Las Vegas and Phoenix are linked by a road that narrows to two lanes, hits stoplights in a Depression-era town and until recently backed up traffic over the Hoover Dam.
Despite being two of the largest cities in the Southwest, they aren't directly connected by an interstate freeway.
There have been halting advances toward creating a slick, new highway to cover the 300 miles of desert between Sin City and the Valley of the Sun, but if it's ever going to happen, according to Steve Betts, leader of a coalition of project supporters, "everyone would have to be very creative."
An effort to improve what's now a 4 1/2-hour drive with a more reliable road has heavy-hitting allies, including business leaders and the Republican governor of each state. "Long-term jobs are created by our connectivity," Betts said, noting that the stretch would be the first piece of a new shipping route between Mexico and Canada.
But critics ask whether such a multibillion dollar development would be more than a vanity project that would take resources away from more immediate concerns. The cities already "are connected by U.S. 93. Whether they need an interstate is a question," said transportation historian Earl Swift.
That the cities aren't already linked by an interstate is a fluke of timing. The Phoenix and Vegas populations exploded just after the national road-building frenzy that started in the 1950s.
"As good as the planners were in the midcentury, they could not have foreseen the emergence of Las Vegas," said Robert Lang, director of Brookings Mountain West, a University of Nevada, Las Vegas-based think tank focused on the region's economics and public policy.
The Las Vegas metro area, population 2 million, is 40 times larger than it was in 1950. The Phoenix area, population 4.3 million, has grown 13-fold over that span.
"It's very difficult to remedy something like this once it goes wrong," said Lang, an expert in urban and economic growth. "Piecing this back together is a heavy lift."
Highway supporters won a key victory last year when Congress formally designated Interstate 11. The legislation provides no funding, but it allows builders to tap into interstate construction dollars.
Following that designation, transportation departments in the two states took up a $2.5 million study to plot routes and estimate a cost. The project could run about $4 billion using rough Federal Highway Administration estimates, or closer to $10 billion by other calculations.
Outside-the-box solutions could be necessary as the project confronts difficult political and financial realities.
The nation's interstate system grew out of "a groundswell of grassroots demand" that partly involved safety. The national road fatality rate in 1956, when funding for the system was approved, was almost six times higher than in 2011, the most recent year statistics are available.
"The highway death toll was shocking," said Swift, whose book "The Big Roads," explores the history of the interstate system.
Vehicle crashes were "killing more people on the roads every year than we lost (annually) in Vietnam. It was a crisis," he said. "The public was pushing congressmen to do something about it."
Decades later, the Interstate 11 project doesn't have such public urgency, and safety has been a relatively minor aspect of the pitch.
The main talking points now are how the interstate would create more efficient freight flows and boost the regional economy. "It's not a matter of functionality," said Tom Skancke, president and CEO of the Las Vegas Global Economic Alliance.
"Looking from a global competitiveness standpoint, does it compete better globally?" he asked. "Or do we want to bounce along the bottom?"
An interstate could link Los Angeles, Phoenix and Las Vegas as partners in a "megaregion" that competes with other regions, and could open a trade route from Mexico to Pacific Ocean ports and Canada. Arizona and Nevada are currently losing much of that flow and its attendant development to Texas and California, according to Betts, chairman of CAN-DO, an acronym for Connecting Arizona and Nevada-Delivering Opportunities.
Proponents warn about the consequences of doing nothing, pointing to projections that the cities will add a combined 4 million people by 2025.
"We have to get ahead of the curve," Skancke said. "We're reacting to what happened 30 years ago, while the rest of the world is building."
Proponents are pushing nontraditional financing for Interstate 11, suggesting a public-private partnership in which investors would step in to provide funding. Others, such as transportation expert Bob Poole, of the libertarian Reason Foundation, suggest making the entire stretch a toll road.
Such plans acknowledge the reality that the federal highway fund, which pays for road projects, has dwindled under a revenue model that hasn't changed in 20 years. Also, aging roads and bridges demand an increasing share of that money annually.
However the project would be funded, supporters will need to overcome
concerns from those who ask how essential it is that the two cities are linked by an "interstate" -- defined as a multi-lane, divided highway with interchanges and few access points.
Road proposals are sometimes more about prestige than empirical demands, Swift said.
"Kill away all the layered arguments, and an awful lot comes down to `We're too important not to have it,"' he said.
Arizona is well on its way to widening a 200-mile stretch of two-lane road between Phoenix and the Nevada line. All but 40 of those miles are now four-lane, divided highway, addressing most congestion problems there, said Michael Kies, the Interstate 11 project manager for the Arizona Department of Transportation.
In Nevada, county leaders recently approved a local fuel tax increase to pay for the $300 million Boulder City Bypass, aimed at loosening the most notable bottleneck between Las Vegas and the Arizona border. The 12-mile route will allow drivers to avoid Boulder City's quaint but often congested downtown streets. The bypass is expected to be complete in late 2017.
Also, in a joint Nevada-Arizona project, a bridge went up in late 2010 allowing travelers to bypass tourist traffic on top of the Hoover Dam.
Still, other critics worry that pushing further toward the interstate dream would contribute to urban sprawl and hurt the environment.
"The last thing we need is another freeway," said Sandy Bahr, president of the Arizona chapter of the Sierra Club. "We need to look for other transportation modes."