How do they know when you’re driving? They rely on your phone’s GPS to calculate your speed. If it’s more than five or 10 miles an hour, it’s pretty clear that you’re no longer walking. (You could be riding your bicycle, of course. But come to think of it, that’s probably not a great time to be texting, either.)
You could also be in a bus, taxi or train, or just sitting next to someone who is driving. Fail.
--J