More than a year after the government’s top oversight body urged the Department of Homeland Security to develop a way to measure the effectiveness of fencing and barriers along the border with Mexico, DHS has no such tool ready, even as President Trump prepares to pick the winning designs for his $18 billion border wall.

Trump officials in recent weeks have dismissed criticism of their border security plan with a well-established defensive principle and simple retort: “Walls work.”

But a February 2017 report by the Government Accountability Office (GAO) found DHS has no way to measure how well they work, where they work best or whether less-expensive alternatives could be just as effective. 

Despite the assumption that illegal traffic enters through areas where fencing is absent, the report identified several sectors where more arrests occur in locations that have existing barriers.

U.S. border agents collect “geotag” data, electronic markers that assign geographic locations, to map illegal crossings and arrests. But DHS has no means to gauge the extent to which those incursions are impeded by “tactical infrastructure,” the report noted, undermining the agency’s ability to avoid wasteful spending.

“An assessment of border fencing’s contributions to border security operations could help position [U.S. Customs and Border Protection] to identify the cost effectiveness of border fencing compared to other assets the agency deploys,” the report said.

DHS officials said last week they are working with the Johns Hopkins University Applied Physics Laboratory to develop such an evaluation system, and it may be ready later this year. 

Trump is moving forward anyway. His public statements have demonstrated a keen interest in the aesthetic properties of the wall, along with its height. His administration has budgeted $1.6 billion for wall construction this year.

Trump is scheduled to travel to San Diego on Tuesday to view eight prototypes and is likely to announce one or more winning designs. The trip will be Trump’s first as president to California, a state his administration is suing for refusing to assist with federal immigration enforcement.

Trump’s wall-building plan — which is stalled in Congress — would spend $18 billion over 10 years to add 316 miles of new barriers and replace aging fencing along 407 miles.

The 30-foot steel and concrete prototypes showcased in San Diego are far taller and more formidable than anything in place along the border. They extend six feet underground to deter burrowing and feature an array of anti-climbing configurations. One is crenelated with metal spikes. 

DHS officials say their testing teams found the structures exceedingly difficult to scale or break through. The prototypes cost as much as $486,000 each to build, and DHS has not said if the $18 billion overall cost projection is based on one or several of those designs. 

Instead, DHS officials have defended the expenditure by pointing to major decreases in arrests for illegal crossings in areas where tougher fencing was installed. In a new promotional video titled “Walls Work,” CBP said illegal traffic dropped 87 percent in San Diego after its two-layered barrier system was installed.

“Border walls have proven to be extremely effective in preventing the flow of drugs and illegal aliens across our borders,” DHS spokesman Tyler Q. Houlton said in a statement this month, after a court victory allowing Homeland Security officials to move forward with fast-track construction plans. “Simply put — walls work,” he said. 

But when the independent, nonpartisan GAO launched its study in 2015, it determined that the efficacy of walls and fencing varies widely across the 2,000-mile border, depending on a variety of factors that include topo­graphy, proximity to urban areas, and the ancillary presence of tools such as cameras, sensors and enforcement agents.

GAO researchers analyzed the location of illegal entries between 2013 and 2015 and found sectors of the border in California, New Mexico and other areas where more arrests occurred were in places that already have fencing. In southern Arizona, about half of the illegal “drive-throughs” by unauthorized vehicles occurred in places with barriers to prevent exactly that.

Some of the most robust fencing along the border has been installed in urban and semi-urban areas adjacent to U.S. border cities such as Calexico, Calif., Nogales, Ariz., and El Paso. But those who illegally cross the border sometimes prefer those areas because they can quickly blend into urban surroundings if they manage to get through. 

DHS officials recorded 9,287 breaches of border fencing between 2010 and 2015 in areas with “pedestrian” barriers that are designed to be more forbidding than “vehicle” fencing. Areas with older “legacy” fencing were nearly six times more likely to be breached, the GAO report noted, and presumably many of those sections will be first to be replaced by the taller and tougher ramparts on display in San Diego.

Tougher fortifications help channel illegal traffic toward more remote, isolated areas away from U.S. cities and highways, DHS officials say, giving agents more time to catch up to illegal entrants when ground sensors and aerial surveillance tools detect suspicious activity. 

“Infrastructure on the U.S. side is a key factor that determines how easy it is to get from the border to being able to disappear into a vehicle or into a city, and that affects how much time we have to respond,” DHS statistician Marc Rosenblum said.

Critics of the president’s border security plans say their concerns have less to do with the physics of huge walls than with the fiscal prudence of building them at a time of ballooning deficits.

“We’re spending money like a drunken sailor,” said Sen. Thomas R. Carper (D-Del.), a former naval officer and one of the members of the Senate Homeland Security and Governmental Affairs Committee, which commissioned the GAO report. “We cannot continue to waste money, so we need to find out what works and what doesn’t.” 

Trump campaigned on a promise to build a border wall and oblige Mexico to pay for it. Top DHS officials in the Trump administration praise the proposal. 

But before Trump’s presidential run, Border Patrol agents and officials did not say they wanted a wall, Carper said. Instead, they have long emphasized a flexible “layered” approach combining barriers, technology and personnel in configurations that can adapt to changing security needs. 

“It’s no one thing,” Carper said. “It’s a combination.”

In an interview, the head of the Border Patrol’s Strategic Planning and Analysis Directorate, Benjamin “Carry” Huffman, said that after a career in the agency, he doesn’t need a yardstick to know that walls and fencing are effective. 

“Having done this for 33 years, I can tell you a wall is essential in gaining operational control capability,” Huffman said. “And having worked the border with a wall and without it, I can say you want to work the border with it.”

Look at San Diego, Huffman continued. “It’s a pleasant place to be, one of the finest cities in America. In 1985, it was quite a different place. You had 1.6 million people coming across the southern border. . . . South San Diego was practically uninhabitable. Property values were in the tank,” he said. 

“Fast forward a few years and we started adding this infrastructure,” Huffman continued, describing the addition of primary fencing backed by another “secondary” fence with Border Patrol roadway in between, creating a no man’s land where undocumented crossers could be trapped. 

“We changed the whole environment in that area,” said Huffman. “The U.S. government literally made millionaires and billionaires down there. They had property that was practically unusable, and it changed dramatically.”

Last year, the number of people arrested along the border with Mexico dropped to a 49-year low, and Trump has touted the decline as proof his border security strategies are working.

But illegal crossings have been falling for most of the past decade, and migration experts say tougher border security is only one of several factors. Birthrates in Mexico have plunged since the 1960s, leaving the country with far fewer unemployed young people, while the domestic labor demands of Mexican manufacturing have grown. 

Today, the majority of unauthorized border crossers are from Central America, not Mexico, including many families and unaccompanied minors who turn themselves in to U.S. agents to request asylum, citing threats of chronic gang violence back home.

Most of the Central American migrants cross the Rio Grande in South Texas. DHS officials have prioritized that area for a surge of new wall construction along the winding riverbanks.