1913 trunk line system

In 1900, the State Highway Department proposed a statewide system of trunk line routes. By 1913, the system consisted of 10 north-south highways and 4 east-west highways, including the lower Boston Post Road. The system covered roughly 1,400 miles (2,300 km). The 14 trunk lines were numbered on paper but were never actually signposted. The 14 trunk line routes were:

• Route 1: east-west from Greenwich to Stonington, roughly modern U.S. Route 1
• Route 2: north-south from Stamford to Salisbury, roughly modern Route 106, Route 33, Route 35, U.S. Route 7, Route 341, Route 41
• Route 3: north-south from Norwalk to North Canaan, roughly modern U.S. Route 7, Route 107, Route 53, Route 37, U.S. Route 202, Route 45, U.S. Route 7
• Route 4: north-south from Bridgeport to Colebrook, roughly modern Route 8
• Route 5: north-south from New Haven to Suffield via Cheshire (concurrent with Route 6 north of Hartford), roughly modern Route 10, Route 4, Route 159
• Route 6: north-south from New Haven to Suffield via Meriden (concurrent with Route 5 north of Hartford), roughly modern U.S. Route 5, Route 71, Route 159
• Route 7: north-south from New Haven to Enfield via Middletown, roughly modern Route 17, Route 99, U.S. Route 5
• Route 8: north-south from Old Saybrook to Hartford, roughly modern Route 9, Route 99
• Route 9: north-south from New London to Colebrook, roughly modern Route 85, Route 2, Route 189, Route 20
• Route 10: north-south from New London to Thompson, roughly modern Route 32, Route 169, Route 12, Route 193
• Route 11: north-south from Stonington to Woodstock, roughly modern Route 2, Route 32, U.S. Route 6, Route 198
• Route 12: east-west from Salisbury to Putnam, roughly Twin Lakes Road, modern U.S. Route 44
• Route 13: east-west from Kent to Sterling, roughly modern Route 341, U.S. Route 202, Route 118, Route 4, U.S. Route 6, Route 14
• Route 14: east-west from Sherman to Killingly, roughly Route 37, Route 67, Route 317, Route 64, Route 322, Route 66, U.S. Route 6

New England road marking system

The first public route numbering came with the advent of the New England road marking system of 1922. This highway numbering system was used throughout New England and consisted of 25 routes (with route numbers from 1 to 32). A total of 9 of the routes passed through Connecticut (Routes 1, 2, 3, 4, 8, 10, 12, 17, and 32). In this system, inter-state routes would be numbered 1–99 and state routes numbered 100 and up. The New England route system was soon eclipsed by the national U.S. highway system.

1922–1931

The State Highway Department classified state roads as either State Highways (SH) or State Aid Roads (SA). These roads were given number designations – 100–299 for primary routes and 300+ for secondary routes. Some state roads were signposted and some were not.

1932 renumbering

The state abandoned its old numbering system and renumbered almost all of their state highways in 1932. Most of the present route numbers were formed during this renumbering. The only route numbers that survived were U.S. Routes and a few state highway routes. For route numbers established in 1932, the new numbering system used odd numbers for north-south routes and even numbers for east-west routes, matching the U.S. Highway numbering system. The New England routes that were grandfathered into the highway system (Routes 8, 10, 12, 32) did not follow the new system. The state also assigned new route numbers in clusters, with routes in the same general location having numbers close to each other as well. Shortly after the renumbering, in 1935, two new U.S. Routes were commissioned: US 44 (taking over part of old New England Route 17) and US 202.

1963 renumbering

In 1963, the state passed the Road Reclassification Act to fix the by now fragmented state highway system. Many state highways had state maintenance gaps and several highway segments were even isolated from the rest of the system. State highways were classified into primary, secondary, and service roads. Primary routes were essentially left unchanged, while minor realignments, additions/deletions, and extensions occurred in many secondary routes. About 1/3 of all routes were changed to some degree by this renumbering. The current system of unsigned ("secret") routes, including the special service roads, was also created during this renumbering. The state highway system has not had any major changes since then. The state completely abandoned the odd/even numbering scheme established in 1932 with new numbers in 1963 assigned without regard to their direction or general location.