LEDs might help more than hurt because nearly 100% of the light can be directed exactly where you want it. HPS/LPS lights direct a fair percentage of their output upwards. When it snows or is foggy, I noticed the sky has a hazy orange glow. Also, due to the higher blue content, the apparent brightness of LEDs at any given lux level is higher. Therefore, you can reduce lighting levels without sacrificing perceived output. That should also help. The point here is we really don't know how LED will affect the light pollution situation until a large city has mostly LED streetlights.
There is nothing inherent in LPS/HPS that the light has to go upwards. Many progressive areas have been installing full cutoff HPS fixtures for some time and I am finding more and more cities, towns, etc. are insisting on at least semi-cutoff if not full cutoff fixtures for all installations no matter the technology. I have not seen movement to the more modern BUG standard yet, but it should come.
Apparent brightness does not impact used lighting levels at all though. Apparent brightness and real world impact do not always coincide.
For roadway lighting due to the speed of movement, the central cone of vision is the most important, hence photopic or at a bare minimum mesopic levels are used. For slow speed residential roads you may be able to reduce lighting levels 25%(more?), with the advantage diminishing as the speed goes down.
Where you could get the most benefit is parking lots, but I find that more newer parking lots are being lit with blue rich metal halide already and would see now advantage (spectrally) going with LEDs. What they could do though is not make them so bloody bright!
Completely agree with placement accuracy of LEDs having the ability to reduce light pollution. It is not unusual to be able to hit the required lighting levels on a road with 30-50% less lumens than an HPS (measured at end of life in both cases). LED lighting is also far more amenable to electronics controls. Turning down streetlights at night does not impact safety as the traffic levels are less and hence not as much light is needed.
As LED efficiency improves, I think you will see a move towards 4000k(ish) as the standard for outdoor LED lighting. 5000K+ is not really justifiable. It does not enhance vision appreciably but contributes to added glare.
Semiman