when did its construction begin?
September 21, 1927
George Washington Bridge
Montgomery B. Case, chief construction engineer, began construction on the Hudson River bridge began on September 21, 1927 with groundbreaking ceremonies held at the sites of both future suspension towers. 
what route is the george washington bridge on Interstate 95 (I-95) and U.S. Route 1/9 (US 1/9, composed of US 1 and US 9)
wiki:13537472
George Washington Bridge [SEP] First contracts
Montgomery B. Case, chief construction engineer, began construction on the Hudson River bridge began on September 21, 1927 with groundbreaking ceremonies held at the sites of both future suspension towers. Each tower was to have a base with a perimeter measuring , and descending 80 feet into the riverbed. The riverbed around the towers' sites was dredged first, and then steel pilings were placed in the riverbed to create a watertight cofferdam. The cofferdams for the bridge were the largest ever built at the time. In early October of that year, the Port Authority received bids for the construction of the bridge deck.