For outdoor lighting the top issues are efficacy, safety and energy efficiency. At least two of these factors play into reduced light polution. It has been shown that using light with the greatest efficacy (and yes it is somewhat blue shifted) allows light levels to be reduced (by as much as 25% compared to HPS) while achieving the same functionality. More importantly, efficiency means putting light where needed and only where needed. The directionality that can be readily obtained with LEDs leads to an optimal use of light. The coverage of well designed LED luminaires is vastly superior in two respects - oblique radiation is much less than with typical'cobra' fixtures; - there is no pronounced downward hot-spot mimimizing the resulting upward reflection of light from the ground. Both of these properties mean that less light is ultimately directed skyward which is likely a good thing. As the data shows, HPS and mercury fixtures produce a very intense downward beam with substantial lateral fall-off with negligible 1/r^2 correction. As a consequence, in order to ensure minimum light levels at the mid-point between luminaires, the overall light level must be increased resulting in unnecessarily high light levels being used. Basically, LED fixtures have been shown to provide better visibility and less glare while also using less light.

When considering the dark sky issue, one should take into account various factors such as surface reflectance, atmospheric absorption and scattering. Clearly, shorter wavelength light experiences greater scattering which results in a more diffuse and lossy atmospheric reflection. Also, atmospheric absorption is slightly higher for shorter visble wavelenghts. Finally, ground reflectance is variable but higher at longer visible wavelengths for pavement, concrete, dirt, sand and even dry vegitation. As I live in the country, I get to experience this when navigating at night since the villages with HPS street lights are clearly identifiable relative to those with mercury based on how brightly they light up the sky.

I'm not sure how one would respond to the circadian rythem issue except that I would consider that anything that keeps you awake while driving at night is probably good for your health. There are also some studies that suggest that extra exposure to blue light during winter months has beneficial effects.

Whether street lighting is provided or not, the motorist carries his own lighting system.

We started outdoor lighting with short poles just taller than a human being and with 40w to 100w incandescent, moved over to 20w fluorescent tubes and later moved over to greater heights as we got sources of high power & light output in HPMV initially followed by HPSV and these were not available in low power such as 5 or 7 watts that would have been ideal to be fitted on short poles and light the pedestrian areas. With LEDs available satisfying most of these requirements we should revert back to short poles and light up the pedestrian paths instead of the road for the vehicles and satisfy the requirements of safety, energy economy, requirements of Dark Sky. With the typical pedestrian area of about less than a third of the road area, the energy requirement will come down to a quarter. From the security angle, lighting is required to remove dark spots. There are no dark spots on the road; but they are present in the shadows (of trees & other objects) cast from the street lighting system. With pedestrian lighting such dark spots which can harbor anti-social elements will be eliminated.

The advent of LEDs has open a window for a real reduction in the use of energy (the US contributes a great deal to the world problem by still burning coal in very inefficient power plants). The IDA lobby is not contributing to solve any of the problems we are facing. In fact their attitude is biased and anachronistic and making thins worst. It will be impossible to turn off lights, its a fact of life. What we need to focus on is instead to redesigning lighting systems in order to become more efficient and better used. The idea of illuminating streets as if they where football fields is a XX century craziness that needs to be reconsidered at length. Maybe the choice is not between artificial lights and stars but Motor Cars or Stars.