I drove in the same areas as you starting 1965, and recall only hearing “101” or “17” with no “the.” I’ve heard “I-5” called “5” but never (by a Northerner) “the 5.” You are a Northerner, Snippe but perhaps you’ve fraternized with ex-Southerners. 
“17” is (was?) also called “The Nimitz,” but I recall that “Highway 17” (or, later, “Old Highway 17”) south of Valley Fair referred to what is now called “South Bascom” or “Los Gatos Blvd.” 
(PS: I’d been living in “Silicon Valley” for 19 years before ever hearing it called “Silicon Valley.” :smack: )