Boston traffic: the only predictable thing is that it’s unpredictable

Traffic on northbound I-93 in Boston. Craig F. Walker/Globe Staff/file/Globe Staff

For six years, Amy Shanler knew the milestones she needed to hit to make it from Sudbury to Boston University to teach her Principles and Practices of Public Relations course, which starts at 10:10 a.m., promptly.

If she left home at 8:38 a.m., made the right onto Trapelo Road in Lincoln by 9, hit Watertown Square by 9:25, merged onto Storrow Drive by 9:30, got off at Kenmore by 9:35, crossed Commonwealth Avenue and pulled into the parking garage by 9:40, she’d be in such good shape she’d have time to get a Diet Dr Pepper.

But lately that schedule hasn’t been working, and on a recent morning, a truck on the side of Storrow caused such a slowdown Shanler had to call the department secretary from her car to make sure her students didn’t take off.

“I’m already allowing 90 minutes for a 25-mile trip,” she said. “How much of a cushion do I need to leave?”

The “cushion question” is gaining urgency as congestion, construction, and Uber and Lyft combine to make the only predictable thing about Boston traffic its unpredictability.

Of the many ways Boston traffic tortures its motorists, variability is perhaps the cruelest. It plants the seeds of uncertainty before any drive: Is a 15-minute cushion enough? Maybe 30’s better. But what if a truck has overturned? I’m already doomed.

Unpredictability pits spouses and parents and teens against each other, as they bicker over how early is too early to leave for the airport. Cue the eye rolling when dad insists on a two-hour cushion, and the trip to Logan turns out to be traffic-free. It forces young parents into a daily day-care gamble: Leave work when there’s still work to be done or risk a late fee?

So real is the fear of unexpected traffic that being able to leave a cushion — namely, having a flexible job or life — is a new signifier of status.

At Massachusetts General Hospital, traffic has started making about 20 percent of his patients late, said psychiatrist Timothy Wilens, chief of the Division of Child and Adolescent Psychiatry and codirector of the Center for Addiction Medicine.

About six months ago, the situation became so disruptive that it prompted him to change the way he operates. Now, if the 9 a.m. patient is running 10 or 15 minutes late, and the 9:30 is already there, he will see the 9:30 person first, lest the entire day run behind.

“You could say the patients should be more mindful of the traffic,” Wilens said, “but it’s not such a simple issue. You can’t expect someone to budget in an extra 45 minutes’’ — on top of the predicted traffic — “to accommodate extreme traffic issues.”

Wilens is understanding, but he is, after all, in a helping profession. Airlines, employers, and day-care centers aren’t typically so forgiving, and that’s ramping up stress, as people already on tight schedules don’t necessarily have an extra 45 “just in case” minutes to spare. Or even 15.

In East Boston, Blythe Berents is already awakening her 14-year-old daughter at 5:30 a.m. so they can leave at 6, which should be plenty of time to make it to Boston Latin Academy by the start of her first class at 7:20. But in April, twice in one week, massive traffic jams — one triggered by an overturned cement truck, the other by an accident in the tunnel — made Giselle and a fellow carpooler late anyway.

The school takes tardies very seriously — unexcused, they can lead to a lack of credit — but with sleep crucial for teenagers, Berents says her options are limited.

“I’m not going to get her up at 5,” she said.

At this point, no one in Boston is surprised that there is a lot of traffic, particularly during rush hour. Earlier this year, yet another study confirmed we’re in trouble. “The worst gridlock in the US is right here in Boston,” the Globe headline read.

But here’s something less discussed. When the roads are at or near “capacity” — a traffic engineering term to describe the maximum number of motor vehicles a road can handle in an hour under normal conditions — even the smallest thing (or non-thing) can congeal traffic, said Sam Schwartz, a former New York traffic commissioner and the man who coined the term “gridlock.”

Schwartz, now a consultant, described a common scenario: The road is full but traffic is moving, when a driver almost spills his cup of coffee, and in the process of grabbing it, taps the brakes, triggering a chain reaction that slows the traffic from, say, 45 miles per hour to 25 miles per hour, and causes a slowdown that can last for an entire morning commute.

“People sometimes refer to that as a phantom traffic jam because they don’t know what caused it,” Schwartz said. “There was no crash. No broken-down car.”

Another issue: Boston’s famed schools and ridiculously winning professional sports teams are doing us in. Postseason games, victory parades, move-in and move-out days, and graduations all spawn congestion and can come as a surprise to drivers not focused on Northeastern’s or Harvard’s calendars, say, or the Bruins’ playoff schedule.

“There are so many events outside the norm,” said Scott Peterson, acting executive director of Boston Region Metropolitan Planning Organization’s Central Transportation planning staff. “Any one in isolation is not a big deal, but they add up.”

How will we tell when unexpected traffic becomes so common it’s no longer, technically, unexpected?

Urban transportation consultant Bruce Schaller offered what might be called a facial-expression metric:

“If I get to a meeting late and I say, ‘the traffic was messed up,’ do people say, ‘I’m sorry to hear that,’” he said, “or do you see in their faces, ‘You should have planned for this kind of thing.’”