This app makes sure you never walk home alone

"Text me when you get home" is a frequent farewell among friends and family.

But what if you could actually track their journey home, check in if they're feeling unsafe on their commute, and ensure that they can easily contact the police in an emergency?

That's all possible with a free app called Companion. Created over a year ago by a group of five students at the University of Michigan, the app has been gaining a ton of momentum after launching to the public two weeks ago.

The app, available on Android or iOS, lets users send requests to phone contacts to virtually track their trips using GPS.

The app has seen 500,0000 new users in the past week alone.

Why is everyone so excited about a safety app? I tried it out for myself. A friend of mine graciously agreed to let me track him as he walked to a coffee shop about 18 minutes away. (I also took my turn being the walker, but it was more interesting to be on the other end.)

I received an SMS text requesting that I be his companion. A link in the text sent me to the app -- though you don't need it to track someone. If not, it would have directed me to a map in a browser. Once I accepted, I could see exactly where my friend was.

The app uses the phone's built-in sensors to detect changes in movement- like if the user starts running or the headphones come out. If that happens, the app asks users to confirm that they're OK. If they don't do so within 15 seconds, the app notifies your companion who has the option to call the police. At the same time, the app will also go into alert mode for the walker, emitting siren-like noises and displaying a button to also call the police.

If I had been concerned about something with my friend -- like why he wasn't walking faster! -- I could use the app to call him. Alternately, he could select, "I feel nervous," which would have prompted me to check in on him. Once he reached his destination, I got an alert that he was safe.

"We've seen so many amazing use cases," said 21-year-old Lexie Ernst, a co-founder who's a senior at the University of Michigan. She said everyone from senior citizens to kids studying abroad are using the app.

That wasn't exactly anticipated: The app was initially built to help tackle crime on college campuses, which the founders say is "way too prevalent."

The first version of Companion rolled out in November 2014 for students at the University of Michigan. The founders, three of whom are still in college, incorporated early feedback into the app's current version.

"We asked, 'What do people actually want to use,'" said co-founder Nathan Pilcowitz, 22.

The founders plan to add more features in the future -- like touch-ID technology so that only the phone's owner can hit the "I'm OK" button. That way, someone can't steal your phone and falsely tell your friends that you're fine.

Because Companion is collecting anonymous data on users' paths -- and where they "feel nervous" -- it's a potential goldmine of information that universities and cities could analyze to make streets safer.

For now, they're just working with University of Michigan, although Pilcowitz said others have reached out about potential partnerships.