Southern Californians have a distinct way of giving directions. take "the" 10 to "the" 110 to "the" 101. Burbank to San Diego? "The" 134 to "the" 5. And, if you can, always avoid "the" 405.

Why the definite articles? After all, a resident of the Bay Area enjoys coastal drives along "101" or takes "80 east" to Sacramento. Most of North America, in fact, omits the "the" before route numbers.

 The answer begins with the region's early embrace of the freeway. Long before the National Interstate and Defense Highways Act of 1956 gave most U.S. cities their first freeways, Los Angeles had built several.

These were local routes, engineered to carry local traffic and (partly) paid for by local funds. It only made sense that, as they opened one by one, they'd get local names that denoted their route like the Cahuenga Pass thus became the Cahuenga Pass Freeway, and the freeway to San Bernardino became the San Bernardino Freeway.

State highway officials did affix route numbers to these freeways. In their early years, the Pasadena Freeway, for example, was Route 6, 66, and 99, all at once. The Harbor Freeway carried both Route 6 and Route 11. Who wouldn't prefer the simplicity of a name over a confusing array of numbers?

 Around the same time, a flurry of new construction added unfamiliar freeway names to the region's road maps. Drivers found it easier to learn new numbers like the 605 or the 91 rather than new names like the San Gabriel River Freeway or the Redondo Beach Freeway.

Soon a shorthand emerged when the state simplified its highway numbering system, ensuring that, with few exceptions, each freeway would bear only one route number.

http://www.kcet.org/updaily/socal_focus/history/la-as-subject/the-5-the-101-the-405-why-southern-californians-love-saying-the-before-freeway-numbers.html