exit 10, while US-20 directly interchanges with Interstate 395, which becomes Interstate 290 north of the Mass Pike interchange. US-20 continues to the northeast, entering Millbury after briefly crossing into Worcester, where it interchanges with the Mass Pike at Exit 10A, as well as with Route 146 (which at this point is overlapped by Route 122A). The route re-enters Worcester, crossing over Route 122, which provides access to the Mass Pike at Exit 11. Leaving Worcester, US-20 enters the town of Shrewsbury, interchanging with Route 140 south of town. The highway continues to the northeast, passing southwest of Shrewsbury, and interchanging with Route 9. Heading almost due north at this point, US-20 crosses into Northborough, and turns back eastward, intersecting the western end of Route 135 in the town center. Two local roads, Church Street and Hudson Street, provide access to nearby I-290, which runs to the north. US-20 proceeds out of Northborough to the northeast, crossing into Marlborough and Middlesex County.

US-20 enters Marlborough and interchanges with Interstate 495 (also referred to as the Outer Circumferential Highway) at Exit 24 west of town, effectively crossing into Boston's western suburbs. The route directly serves the center of town, crossing Route 85 in the process. It is in eastern Marlborough where US-20 begins to be famously referred to as Boston Post Road, and the road continues eastward into the town of Sudbury. US-20 passes through the southern part of the town, crossing over the Route 27/Route 126 concurrency in Wayland, and continues into Weston. US-20 does not intersect any major roads or numbered highways in Weston, but does follow an approximately mile-long bypass of the Boston Post Road (appropriately named Boston Post Road Bypass) between Highland Street and Wellesley Street, where US-20 rejoins the road and continues east into Waltham. The entire length of US-20 in Weston is designated as the Boston Post Road Historic District, since it retains much of its rural 18th-century character. Immediately upon crossing into Waltham, US-20 enters a rotary interchange with Interstate 95 (Route 128) at Exit 26, marking the change from suburban landscape to the urban landscape of metro Boston and having access to the Mass Pike. US-20 meets the eastern end of Route 117 and cuts through the center of Waltham, serving as Main Street east of Route 117. US-20 also meets the western end of Route 60, an urban highway serving Boston's northern suburbs, in Waltham. The highway continues into the city of Watertown serving as its Main Street to Watertown Square, where it has a major junction with Route 16, which passes west and north of Boston via various parkways on the edges of the city, through Cambridge and Malden. US-20 becomes North Beacon Street at this point and continues southeast as it crosses into the city of Boston over the North Beacon Street Bridge.

US 20 eastbound approaching Kenmore Square, Boston

Eastern terminus sign of U.S. Route 20 in Boston, MA

US-20 briefly continues as Leo F. Birmingham Parkway upon crossing into the Boston neighborhood of Brighton, immediately intersecting the western end of Soldiers Field Road Extension, before it turns right and continues as North Beacon Street again, and crosses under the Mass Pike (no interchange). US-20 continues towards downtown, passing through the Allston neighborhood of Boston. Upon crossing Cambridge Street, US-20 becomes Brighton Ave for a short distance. US-20 soon reaches Commonwealth Ave, and takes the eastbound routing over for Route 30, which ends at the intersection. At this point, the Brookline/Norfolk County border lies just feet south of the road, and US-20 passes through Boston University.[5] US-20 crosses Route 2 at the southern end of the Boston University Bridge, crossing over the Mass Pike for the final time. Route 2, via Mountfort Street, Park Drive, and Beacon Street, turns from south to east to meet US-20 at Kenmore Square. Upon meeting Route 2 again, US-20 meets its eastern terminus, with Route 2 taking Commonwealth Ave its final stretch to the Boston Public Garden and Route 28, where Route 2 meets its own end.[6]

U.S. Route 20's official eastern terminus is in Kenmore Square at the intersection of Commonwealth Avenue, Beacon Street, and Brookline Avenue.