Route 166 is a minor state highway

State highway

State highway, state road or state route can refer to one of three related concepts, two of them related to a state or provincial government in a country that is divided into states or provinces :#A...

 in southern Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

 running from Westbrook

Westbrook, Connecticut

Westbrook is a town in Middlesex County, Connecticut, United States. The population was 6,292 at the 2000 census. The town center is also classified by the U.S. Census Bureau as a census-designated place .-Geography:...

 to Old Saybrook

Old Saybrook, Connecticut

Old Saybrook is a town in Middlesex County, Connecticut, United States. The population was 10,367 at the 2000 census. It contains the incorporated borough of Fenwick, as well as the census-designated places of Old Saybrook Center and Saybrook Manor.-History:...

. The route primarily serves as a connector from I-95 to the Saybrook Manor

Saybrook Manor, Connecticut

Saybrook Manor is a community and census-designated place in Old Saybrook, a town in Middlesex County, Connecticut, United States. The population was 1,133 at the 2000 census. The Saybrook Manor section is generally the area south of U.S...

 section of Old Saybrook.

Route description

Route 166 begins at an intersection with Route 153

Connecticut Route 153

Route 153 is a Connecticut state highway in the Connecticut River valley running from U.S. Route 1 in Westbrook center to Route 154 in Essex Village in the town of Essex.-Route description:...

 in eastern part of the town of Westbrook

Westbrook, Connecticut

Westbrook is a town in Middlesex County, Connecticut, United States. The population was 6,292 at the 2000 census. The town center is also classified by the U.S. Census Bureau as a census-designated place .-Geography:...

. It heads east, roughly paralleling I-95 on the north, into the town of Old Saybrook

Old Saybrook, Connecticut

Old Saybrook is a town in Middlesex County, Connecticut, United States. The population was 10,367 at the 2000 census. It contains the incorporated borough of Fenwick, as well as the census-designated places of Old Saybrook Center and Saybrook Manor.-History:...

. Along the way, it passes by the Westbrook Block company just before crossing the town line. In Old Saybrook, it turns south, crossing I-95 along the "Rosario Aloisio Memorial Bridge", with an interchange at Exit 66. It continues southeast over Amtrak

Amtrak

The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...

 railroadtracks to end at an intersection with US 1

U.S. Route 1 in Connecticut

In the U.S. state of Connecticut, U.S. Route 1 is a major east–west state highway along Long Island Sound. It has been replaced by Interstate 95 as a through route, which it closely parallels, and now primarily serves as a local business route...

 in the western part of Old Saybrook. The entire route is known as Spencer Plains Road and is classified as a minor arterial road carrying average daily volumes of 5,600 vehicles per day.

History

Route 166 was established from town roads in 1935, and has had no major changes in alignment or designation since.

Junction list

Town	Road names	Junction	Milepost
Westbrook Westbrook, Connecticut Westbrook is a town in Middlesex County, Connecticut, United States. The population was 6,292 at the 2000 census. The town center is also classified by the U.S. Census Bureau as a census-designated place .-Geography:... (0.89 miles)	Spencer Plains Road		0.00
Old Saybrook Old Saybrook, Connecticut Old Saybrook is a town in Middlesex County, Connecticut, United States. The population was 10,367 at the 2000 census. It contains the incorporated borough of Fenwick, as well as the census-designated places of Old Saybrook Center and Saybrook Manor.-History:... (0.73 miles)			1.28
			1.62