Connecticut started numbering New England Interstate routes in 1922, and its own state routes in 1923. Over the next several years, the system grew into dozens of routes, mostly in the 100s and 300s, with little geographical order or congruence with routes in neighboring states. For details, see Connecticut Routes in the 1920s.
On Jan. 1, 1932, the state's new numbering system went into effect, creating some routes that have changed very little over about 90 years. Route 128, for example, has been repaved from time to time, but otherwise is the same Route 128 born in 1932.
However, the system did not, unlike Athena, emerge on January 1 fully formed. There was discussion, some alternatives, and some hard-to-find (if they still exist) documents about ideas that were rejected or revised. Fortunately, an online document of early correspondence between Connecticut officials and the American Association of State Highway Officials (AASHO) included a high-quality scan of a blueprint dated Nov 10, 1931. More than 90% of it survived as the 1932 plan. But the differences are interesting.
The original Route 15, from New Haven to Union, is marked as Route 9. The original Route 9 is instead Route 17. A short Route 171 (later part of Route 3) connects Route 9, er, 17, and Route 160. Route 94 in Glastonbury is Route 180.
Some of the outcomes here are "welp, they picked a different number" without much historical significance. But some may be clues that carried over past 1932. A short-lived Route 17 in Essex (1939-1940) might have gotten the number from an area engineer fond of the number from 1931. The Route 9 to Union plan would not technically have caused a conflict in Massachusetts with its own Route 9, since MA 9 debuted in 1933. But the Bay State's future plans likely came up in neighboring state discussions.
It gets even better: a few routes on the map have erasures that (vaguely or clearly) hint at earlier planned numbers that had been changed before late 1931. Route 137 was once marked as Route 27 on the map; and less legible erasures for Route 104 and Route 123 hint at routes 23 and 31. This implies that the geographical ordering of routes probably started not at old Route 29 but at 23 (or a 21 in Greenwich we haven't seen yet), continuing with the routes 33, 35, 37 and so on that we see today.
Here's a summary of all the 1931 routes that were changed by Jan. 1, 1932. Routes without significant changes (1931 to 1932) are left out.
For each 1931 route on the left, we explain what happened to it in 1932. Key: [SL - state line; UC - under construction; RI - Rhode Island; MA - Massachusetts; amb. - ambiguous; "Did not exist" means no route, with any number, in that alignment.]
|4||Briefly (erased) would have followed CT 343 to the state line instead of today's CT 361.|
|9||Became CT 15 instead.|
|15||The 1932-era CT 15 was originally planned as CT 9.|
|15||Briefly (erased) shown for 1932-era CT 15 from Stafford to Union. This became proposed CT 9 in late 1931, but was CT 15 in 1932.|
|17||Briefly (erased) shown for CT 19.|
|19||Became part of CT 32 instead. (In the 1931 plan, CT 32 followed NE 32, which used today's CT 19 to enter Massachusetts.)|
|21||A stretch, but the route numbering pattern might have started with a CT 21 in Greenwich.|
|23||Possible draft number (erased, illegible) for CT 104.|
|27||Draft number (erased) for CT 137.|
|31||Possible draft number (erased) for CT 124 and CT 123 in Darien and New Canaan.|
|32||Followed NE 32, which used today's CT 19 to enter Massachusetts.|
|47||Followed CT 131 from Roxbury to Washington, then CT 47 to CT 25.|
|63||Existed south of CT 14 only.|
|69||Continued past Waterbury along CT 73 to Watertown.|
|73||Part of CT 69.|
|74||Did not exist; planned as CT 192.|
|86||Planned as CT 155.|
|94||Planned as CT 180.|
|97||Did not exist. A small portion (Norwich to Scotland TL) was planned as CT 209.|
|101||Continued west to CT 41 instead of ending at US 7.|
|104||Possibly planned as state route 23.|
|118||Planned as part of CT 136.|
|130||CT 341, from CT 45 to CT 25, Warren. (The 1932 version did not exist.)|
|131||Did not exist.|
|134||Followed CT 47 from US 6 in Woodbury to CT 47 in Washington.|
|136||The 1931 version of CT 136 also included CT 118 in Darien.|
|137||Planned as CT 27|
|155||Became CT 86.|
|162||Did not exist.|
|164||Did not exist.|
|171||Would become part of CT 3 (between CT 160 and Mill Street) in 1941.|
|176||Did not exist.|
|180||Became CT 94.|
|186||Did not exist.|
|190||Did not exist.|
|192||Became CT 74 instead. (Did not exist in Enfield.)|
|194||Line on the map, but no number.|
|195||Oops... defined in Mansfield and East Hampton (became CT 196).|
|196||Possible typo as CT 195.|
|197||Existed only east of CT 93.|
|198||Did not exist.|
|209||Became CT 97 from Norwich to Scotland TL. A few post-1932 maps still show this Route 209 here.|
|341||Existed west of US 7 only.|