Phoenix and Tucson may still merge by 2040.
A 2005 prediction for the cities with downtowns separated by 120 miles didn't seem far fetched when proclaimed by growth experts during the housing boom.
But when the bust started in 2007 and stretched to 2011, such a prediction seemed unlikely.
Now, metro Phoenix is on another growth streak.
New census data shows Maricopa County grew faster than any other county in the nation for the second year in a row.
Population projections for metro Phoenix could mean it will join with Tucson somewhere in Pinal County in about two decades, if the numbers are right and the Valley's growth engine doesn't stall again.
Urban researchers began identifying what they believed would be "super-sized" metro areas or megapolitans 15 years ago when growth was rampant in the Valley and several other parts of the country.
The forecast for Arizona was for a swath nicknamed the Arizona Sun Corridor from Prescott in the north all the way south to Sierra Vista and the Mexico border to emerge with more than 10 million residents by 2040.
That seemed like a lofty goal a decade ago during the recession.
Tucson and Prescott aren't growing as fast. But still the possibility of Arizona's growth corridor doubling in population in a decade doesn't seem unbelievable.
With that said, the formula for Arizona's population growth has been wrong before.
Housing has played an outsize role in metro Phoenix's population forecasts since the 1950s when the area's affordable new housing began drawing lots of residents.
But during the boom of 2004-06, speculators bought multiple homes with no plans to move into them. Some lied on mortgage documents and got Arizona driver's licenses even though they lived out of state.
Those new homes and home sales artificially drove up prices and were calculated as part of metro Phoenix's projected growth.
When home prices began to fall in 2007, many of those speculators walked away from Valley houses.
As many as 50,000 new homes in the Phoenix-area sat empty in 2008-2009. Those houses had been tracked during the boom as part of the area's expected explosive population growth.
The recession hit, and the fake demand for homes and bad subprime loans backed by Wall Street led to a record number of foreclosures.
New neighborhoods in the Valley's edge suburbs from Pinal County to Buckeye were half built and almost empty by 2008. Those "ghost town" neighborhoods became a symbol of the housing crash.
In 2007, the forecast was for 105,000 people to move to metro Phoenix in 2008. That didn't happen, and some growth watchers still think the Valley might have lost more people than it gained in 2008-2009.
Arizona estimated the state had 6.7 million residents in 2009. But census data released in 2010 showed only 6.4 million.
The overly optimistic forecasts also led to too many new schools, shopping centers and roads being built on the edges of metro Phoenix.
In 2010, Arizona hired its first demographer to find out what had gone wrong with the state's growth projections.
Now, the state's population growth numbers aren't as dependent on housing, and tallies during the crash have been revised.
Arizona's population, now at almost 7.2 million, is expected to grow 1.6 percent in 2018 and 1.5 percent 2019, adding a bit more than 100,000 residents both years.
Most of Arizona's residents live in metro Phoenix and Tucson.
To hit those six-digit growth forecasts, metro Phoenix must keep enticing residents with housing they can afford and jobs.
Right now Casa Grande, almost in the middle of Phoenix and Tucson, has new homes priced at $170,000. That's $150,000 less than what the typical new home in metro Phoenix is selling for now.
And many of the new home buyers in Casa Grand work in nearby metro Phoenix suburbs, Gilbert, Chandler and Mesa.
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