In 1922, the alignment of modern Route 20 was designated as two separate state highways: Highway 133 from Route 8 in Colebrook to Route 10 in Granby, and Highway 343 from Route 187 in East Granby to Route 159 in Windsor Locks.
In 1922, the current route of Route 20 was split into two distinct state highways: Highway 133 running from Route 8 in Colebrook to Route 10 in Granby, and Highway 343 stretching from Route 187 in East Granby to Route 159 in Windsor Locks.