The new world atlas of artificial night sky brightness

"Artificial lights raise night sky luminance, creating the most visible effect of light pollution—artificial skyglow. Despite the increasing interest among scientists in fields such as ecology, astronomy, health care, and land-use planning, light pollution lacks a current quantification of its magnitude on a global scale. To overcome this, we present the world atlas of artificial sky luminance, computed with our light pollution propagation software using new high-resolution satellite data and new precision sky brightness measurements. This atlas shows that more than 80% of the world and more than 99% of the U.S. and European populations live under light-polluted skies. The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans. Moreover, 23% of the world’s land surfaces between 75°N and 60°S, 88% of Europe, and almost half of the United States experience light-polluted nights."

Link here.

"The dark gray level (1 to 2%) sets the point where attention should be given to protect a site from a future increase in light pollution. Blue (8 to 16%) indicates the approximate level where the sky can be considered polluted on an astronomical point of view, as indicated by recommendation 1 of IAU Commission 50 (9). The winter Milky Way (fainter than its summer counterpart) cannot be observed from sites coded in yellow, whereas the orange level sets the point of artificial brightness that masks the summer Milky Way as well. This level corresponds to an approximate total sky brightness of between 20.6 and 20.0 mag/arcsec2 (0.6 to 1.1 mcd/m2). With this sky brightness, the summer Milky Way in Cygnus may be only faintly detectable as a small increase in the sky background luminosity. The Sagittarius Star Cloud is the only section of the Milky Way that is still visible at this level of pollution when it is overhead, as observed from southern latitudes. Red indicates the approximate threshold where Commission Internationale de l’Eclairage (10) puts the transition between scotopic vision and mesopic vision (1 mcd/m2). Also inside the range of the red level, the sky has the same luminosity as a pristine uncontaminated sky at the end of nautical twilight (1.4 mcd/m2) (11). This means that, in places with this level of pollution, people never experience conditions resembling a true night because it is masked by an artificial twilight."

Here's a picture of US light pollution.