That map is completely exaggerating how rugged the central US is (and the US in general), to an almost comical extent. The majority of the US is tediously flat. The map I linked is *much* more accurate. Or even better: simply
go to Google Maps and turn on terrain.
Trust me, I've spent most of my life in multiple locations that they draw as mountainous in your map, where in reality you'd have to drive 1000+ miles to see an actual mountain. Google is correct. The entire Gulf coast and southeast used to be *underwater* for crying out loud, and in the Great Plains the soil is so deep (leveling everything out) that anywhere you can find bedrock, they make it into a state park.
One of the places I lived (among many) was Houston. We were something like 60 miles from the ocean but only 60 feet altitude. One foot average incline per mile. And not much local variation therein. Also lived in Dallas (not much different from Houston), Nederland (if anything, worse than Houston), Indiana (most parts not much different; slight slopes in minor areas, and a couple *very confined* areas of actual terrain), Iowa (very gently rolling hills). Of places I've lived, only SoCal had meaningful slopes - and even there there were long many long routes of flat terrain as well.
Beyond places I've lived, I road tripped very extensively. From the south to the north in the Great Plains, the only place with significant contours is the Ozarks, and even there it's not that bad (they're not really mountains, just hills). Minor slopes in states around the northern Mississippi, but not too significant. The region is called the Great Plains for a reason, by the way. Flat all the way up to the eastern Rockies, which are 2/3rds of the way from the East Coast. Except in New England, there's a wide band across the Atlantic seaboard that's flat; the Appalachians are just a relatively small band interrupting coastal flatland's transition into interior plains flatland.
Even in regions that have mountains, there are long stretches of interstate with only mild slopes; the interstates are designed to avoid as much as possible going over rugged terrain. For example, the Appalachians are highly folded along a southwest/northeast axis. So I-81 is laid out to perfectly follow that axis so that it doesn't have to keep going over ridges. Meanwhile, interstates that have to go the other direction (such as I-64) take it perpendicular so that they're going over ridges for as short of a period as possible. With the Rockies, interstates either try to divert around the mountains (ex: I-80, I-40), or beeline through them for the first flatland (ex: I-70 beelining for Grand Junction).
The point is: while there certainly are lots of places where interstates have to climb significant grades, they are *by design* only a very small fraction of the total mileage of the US interstate system. And in most of the US don't exist at all, because there simply does not exist anything more than ever-so-gently-rolling hills (if that) in most of the US.
