Okay, maybe thinking this through will help us brainstorm in new ways.
My guess is Tad hadn't tried to, say, drive to Mexico lots of times previously, so he'd still be relying on the same tools you and I would. He'd probably Google Map it. Google Map offers 3 routes from Columbia, TN to Mexico when just trying to get to the border in the fastest way (24 to 25 hour drive). Only one of the routes goes through Decatur. I am pasting that one below. Click on it to enlarge it.
I'm with Steelman on the fact that the delay on the Amber Alert may have bought them all the time they needed to, at least, temporarily disappear.