[feint]

I'm guessing floor tiles/fake stone benchtops. Huge market that would require a lot of glitter and it's not something you initially think would contain glitter. The expensive fake stone benchtops would feel "cheaper" if you knew that it was just glitter making them look good.

[salgernon]

I was thinking asphalt and concrete - it seems that New York uses a specific percentage of mica flakes in their sidewalks, but I wouldn’t be surprised to find that roadways would use something to aid reflectivity as well.

[anon_cow1111]

I looked road markings up earlier because it was also my first guess, but everything I've seen indicates they just use plain glass microspheres in reflective paint and such.

For the roads themselves, more reflective might be a bad thing anywhere that sees heavy snowfall. Less sunlight absorbed=colder roads=more snow buildup and many other related problems.

None of these uses would need to be kept secret though.

[lotyrin]

Yeah, the microspheres have the property of being very retroreflective (shine headlamp light directly back toward a driver efficiently) and so get used in lots of automotive visibility scenarios.

[titanomachy]

Why would a local government be secretive about what they put in roads?