Route 159 is a primary north-south highway following (within a mile or two) the left bank of the Connecticut River. It intersects I-91 twice, in Windsor and Windsor Locks.
Commissioned in 1932, the original Route 159 was Cherry Hill Road in Middlefield, a 2.58-mile north-south road from Route 157 to present-day Route 147, which was back then part of Route 157. In 1963, this road was handed over to the town.
The modern Route 159 has had a few numbering changes. In the 1920s, much of it (except for a western jog along the original Route 75) was called state highway 110. In 1932, SH 110 became US 5A, a northerly route second in importance only to US 5, on the opposite side of the Connecticut River.
In 1940, plans for the four-lane divided segment of US 5A, leading from the Hartford city line to Windsor Center, were finalized. The 3.5-mile stretch of four-lane US 5A would not only support more traffic than the present two-lane road, but several dangerous curves would be softened or eliminated. The work went to bid in July 1940, and completed in 1942. At the same time, a relocated four-lane US 5 was under construction on the other side of the Connecticut River.
In 1968, the state announced it was considering changing US 5A's designation to state route 159. In Massachusetts, the route would continue as MA 159. The reasoning:
If there were no adverse comments from businessmen or residents, US 5A was to have been changed in 1969, in time for that year's new tourist maps. As it turned out, the number change took effect earlier than that, on Oct. 24, 1968. This was the last of several historical US 5A routes to be renumbered or decommissioned.
1960s long-range regional highway plans included a freeway leading north from the proposed Route 190 freeway into Massachusetts, approximately along Route 159.
In 1967, the Capitol Region Planning Agency proposed a serpentine expressway replacing parts of Routes 189, 187, 190, and 159, leading from I-291 to the Windsor - East Granby line, then leading west and north around Bradley International Airport to approach the Route 190 bridge over the Connecticut River. Finally, the highway would turn north between Routes 75 and 159 in Suffield to enter Massachusetts.
In 1972, the Windsor Town Council debated removing the median from Route 159, prompted by business owners who believed it was hindering access. However, residents opposed this move, citing it as a safety feature for pedestrians; the council voted to keep the median.
The reconstructed intersection with Route 75 and Route 305 in Windsor won an Award of Merit from the FHWA in 1990: "... [early involvement with local businesses], underground utilities, landscaping, and selective placement of street and pedestrian lighting complement the [Windsor village] green and retain the historical character of the community."
Similar work was planned for Route 159 south of there, to the Hartford city line. The Wilson/Deerfield Advisory Committee held a public meeting in May 2001 regarding the Route 159 Wilson Corridor Improvement Project: plans for roadway and streetscape improvements.
In 1970, a candidate for state representative pledged to advocate for a rotary at that intersection. At that time, this most likely would have been a Massachusetts-style rotary instead of a more modern roundabout. This was never built.
Renumber as part of Connecticut's Great River Road, Route 99. Coerce Massachusetts into going along (even though they already have a Route 99 elsewhere).