You can be on the clock for more than 16 hours. It's just an emergency extension for the once a week, 14 hour rule. A few years back, a bunch of us ran into a major snow storm going to our first leg hub. I sat on the interstate for over three hours on the way to my destination. On the way back, I had to pull over over 70 miles from my home hub because my 11 hour drive time was up. I waited two or three hours for a ride. When it was all said and done, I eventually clocked out being on the clock for nearly 19 hours. Another driver logged 22 hours. The extra time is considered off-duty. But yes, in emergency conditions, you can go over 16.