PHOENIX – Twenty-five, years ago, the last segment of Interstate 10 was completed and it happened in Phoenix.

When the Arizona Department of Transportation opened the “Final Mile” between Third Avenue and Third Street, including the Deck Park Tunnel, it created the nation’s second coast-to-coast interstate, stretching 2,460 miles and across eight states, from Santa Monica, Calif., to Jacksonville, Fla.

“This opening of I-10 is truly a milestone in transportation history,” Thomas Lane, head of the Federal Highway Administration, said at the tunnel’s dedication ceremony on Aug. 10, 1990. “Today, we mark the completion of a major transcontinental route.”

Fast-forward to 2015, on Monday, Aug. 10, when ADOT observes the Deck Park Tunnel’s 25th birthday.

While the opening of the Deck Park Tunnel marked the completion of I-10 as a transcontinental interstate, it also signaled the beginning of the Phoenix-metro area’s modern freeway system, which continues to grow today. Putting the age of the freeway network into perspective, “The Simpsons” has been on television longer than the Deck Park Tunnel has been open to traffic.

When the tunnel opened, construction of the Loop 101 and state routes 51 and 143 had just begun, and the Loop 202, Loop 303 and State Route 24 existed only on planning maps. Phoenix’s population boom made the expansion necessary. The 20th-largest city in the United States in 1970, Phoenix would rise to No. 6 by 2000, according to U.S. Census data. The completion of the Deck Park Tunnel connected the metropolitan area’s east and west valleys, allowing for quicker and more convenient travel across the metro area. The tunnel also emerged as a linchpin in Arizona’s economic development, supporting the efficient movement of goods and commerce into and through the state. Since it opened, an estimated two billion vehicles have passed through the tunnel’s tile-lined walls.

“The Deck Park Tunnel is more than simply a way to get through downtown Phoenix,” ADOT Director John Halikowski said. “It is part of an interstate Key Commerce Corridor that is integral to Arizona’s continued economic growth and development.”

Decades before it was built, transportation officials recognized the need for the Papago Freeway – the stretch of I-10 that passes through Phoenix. The first plans for the freeway were formalized in 1960. However, a tunnel wasn’t included in the original design.

The tunnel was a part of a solution to opposition that did not want the Papago Freeway built near the Phoenix city center, unsettling neighborhoods established before interstates existed. In 1969, plans called for an elevated freeway with wide, arcing “helicoil” ramps that were designed to minimize disruption of city streets and the utility grid. But a public vote to build the freeway was defeated in 1973. Two years later, the elevated freeway was scrapped in favor of a below-grade design, which included the tunnel, and Phoenix voters approved the measure. Engineers devised an innovative plan that set the freeway below street level for six blocks – from Third Avenue to Third Street. Above the freeway, 19 bridges would be lined up side by side, creating a tunnel effect for motorists, even though it does not meet the Federal Highway Administration definition of a tunnel. A 30-acre park would be built atop the bridge decks. That’s how it came to be known as the “Deck Park Tunnel,” though its official name is the Papago Freeway Tunnel.

Still, the Papago wasn’t yet a “go.” Freeway opponents put the issue on the ballot again in 1979, but citizens voted overwhelmingly in favor – 3-to-1 – of building the Papago Freeway and construction began in 1983. When it was finished in 1990, the Papago Freeway ranked as the most expensive highway project to date in Arizona at a cost of $500 million, plus $150 million for right-of-way purchases.

Not surprisingly, public interest in the freeway was high as the opening neared. According to a report in The Arizona Republic, more than 100,000 people attended a three-day “open house” at the Deck Park Tunnel, riding bikes and running footraces in yet-to-be-driven-on traffic lanes. At the dedication on Aug. 10, 1990, Federal Highway Administration official Thomas O. Willett addressed the obstacles overcome in the previous three decades to build the freeway.

“Completion of the Papago Freeway is far more than construction of concrete and steel,” Willett said. “It represents a successful culmination of a state, city and federal partnership forged by the challenge of a concerned public.”