Much of this dusty border city is separated from Mexico by a fence consisting of 12-foot vertical metal bars, spaced inches apart. Surveillance cameras are mounted...

DOUGLAS, Ariz. —

Much of this dusty border city is separated from Mexico by a fence consisting of 12-foot vertical metal bars, spaced inches apart.

Surveillance cameras are mounted on nearby towers, and Border Patrol agents posted in the desert scrub and flowering ocotillo watch for anyone who might try to scale, cut through, slip under or sneak around the fence.

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Although the fences are criticized for shifting would-be border-crossers to more dangerous and remote spots, they make it harder for illegal immigrants to reach urban areas where it’s easier to slip away.

Congress is considering putting many more such barriers along the 1,952-mile border, which has 83 miles of fences.

House bills passed in December and again last week would add fences in all four border states. New fences, at a cost of $2.5 billion, would cover 698 miles — almost one-fifth the length of the Great Wall of China, though not in one continuous wall. The Senate has approved 370 miles of fences, mostly in California and Arizona, and 500 miles of vehicle barriers.

Gaps would be policed as many remote areas are now: with motion sensors, cameras, unmanned drone aircraft and Border Patrol agents.

The House calls for a mostly continuous, 361-mile fence from Calexico, Calif., to Douglas. The second-largest piece would be a 176-mile segment in Texas from Laredo to Brownsville.

Immigrant-rights groups say fences waste taxpayer money because would-be border-crossers who are desperate to earn a better living will find a way around or through barriers. The groups cite lower sections of the fence in Douglas, where rods have been welded into place to patch up breaches.

Even some proponents say fencing, without other enforcement efforts, will not stop illegal immigrants.

But the Border Patrol says fences slow immigrants and free up agents to focus on remote areas, where aircraft and ground sensors are used.

“Fencing by itself is not effective, but not having a fence is not effective, either,” said Sen. Jon Kyl, R-Ariz.

Advocates say a 14-mile fence near San Diego, once the nation’s most prolific smuggling center, shows that barriers work. That fence is made of corrugated metal sheets. Behind it is a second fence, made of tightly woven mesh.

Rep. Duncan Hunter, R-Calif., the leading voice in Congress for more fences, said the costs of building fences are much lower than expenses associated with illegal immigration, including jailing immigrants convicted of crimes.

But immigrant-rights advocates say fences prompt migrants to cross in remote areas with dangerous obstacles — rivers where some drown, deserts where some succumb to heat, mountains where some are injured or die.

Also, a large-scale fence could force immigrants to stay in the country longer, instead of returning home, said Angela Kelley, deputy director of the pro-immigrant National Immigration Forum. “If it’s riskier and harder, people don’t leave,” she said.

In Douglas, retired rancher Louis Hahn said the fence reduces traffic in the city. But he said it is simplistic to think a barrier will trump economic forces that prompt fathers to leave their families for a chance at a better life.

“You have got to put yourself in the position of the man crossing the border and what he’s willing to take,” he said.