The problem is no longer light going up from the fixtures, but light so bright it reflects off the ground, lighting up the sky. LED fixtures are mostly pretty good at directing their light downward, and this has been one of the selling points for these lights, but for some reason, they always use much much brighter LEDs than the bulbs they replace, promising the stargazers that they are better for light pollution because they direct their light down. I think even the sodium vapor fixtures were pretty good about not shining directly up. The ones that shine up are the "historical" designs, made to look like Victorian era gas lamps, but these aren't widely used. I would recommend a poster suggesting no fixtures of more than 8 watts, and color temperature no more than 2700K.
The good news is that light reflecting up from the ground - all else being equal - contributes significantly less to sky glow than light emitted directly from the fixture.
This is a graph of the average emission intensity verses angle for a typical city. This model is used extensively in predicting sky-glow.
From Garstang, R.H. 1986, Publ. Astron. Soc. Pacic, 98, 364-375.
If you look closely at the graph, you'll see that the overall emissions are a composite of ground reflections (circular emission curve sitting on the orgin) and direct low-angle emissions (the airfoil shaped curves on either side of the orgin near the ground).
Now, the bulk of the ground emissions are directed at the zenith, which means two things: (1) a decent percentage of these emissions will pass harmlessly through the lower atmosphere into outer space, and (2) the sky-glow near the zenith caused by scattering of this light, which typically occurs in the lower 8 km of the atmosphere, will be largely confined to areas in and close to the city.
Turning to the direct emissions, not only are these significantly more intense, but they must pass through a large cross-section of the lower atmosphere, which means a large portion (50% or more) of this light ends up being scattered downward and contributing to sky-glow. Not only this, but they will be traveling through and being scattered by the bottom 8 km of the atmosphere for many miles beyond the city limits.
Thus, it really is worth it to try and have lighting regulations that require full-cutoff lighting. It makes a difference, especially in suburban, exurban, and rural areas.
Edited by earlyriser, 23 April 2018 - 04:44 PM.