A Silver Line Metro train pulls into the Ballston Metro station on Wednesday, February 24, 2016 in Ballston, Va.  (Photo by Pete Marovich For The Washington Post)

When the Washington Metro system shuts down for 24 hours, as officials just announced will happen at midnight tonight, there will be consequences for daily commutes and regional congestion and the entire area's economy. Workers will be stranded in traffic. The businesses — and customers — that depend on them will suffer too. A lot of money will be lost in gridlock.

It's hard to understate what a big shock to the region this will be. But WMATA has actually already envisioned such a scenario.

Four years ago, the agency modeled what would happen to the region if the Metro system didn't exist, as part of an exercise in illustrating to the public the "business case" for why transportation investments are so crucial. As WMATA explained when it introduced the study, "One of the best ways to understand the value of something is to take it away." Justin Antos, who worked on it, explained the results to me at the time:

"It was literally just imagining Washington, and all of a sudden, you wake up tomorrow, and the transit system isn’t there," Antos says. "What would you do?"

People, it turns out, do something very interesting. They stop making long car trips because the traffic is so bad. In one hypothetical scenario, Antos took away the transit but kept the rest of the area’s road infrastructure the same. People were allowed to change their trip patterns — to chose different jobs or shopping centers — and most of them stopped crossing the region to get to those things.

"The congestion was forcing people to regress into a more local economy," Antos says. "We looked at that and realized we were watching the economy splinter. All of a sudden, we weren’t watching a regional economy function where workers could find jobs in the whole region."

People weren’t crossing county lines — or even rivers —  to get anywhere.

This scenario envisions how the regional economy would break down if the Metro disappeared not just for one day, but for good. So the analogy is not perfect: No one's going to go get a new job tomorrow because of a 29-hour shutdown. You won't permanently change where you shop or how you move around the region.

But the scale of the disruption WMATA envisioned in this exercise gives a sense of what we're in for, and it makes the case that the metro system is fundamentally a means to an economic end: The point isn't that the system moves people around, but that helping people move around is central to our economy, and to the creation of millions of dollars in real estate value and tens of thousands of jobs.

If the metro didn't exist, WMATA found, we'd need to add more than a thousand lane miles of arterial roads and highways to the region, costing about $6 billion, just to keep congestion at levels close to what we have today. If everyone who rode transit downtown drove instead, we'd need five-story parking garages covering this much real estate, according to the study:

WMATA

On Wednesday — even if temporarily — all of this will become a lot less hypothetical.

The entire Metro system is shutting down for at least 24 hours on Wednesday, March 16, impacting thousands of commuters. (Alice Li/The Washington Post)