Climbers are forever wittering on about how difficult they find it to ride on the flat, and presenting climbing as virtuous. So I'm more interested in the world's flattest road, because that's obviously more challenging.
Dunno about flattest, but longest straight is I think 90 mile straight in the Nullabor of Australia. 140km of soul destroying dead straight flat nothingness... Nothing higher than a small bush as far as the eye can see...
Closer to home my personal fear is a headwind on the Afsluitdijk. 31km of near dead straight, flat road. The prevailing wind is SW, on a road that runs SW/NE. No shelter, no let up. With a tailwind, it's rather cool, get down on the aerobars, ramp up the gears, and zoooooom. Headwind, it's like climbing a mountain, just without the reward of a great view and coasting down the other side...
Other dead straight Dutch roads that are possibly longer: the road through Flevopolder, maybe.
To give you an idea about the flatness of some of these roads, this is a ride I did from Amsterdam, out, round Flevopolder, and back again. 205km, with just 84m of climbing. Most of it is going over the bridge between the main "land" and the polder... -
https://www.strava.com/activities/1690079118J