March 16, the day Metrorail shut down completely for a safety check, meant a bad trip for many commuters, but it could have been so much worse.

After all, Metrorail accounts for 700,000 trips per weekday, and about 70 percent of them are made by commuters. On March 16, they all had to go somewhere else, right?

Researchers with the region’s Transportation Planning Board wanted to know how we got through it, so they’ve done a preliminary analysis. Here’s the big surprise: At the regional level, the congestion on March 16 was slightly lower, compared to a typical Wednesday. Drilling down below the regional level, the researchers found some interesting variations.

The worst traffic was inside the Capital Beltway. Travel time for roads in the District, Arlington and Alexandria at the peak hour of the morning rush, 7 to 8 a.m., was an average of 12 percent higher than normal that day. Travel times on freeways heading into the regional core were as much as three times longer than normal for the peak hour, the researchers said: “Northbound I-395 in Northern Virginia saw a 125 percent increase in travel times, jumping from 28 minutes on a typical Wednesday to 64 on the day of the shutdown. Northbound I-295 in the District saw a 218 percent increase, jumping from an 18-minute trip normally to nearly an hour on March 16.”

Outside the Beltway, driving conditions were much better. The trip on Interstate 66 from Route 286 to the Beltway, for example, came in at 15 minutes. On a normal Wednesday, it takes 22 minutes.

The evening commute was a snap for almost all drivers. In the core, congestion was about 3 percent lower than usual — just as it was in the rest of the region. Nearly every freeway regionwide had lower outbound travel times.

So from the preliminary analysis, they know time and location made a difference, but they don’t know exactly why. It’s a good bet that the design of the Metrorail system has something to do with it. A key goal in setting up Metrorail was to serve the central employment centers.

“Since the majority of commuters who normally use Metro are traveling to jobs in the core, it would make sense that the major routes entering downtown saw the worst congestion,” the report says. “But it could also be that more commuters who live outside the Beltway already drive to work and were unaffected by the shutdown. Or that commuters living outside the Beltway, those who would have faced the longest drives into downtown, simply opted not to go into the office that day, instead taking advantage of unscheduled leave or unscheduled telework.”

So what happened in the afternoon to make even the commute from the core better? Well, that’s still a puzzler, but the researchers have some ideas: “It‘s possible that after experiencing congestion in the morning, drivers adjusted when they left at the end of the day or made other driving trips like school or child care drop-offs. Another possible explanation is that people decided not to make certain discretionary trips, like shopping or going to the gym, given the unusual and uncertain travel conditions.”

They’re not done. They want to know not just how many people traveled, but also how they adjusted their travel. Did they change their travel time, travel route or travel method? Did they just take the day off?

While they’re working on the answers, here’s my theory, based on watching the traffic patterns that morning: The rush hour on the highways started very early that morning, to the point where I was worried we’d be overwhelmed, but by 9 a.m., it was clear we weren’t going to have a catastrophe on the commute. Personal conclusion: While some people did change travel modes for the day, a lot of people just stayed away, while many others varied their travel times. So overall, we did well, despite the short notice about the Metrorail shutdown.

As we await more data from the researchers, what conclusions have you reached about March 16 and its implications for any extended Metrorail closings in the future?