RI line in Millville to Worcester.

Expressway nearly the full length.

Multiplex with MA 122A in Worcester.

Extension of RI 146.

146 first appeared in 1936, connecting from RI 146 at the border to Uxbridge. This road is now part of MA 146A.

146 remained a relatively minor route until after 1950, when a section of 4-lane was built from west of Millbury to just north of Whitinsville. Another road, 2 lane, was built between this and 146, and the number was applied to the entire length. By 1956, the 4-laning extended all the way to the original section of 146. near Uxbridge (on some maps; others show it as a regular 2-lane road). 146 was cosigned into Worcester with MA 122A (to what purpose is unclear; it ended in the city somewhere; when 122A was put onto the I-290 expressway, 146 appeared to end at the highway.) By 1966, the road was standard 2-lane from Worcester to Millbury, divided from Millbury to Whitinsville, 2-lane divided with controlled access from Whitinsville to 146A, and 4-lane controlled access south of there.

The state is upgrading 146 to be a full expressway from Worcester to I-90. An interchange with the Mass Turnpike opened in 1998; the road is expressway up to the interchange with I-290.