Driving down Hwy 3 from Moorhead to Hwy 49 at Inverness got me thinking about straight stretches of road. This one is about 6.5 miles without a curve, hill, or bobble (other than the road surface)
Assuming Texas probably has the longest stretches.
Interesting Sunday morning brain teaser/Google exercise given the criteria. Tried to think of where the true flatlands are in the US given most of the country has at least some topography and where RoW considerations hasn't put a kink or two in it somewhere. A lot of straight stretches out west but even those have a "surprising" number of kinks in them and not sure about the hills. Several straight runs that I know are flat across South FL and as noted a lot of the total MS delta area.
That said, our friend AI says this is the champ (I don't know their definition of "significant" and actually still has couple kinks in it).
The longest straight road in the US with no significant hills is a section of North Dakota Highway 46, stretching for 121 miles between Oxbow and Streeter. While there are a couple of other contenders for the title of longest straight road, this section of ND-46 is generally considered the flattest and most consistently straight.