When on public transportation, it's not uncommon to see passengers staring deeply into their smartphone or tablet; it happens everywhere you look, whether you're at a coffee shop or just walking down the block. Well, a recent incident in San Francisco highlights how being so absorbed on that tiny smartphone screen can blind folks to what's happening right in front of them.

According to an SFGate report, passengers recently riding on a Muni train were so distracted by their smartphones they didn't notice another passenger, later identified as Nikhom Thephakaysone, 30, was wielding a loaded gun right in front of them. In surveillance footage, Nikhom can be seen repeatedly taking out and putting away his firearm, before shooting and killing 20-year-old Justin Valdez as he exited the train. Authorities say Nikhom was out "hunting" for a stranger to kill.

"These people are in very close proximity with him, and nobody sees this," said District Attorney George Gascón. "They're just so engrossed, texting and reading and whatnot. They're completely oblivious to their surroundings."

While there's an issue here that's much larger than people simply using their smartphones out in public, the point made by Gascón isn't wrong. Folks shouldn't feel ashamed, or even responsible, for reading an email or otherwise staying in touch with loved ones, but there's an argument to be made about how the digital age is changing how we interact with other people, whether they're riding on the same train or thousands of miles away.

Would it have made a difference if someone had noticed Nikhom was very openly wielding a gun? We won't ever know for sure. But it's a little scary to think what can take place right in front of multiple people without them noticing.

"What happens to public places when everybody is talking on a cell phone?" asked Jack Nasar, an Ohio State University professor residing in San Francisco. "Everyone is somewhere else."

Gascón admitted he recognizes the value of smartphones, especially in the circumstance that it helps authorities catch a criminal. However, he also said it makes people vulnerable to crime. In this case, to a gun-wielding lunatic that nobody noticed because they were so preoccupied with checking Twitter, or posting to Instagram.