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 modern route of Route 20 was divided into two separate state highways: Highway 133 connecting Colebrook and Granby, and Highway 343 connecting East Granby and Windsor Locks.