# Where in the world is the longest section of flat road

I want to find the longest section of laser-level flat road. It need not be straight, but allowing for the curvature of the Earth is NOT permitted. To avoid stray chips and allow for repairs, let’s say that the laser level is set at 2 cm above each end; if the road is not straight, assume mirrors at each curve.

Inspired by this thread, of course.

If you want it that level you’d go to the Space Shuttle landing strip in California. They did account for the curvature of the Earth, but I don’t remember if they followed the curvature or went with straight line level.

The Horizon Calculator tells us that for a viewer .02 meters above the earth’s surface the horizon is only 500 meters distant. Gives a whole new meaning to worm’s-eye view. Anyway, that means your laser levels couldn’t be more than 1000 meters apart on a theoretical Earth-sized sphere.

I would suggest that there are 2 likely places to look for this:

• where roads go across deserts, like the Sahara in N. Africa, the Arabian peninsula, northern Australia, or the Utah salt flats area of North America.
• the center of tectonic plates (not the edges), like the midwest (Nebraska, the Dakotas, Manitoba) of North America, or central Russia east of the Urals.

I have to confess I’m not entirely understanding the OP, but the arms of the Laser Interferometer Gravitational-Wave Observatory (LIGO), at 4 km in length, have to be among the longest, straightest, most precisely measured beam paths ever.

Would the track at CERN count, since we’re allowed to bend the path? The “road” that the protons and anti-protons circulate along is flat, isn’t it? Or do the magnets also have to give an upwards force to the proton streams?

That’s the point: on a totally flat road they can be farther apart; on a road that follows the curvature of the Earth, they can’t.

Trouble with the definition is that for roads, well, laser flat isn’t flat. An object will roll into the middle of the road’s length. You would only make a road this shape either by mistake, or because it had some other purpose. The earth is a sphere, and the the geometry is spherical. A Euclidean flat, isn’t flat in this geometry. Salt flats will follow the the Earth’s curvature.

Now I do know of laser flat areas of land. One is a radio antenna test area. About 350m across, and dead flat. No Earth curvature. Whether the service roads for LIGO, or in the tunnels for the LHC, and SLAC are dead flat would be interesting. They probably don’t need to make the roads all that carefully, so they are probably just lumpy.

I’m not sure how relevant this is to the OP, but in general,nothing outdoors is intentionally built perfectly flat.There’s always rain, and you have to design to avoid drainage problems.

Roads are usually builit with a “crown”–so that the center of the road is about 1% higher than the edges at the shoulder. Otherwise, you get puddles in the middle of the road. Curves in roads are banked, just like oval race tracks, but much more mild slopes–about 3-6% higher on the outside of the curve than on the inside.