Uber, realizing it had hundreds of millions of people in its back seats as a captive audience, just announced it will allow third-party apps the ability to serve up notifications, content, or ads to its users during their trips. The notifications would be customized to the length and locations of each person's ride, but only with permission from the user.

Uber is calling it "Trip Experiences," and is billing it as its biggest update to the Uber application program interface, or API, since it announced last March it would allow app developers the opportunity to drop an Uber button into their user interfaces. Essentially, it would allow an app, with the user's permission, to serve up a variety of content during that user's Uber trip, based on the specific details of each ride.

A student on the way to school could get a playlist of songs from their favorite music app that conforms to the length of their trip. A couple on a date could receive recommendations for the restaurant they're about to visit. A family on the way home could get a reminder to turn on the heat from their smart home app.

Of course, these are just the benign possibilities for Uber's Trip Experiences. Others may see this as an opportunity for brands to inundate users with ads and irritating notifications during their Uber rides. Putting your code in the other companies' hands is a slippery slope. Maybe there's a McDonald's nearby to your drop-off location. Here's a McDonald's ad to help remind you of that fact.

Users will be in complete control

Uber says it's aware that with great power comes great responsibility, which is why the company claims that users will be in complete control. "They will need to give permission before their favorite apps can connect to Uber and access their trip details," the company says. "And users will be able to turn off the feature app by app at any point if it's not useful."

Likewise, a provision in Uber's developer terms prohibits developers from using the Uber API to distribute unsolicited advertising or promotions. But it doesn't prevent, say, an ad from playing before an entertainment company's YouTube video. "The point of this experience to provide added value to a user not irritate them," an Uber spokesperson said. "Because we require all apps to be whitelisted with us, we will be able to ensure the user experience is a positive one."

The new feature is the latest in the company's continuing mission to integrate with as many third-party apps as possible, and in the process become the world's de facto transportation service.