Start of trip: "I see you have a 500 mile drive. The speed limit is 65 all the way, so I'll divide 500 by 70 and say you'll get there in 7 hours and 8 minutes."
A quarter of the way through: "You've been driving at 60 mph the whole time. You have 375 miles remaining. I'll divide by 70 mph to get 5 hours and 21 minutes."
90% through: "You've had cruise control at 60 for seven and a half hours straight, just like you've done every previous time you've made this trip, but the speed limit is 65 for the remaining 50 miles, so I'll ignore your obvious behavior and assume you'll drive 70 for the rest of the trip...."
Also. "I've helped you drive down this section of highway 184 times before. 182 of those times you stopped at this exit for 12.4 minutes to go to the Starbucks. This is a useful piece of information for targetting ads, but I don't see any relevance to you ETA calculation."