Even if lawmakers approved that kind of cash this week, the wall almost certainly wouldn’t be complete by the end of Trump’s first term—or even a potential second term. The “iron law” of infrastructural megaprojects, according to a paper by Bent Flyvbjerg published in the Project Management Journal in 2014, is that they will go “over budget, over time, over and over again.”

This is obviously the case in projects where everything that can go wrong seemingly does (think: Boston’s infamous Big Dig highway project). But even for well-managed megaprojects, building major infrastructure always seems to take longer than estimates suggest. Sometimes that’s because a time estimate only pertains to the actual construction—not the time leading up to it, says Andrew Natsios, the director of the Scowcroft Institute of International Affairs at Texas A&M University.

Natsios is a public-finance expert who was hired to salvage the Big Dig project after catastrophic cost overruns. He also has a reputation for stinginess. (Evidence for his frugality, The New York Times reported in 2000, was displayed on a conference room in his office at the time: “a red sign with the simple word ‘No.’”) Natsios told me that he hasn’t studied the wall, specifically, but that it is likely to face the same sorts of challenges that any megaproject bumps up against.

“Some of the wall will have to be built on private property and the owners may challenge the federal government’s taking of the land,” he wrote in an email. “Politics extended the construction of the Big Dig in Boston for years because of innumerable disputes. That will likely be the case in the construction of the wall on the U.S. border with Mexico.”

That said, there are some aspects of the project that make it easier to stick to a timetable. The fact that the wall is to be built on “open land,” rather than in an urban area, is hugely helpful, Natsios told me. “Building on land where there are existing buildings, in urban areas, that takes much longer,” he said.

That’s not to say it will be easy. “The border presents huge topographical challenges to construction,” analysts at the research firm AllianceBernstein wrote in a report last year. “It runs through remote desert in Arizona, over rugged mountains in New Mexico and, for two thirds of its length, along rivers... These difficulties were illustrated during the construction of the existing fence which was beset by delays, surging costs and disputes with private land owners.”

Overrun models demonstrate that delays to major infrastructure projects often beget more delays. This is an easy concept to understand, but a difficult one to quantify. “The potential to lose productivity as work slips from its initial plan is generally understood but rarely considered explicitly in planning and estimating,” wrote Stephen Grey, an expert in uncertainty models, in a 2010 paper on risk management. “It raises the hypothesis that some projects lose control over their schedule because insufficient effort was devoted to understanding the effect of small deviations from the plan—and to preparing to prevent it from snowballing into a progressive decline.”