Uber’s decision to roll this out was made as a response to the problem of drowsy driving and driver fatigue, both of which are issues that continue to affect people on the road, even if driving while using mobile devices and intoxicated driving get more press and scrutiny.

The Uber feature implementation will trigger when a driver has driven 12 hours without taking a continual, six-hour break at any point between. Drivers will have full visibility into how much driving they’ve done according to Uber, which measures based on a number of factors, and will count things like when you’re stopped at a stoplight (your brain is still engaged in the driving activity, even if you’re temporarily stopped), but won’t count time spent waiting in an airport parking lot to be called for a pickup, for instance, since many drivers use these as napping and rest opportunities.

Uber’s Head of Safety Product Sachin Kansal explained that the company relied on its ample experience with drivers and working with road safety organizations in determining what does and doesn’t count towards a user’s total driving time.

“There’s definitely a lot of third-party expertise that has gone into our thinking,” Kansal said in an interview. “But it’s also that we know how our drivers drive, we know road conditions, so we have baked all that into it as well.”

This limit likely won’t impact the majority of drivers on its platform, the company notes, because around 60 percent of its drivers don’t even use Uber over 10 hours per week, but Kansal tells me that a relatively small number do tip the scales as heavy users. The company wants to do its part to address this safety issue, however, Kansal tells me, and to do so proactively, even where it’s not specifically required by local bylaws.

Uber has studied the feature where implemented in other markets (including Australia, where it launched previously) and built this U.S.-facing version with a lot of feedback in mind. That’s why the app will provide notifications when you’re nearing that 12 hour limit, effectively counting down so that it’s fully transparent and not surprising to a driver when they max out. When the six-hour break is over, the app will once again unlock itself for bookings. Also, where different rules are required by local law, those will apply instead of this new cross-U.S. limit.

Rival Lyft has a driver limit in place, too, which mandates a six-hour break for every 14 hours spent in driver mode, but it’s not as granular as Uber’s. Uber says it also plans to evaluate continued international rollout on an ongoing basis, and to expect this change to be introduced gradually across the driver app in the U.S. over the next few weeks.