The old Spanish Trail, from San Diego California downtown to St Augustine in Florida

Located in Horton Plaza, under the garish glare of a Planet Hollywood sign, the Pacific Milestone seems painfully out of place today. However, on November 17, 1923, the plaza was the site of several thousand people gathered to honor Fletcher’s ceaseless work to bring a southern national highway to San Diego. President Coolidge began the ceremony, pressing a button in the White House which set off a bell in the plaza amid the cheers of thousands. Launched in 1915 in Mobile, Alabama as a connector route between New Orleans and Florida, the Old Spanish Trail soon expanded to a transcontinental trail, linking St. Augustine, Florida, to San Diego, California.

Spanish missions, forts, and trails had indeed historically occurred along many sections of the highway. And historic expedition routes used by De Soto, De Vaca and De Navarez, as well as lesser-known mission trails in Florida and Texas, did roughly align with the Old Spanish Trail. With these in mind, the Old Spanish Trail was publicized for tourists with an exotic narrative of the Trail following the “footsteps of the Padres and Conquistadores” they were also an early effort to get motorists interested in roadside history; a goal later realized by the WPA state guidebooks.
Begun with a bang, progress on the highway stalled in the late teens due to WW1 and the considerable expense of building bridges across the numerous waterways emptying into the Gulf.

The biggest obstacles to developing the transcontinental highway were the ferries. As late as 1927, only 100 cars per day could travel across Mobile Bay by boat.

Numerous major waterways needed to be spanned between Florida and New Orleans. These formidable physical obstacles included two-thirds of the drainage waters of the United States and 125 miles of delta formation east and west of the Mississippi River.

To cross these waterways most state and local highway authorities relied on 30 private ferries between Houston and Florida. The majority of the ferries worked on limited schedules and charged exorbitant fees, a situation not conducive to building a transcontinental highway.

High tolls between Florida and Louisiana were causing motorists to avoid the Louisiana section of the OST entirely.

The United States’ ongoing experience in Europe during World War I, as well as Pancho Villa’s bloody incursion over the border in 1916, had awakened America to the need for dependable military roads. In reaction, numerous trails associations and good roads groups cast their publicity and promotion to reflect importance to defense in the hope that the government would take over the road and build it. A 1917 National Geographic article entitled “The Immediate Necessity for Military Highways” urged Americans to build a national highway system

After several years of near inactivity, the highway project shifted west to Texas (1/3rd of the road), and took on new vigor. The Old Spanish Trail Association successfully brought the highway to completion in 1929.

To celebrate Old Spanish Trail’s completion, St. Augustine hosted a three-day gala, including the dedication of a six-foot diameter coquina stone monument marking the beginning of the trail. (see below for info about this)

The OSTA made one final trip from Florida to San Diego in October 1929. The official itinerary for the trip encouraged members to form the “biggest motorcade ever staged.” By the time they reached Lordsburg, New Mexico, the motorcade “owing to rains and heavy floods through the south” consisted of only fifteen cars

The lure of the OST continued to captivate travelers until the early 1960s, when new interstates redirected traffic off the old road.

Despite the publication of thousands of brochures, maps, and the release of a feature-length promotional film, the completion of Interstates 8 and 10 in the late 1960s doomed the Old Spanish Trail to extinction. The new interstates, which provided a straighter and faster course across much of the Southwest, left many sections of old U.S. 90 and 80 to fade into obscurity.

http://drivetheost.com/history.html is the source of the above info and the map
http://ezinearticles.com/?St-Augustines-Zero-Milestone---A-Not-So-Spanish-Old-Spanish-Trail&id=2363462 for the below info

The zero milestone marker is one of the most misunderstood landmarks in St. Augustine.

The marker is a six foot diameter coquina stone ball with a bronze plaque attached to it. Only the year "1928" which is inscribed on the plaque prevents the visitor from including the stone with St. Augustine's nineteenth or eighteenth century historic lore.

 The plaque simply states that the monument marks the beginning of the Old Spanish Trail between St. Augustine and San Diego, California. Many tourists conjure up a vision of Spanish missionaries and soldiers slogging their way from this marker across the United States to San Diego. The fact that the marker is dated 1928 does little to change their speculations.

The facts are that the Old Spanish Trail did not have its origins in Spanish St. Augustine but in Mobile, Alabama. The City of Mobile developed as a French, not Spanish colony at Fort Louise de la Mobile. 

To enhance and romanticize the road it was called the Old Spanish Trail. Although it is true that the road would connect many Spanish initiated settlements the purpose was what today we would call marketing hype.

http://www.atlasobscura.com/articles/resurrecting-the-original-road-trip-on-americas-ghost-highway