However you measure it, Amazon’s impact on Seattle is indisputable. Here’s a new metric: summer interns. Amazon has so many that King County Metro has adjusted bus service just for them.

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At 8:18 Wednesday morning 23 people with identical black backpacks — fold-over top, metallic buckle, five leather zipper pulls, water-bottle sleeve — boarded a Route 70 bus at the University of Washington.

Eight minutes later 13 more people with the same backpack boarded another 70 bus.

At 8:40, 21 matching backpack-carriers got on the 70, and at 8:42 15 more.

At 8:55, two number 70 buses arrived at the same time and both were swamped with people carrying the black backpack — too many to count, both buses packed to the gills.

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Each one of those backpacks also had one more identifying characteristic, a small tan arc with an arrow on the right pointing upward — the Amazon Smile.

Each of Amazon’s summer interns this year received the same backpack.

From booming jobs to gentrification, from omnipresent cranes to housing shortages, there’s any number of ways to measure how Amazon’s presence has reshaped Seattle. But here’s another one: Amazon’s summer interns — just the interns — can overwhelm bus routes in the city, causing King County Metro to send more buses to accommodate them.

Beginning Wednesday, Metro added two additional buses to Route 70, which runs between the University District and downtown via Amazon’s home in South Lake Union.

Those added buses, which bring the route’s total to 18 on weekdays, will run indefinitely, Metro said.

And while Metro is reluctant to pin the change solely on the influx of interns, it’s not hard to make the connection.

“Amazon did inform us that their internship program was growing,” Scott Gutierrez, a Metro spokesman, said.

How many Amazon interns are there? More than 900 will be living in University of Washington dorms and apartments this summer, Victor Balta, a university spokesman said. Amazon is paying UW $2.8 million to house the interns, Balta said.

But not all the interns — maybe not even half — are staying at UW. An Amazon spokesman did not respond when asked how many interns the company has in Seattle.

A half-dozen interns waiting for buses gave varying answers, anywhere from 1,500 to 3,000 summer interns in Seattle, with the consensus being around 2,000.

A Facebook group restricted to Amazon Seattle 2017 summer interns has more than 1,700 members.

The company has more than 25,000 local workers, and last year it had more than 8,000 job openings in Seattle.

Amazon runs four private shuttles between the two campuses — UW’s in Montlake and its own in South Lake Union — and expects that number to increase. But there were long lines and full seating on those shuttles Wednesday morning.

So what’s the quantitative impact of the interns on Route 70 — generally the 14th busiest of Metro’s 185-or-so routes?

Assume that only half of Amazon’s UW-based interns ride the 70, but ride it to and from work each day. That’s 900 rides, or a 12 percent increase in the route’s daily 7,400-rider average.

Metro added buses after reports of overcrowding earlier this week. Some riders farther down the line reported as many as six buses passing them by in the morning, unable to stop because they were full with passengers.

This is the third summer Amazon interns have stayed at UW.

“I take it year-round, and this time of year buses start passing by,” said Kate Duran, a researcher at Fred Hutch, just as a full bus passed by her stop on Eastlake Avenue, a mile downstream from UW.

“Once crowds start to build up, it can be like three buses going by,” said Ryan Gorup, himself an Amazon employee (full-time), as he waited for the bus on Eastlake with his shepherd-husky mix, Toby. “It really snowballs.”

One of those two buses that arrived at the UW simultaneously at 8:55 — and was subsequently swamped with backpack-toting summer Amazonians — was one of the additional buses that Metro added to the route Wednesday.

As he crossed the University Bridge the driver announced that because the bus was full he’d be making no stops unless someone rang the bell.

He drove a standard single bus, not one of the articulated double buses that bend accordionlike in the middle and can carry more passengers.

“Next time they’ll probably send a 60-footer instead of this 40-footer,” he said.

Information in this article, originally published June 22, 2017, was corrected June 22, 2017. A previous version of this article incorrectly said that Metro bus route 70 travels over the Montlake Bridge. It takes the University Bridge.