The prototypes for President Donald Trump’s proposed border wall are currently sitting on a site near the existing border wall in San Diego.

The eight concrete and metal slabs are prototypes for the Trump administration’s vision for a border wall that could cost between $20 billion to $60 billion to build.

However, in a new report, the Washington Office on Latin America (WOLA), a leading research and advocacy organization advancing human rights in the Americas, points out that the section of the border where the prototypes sit—Customs and Border Protection’s San Diego sector—is a perfect example of how limited walls, fences, and barriers can be when dealing with migration and drug trafficking challenges.

This sector has 60 miles of border and 46 of them are already fenced off.

According to the report, the San Diego sector shows that fences or walls can reduce migration in urban areas, but make no difference in rural areas.

In densely populated border areas, border-crossers can quickly mix in to the population. But nearly all densely populated sections of the U.S.-Mexico border have long since been walled off. In rural areas, where crossers must travel miles of terrain, having to climb a wall first is not much of a deterrent.

The report also points out that walls do not deter people who seek protected status. Some asylum-seekers climbed existing fence at the prototype site while construction was occurring, according to the report.

In San Diego, asylum-seekers include growing numbers of Central American children and families. Last year in the sector, arrivals included thousands of Haitians who journeyed from Brazil, many of whom now live in Tijuana. The presence or absence of a fence made no difference in their decision to seek out U.S. authorities to petition for protection.

“The border doesn’t need a wall. It needs better-equipped ports of entry, investigative capacity, technology, and far more ability to deal with humanitarian flows,” the report reads.

The report also mentions that fences are irrelevant to drug flows. Of all nine-border sectors, San Diego leads in seizures of heroin, methamphetamine, and cocaine. Authorities find the vast majority of these drugs at legal border crossings—not in the spaces between where walls would be built.