Route 175 was commissioned in 1932, along the following route, west to east:
- Allen St, East St, and Cedar St, from Route 71 to Route 176 (this portion is also the modern alignment)
- Main St, Hartford Ave, and Newington Ave, into Hartford (current Route 176, and its older extension)
Route 176 at the time followed Main Street throughout Newington, and overlapped Route 175 between Cedar Street and Hartford Avenue.
Around 1940 the north-south section of Route 175 was relocated to Wells Road, to end at the Silas Deane Highway in Wethersfield: its current alignment.
Widening an important arterial
For decades, Route 175 has served as a critical east-west route, with few alternatives between New Britain Avenue to the north, and the old Route 72 to the south. In the 1960s, it became apparent that the road, two lanes wide at the time, would have to be widened.
In 1967, State Sen. Paul Amenta submitted a bill asking for $2.5 million to widen the route between Stanley Street and Hawley Street (east of Route 176). Gov. Dempsey had already budgeted $1.3 million to fix a shorter segment, between today's Fenn Road and Maple Hill Avenue, including two narrow railroad overpasses. However, this plan would lay fallow for several years.
In the east side of Newington, Route 175 was significantly improved. On Oct. 21, 1968, a relocated segment of Route 175 opened in Newington and Wethersfield. The four-lane roadway lies to the north of the old alignment, which includes today's Patricia M. Genova Dr. and an abandoned section of former Route 175 leading up Cedar Mountain. The new roadway, with a safer, gentler curve up the ridge, also overpasses Mountain Road (SSR 405).
At some point between 1971 and 1975 the original widening plan was expanded so that Route 175 would be four lanes wide throughout Newington; but it was then cut back, so the four-lane section would only extend as far east to nearly reach Alumni Road. Local officials protested the plan, saying it would only create a bottleneck where the road narrowed. Town and state came to an agreement later, and public hearings were held in 1980 to discuss a continuous four-lane profile throughout Newington.
In November 1983, a bid was awarded to widen Route 175 for about 1.5 miles, from 400 feet west of Maple Hill Ave. to Hawley Street, completing the four-lane section. Work finished around 1985.