The alignment of Route 99 was originally designated as part of New England Interstate Route 10 in the 1920s. The Silas Deane Highway was built in 1930 and New England Route 10 was shifted slightly west to use the new highway.
Route 99's alignment was originally designated as part of New England Interstate Route 10 in the 1920s. The construction of the Silas Deane Highway in 1930 led to a shift in the designation of New England Route 10, which now utilized the new highway.