If Route 70 looks like a splicing of two different roads, then its origin will not surprise: in the 1920s, State Highway 323 once led from Waterbury to Cheshire, and SH 325 led from Cheshire to Meriden. Both were secondary routes.

In 1932, Route 70 was created as a 13.75-mile route, incorporating both earlier routes. Its original alignment, from west to east:

• E. Main St, from Meriden Road (Route 14 at the time) in Waterbury to the Cheshire town line;
• Waterbury Road (a continuation of E. Main St.) and W. Main St. to Route 10; this alignment starting at I-84 is part of today's Route 70
• Overlap with Route 10 heading south (also part of today's Route 70)
• Academy Rd, S. Meriden Rd, River Rd, and Main St. to end at Hanover Avenue, the original alignment of Route 71. All of this segment is also part of today's Route 70.

At that time, Route 68 extended only from Route 8 to Route 70, which continued through Cheshire by itself.

On Sept. 11, 1962, Route 71 was realigned in Meriden. Route 70 was extended southeast along Hanover Ave., former Route 71, to end at the new Route 71 at Old Colony Road.

In 1964, the portion of Route 70 from downtown Waterbury to I-84 was changed to unsigned SR 801.

The original interchange at I-84 was a modified cloverleaf, with two loop ramps on the north side, and a weave on I-84 westbound. Around 2006, when I-84 was widened to six through lanes in the area, the interchange was reconfigured to a simple diamond.

There is an Old Waterbury Road section along Route 70 east of I-84, but that section predates the 1950s and I have no other information on it.