Well, maybe not twice as much. But computers aren't human, and you're not a robot. You need to stop for fuel, food and bathroom as well as for red lights, slow down for construction and congestion, and occasionally strike an accident up ahead. Computers travel at or above the speed limit for 100% of the time, never need to slow down or stop. At the times they quote, they don't even have time to take a photo, but expect you to stop to take a photo and load it onto their site. It's no way to 'enjoy' a roadtrip.
We always recommend to add about 20% to the quoted times.
Lifey