Whats the Ideal solution (apart from no lights at all)
It's a combination of things. Using no lights at all is appropriate in some cases - I can certainly think of places where having streetlights provides no benefit!
Where streetlights are needed then the type of light and control of the light needs to be looked at. As for the type, the ideal is the most efficient FCO. Some studies show that insets are drawn to blue light, and melatonin production in humans is stopped more by blue light. However its easier for humans to distinguish things using colour. So the high pressure sodium that is what is currently being installed is a good option - they provide just enough blue to give colour perception.
The problem of skyglow is not so much which light emits which spectra, its almost all down to the angle that the light leave the luminarire at. Dr. Chris Baddley (CfDS) recently presented a paper on this to the ILE in Dublin showing that due to scattering the luminaires that contribute most to skyglow are the ones that emit light within a few degrees of the horizon.
The next aspect to consider is the timing and strength of streetlights. On a winters evening it may be desirable to have all our streetlights on as people come home from work and to help them find their way home after a pint or two. If there is no nightclub in the town, the place will have nobody walking around after midnight or so. So do we need the streetlights on? Can they be turned down or off completely until people are begin to go to work? I don't think this technology is being used in Ireland yet.
Basically you can't go too far wrong advocating that whatever lights are used they should be FCO and should only be used when they are needed.
  