For an Oahu agency charged with predicting population growth around the island — how much and where it’ll be — fruition of the rail transit project has been the key predictor.

Still, the massive 20-mile transportation system is not the only factor. Along with housing and other development near the track route, large-scale housing projects such as Ho‘opili in Ewa and Koa Ridge in Central Oahu are included in projections by the Oahu Metropolitan Planning Organization (OMPO).

Complicating the landscape, though, are legal challenges that have ensnared all three massive planned projects: Ho‘opili, Koa Ridge and, most loudly, the rail.

"If the city for whatever reason decides to stop rail, to not go forward, that will require us to go back, look at our long-range transportation plan pretty thoroughly and then figure out, now what?" said Brian Gibson, executive director of OMPO.

For now, rail is assumed to be in the offing and, with it, nearby development.

"I think the way we look at the rail project is as guiding or shaping development," Gibson said. "I don’t think people are going to move to Oahu to ride a train."

In the organization’s State of Congestion on Oahu updated last November, it predicted that the island’s population would grow from 905,500 in 2007, to 1,113,500 in 2035. The U.S. Census Bureau counted 953,207 people on Oahu in 2010 and estimated 963,607 last year.

The growth predicted over the next two-and-a-half decades is minimal on most of urban Oahu —with the major exception of Kakaako. Residents there are expected to more than triple, from 10,400 to 37,300, by 2035. The prediction was based on "the general consensus that Kakaako is ripe for development," Gibson said.

Terrance Ware, Transit Oriented Development administrator in the city Department of Planning and Permitting, notes that his department has its own projection — and it envisions an even more rapid growth for the island, predicting 1,117,322 residents by 2030, most of it from growth in Ewa and Central Oahu.

"Are people demanding to go there and developers building in response to that demand?" asked Ware about those areas. "I’d say that’s generally the case because development is such a high-risk endeavor. Out in Ho‘opili and Koa Ridge and those other projects, developers see that there’s a market demand and they’re kind of responding to the market demand, so it’s kind of hand in hand.

"The numbers we’ve seen say 79 percent of the population prefers to live in single-family housing," Ware said, "and so that’s what’s driving a lot of the growth that you see in those areas where it’s flat; they can afford to build infrastructure, etcetera."

Ware said his program is involved in preparing areas within a half-mile of the rail’s 21 stations. The population is not likely to expand around every one of those stops, he said.

"I think in most areas, particularly in the core urban areas, starting with Ala Moana, Kakaako, civic center, downtown, we anticipate that there will be population growth," he said. "In many of the areas in between — Iwilei, Kalihi, Lagoon Drive, Pearl City — without additional growth and residential development in those areas, probably very minimal growth."

The Hawaii Community Development Authority "is certainly looking down that road" of significant growth in Kakaako, OMPO’s Gibson said. At the time of the prediction, HCDA had yet to reveal its plan for numerous skyscraper condominiums, but "we assumed pretty significant growth for that area."

But huge population growth is expected surrounding the western end of the rail, near what is now a fairly barren area near Kapolei: From 18,300 in 2007, to 51,300 in 2035 at Kapolei-Ko Olina-Kalaeloa; from 53,600 to 102,200 at Honouliuli-Ewa Beach; 15,600 to 29,900 at Makakilo-Makaiwa; and 11,900 to a whopping 46,700 at Waiawa-Koa Ridge.

"There’s an intimate link between transportation infrastructure and land-use development," Gibson said. "That’s clear. You build a road to an empty parcel, suddenly that parcel becomes very attractive for development. So if you increase the mobility of people to move between Ewa and downtown or Ewa and Waikiki, then you lower the transportation costs and their time costs, frankly, which are more important to these people.

"Then development in Ewa — living in Ewa or Kapolei — becomes more attractive," he said. "That’s sort of the point, to concentrate growth on sort of the southern coast of Oahu, which is one of the reasons the rail project was developed in the first place."

Gibson added: "We see the rail as helping shape how we grow, not necessarily increasing our growth."

Gibson pointed out that the population target for all of Oahu is developed by the state Department of Business, Economic Development and Tourism, and all state and city departments use those projections. Where those future populations can be expected to occur rests with OMPO and the city Department of Planning and Development, with or without the rail project.

The city agency also determines the factor of basic needs such as water and sewer and services such as police and fire departments.

"We see rail as shaping how the island develops," Gibson said. "For example, if the forecast of DBEDT is 100,000 people between now and 2035, without the rail, they’ll allocate themselves around the island in a certain way. With the rail, they’ll allocate themselves around the island in a different way, given the greater mobility that’s provided by the rail."

Without rail, he said, more new residents may gravitate to areas such as "Central Oahu, Kailua, wherever."

While subject to change if plans for rail or other forms of transportation are chosen or rejected, Gibson sees his work as potentially useful, although he rarely receives queries.

"I can imagine that the information might be useful for Realtors, for businesses that look to expand and wondering where the city is expecting to grow," he said. "I think there’s a lot of potential uses out there."