In a US trial, local drivers and pedestrians perceived better visibility, safety, security, brightness and colour-rendering when conventional 100W high-pressure sodium streetlights were replaced with 55W induction lamps.
The reason, said trial organiser the Rensselaer Polytechnic Institute, was that the new lamps better matched the spectral response of the eye in dim conditions.
"By replacing traditional, yellowish high-pressure 
sodium lights with cool white light sources, such as induction, fluorescent, ceramic metal halide, or LEDs, we can actually reduce the amount of electric power used for lighting while maintaining or even improving visibility in night time conditions," said Peter Morante, director of the Institute's Lighting Research Center (LRC).
According to the LRC, lighting levels in artificially lit 
streets tend to be in the mesopic region where both rod and cone-type cells in the eye operate.
Above this level is photopic vision, where only the relatively-insensitive colour-seeing cones operate.
Below this level is scotopic vision, where only the rods function - rods are highly sensitive, monochromatic, most sensitive to green light, and cannot see deep red at all.
The induction lamps in the trial were 6,500K colour temperature types from Philips, designed to appear white to cone receptors, and have plenty of energy around 500-550nm where rods are most sensitive.
They have a scotopic to photopic ratio of 2.88 compared with S/P=0.63 for high-pressure sodium (HPS) lamps.
Energy savings were sufficient to cover the cost of the retrofit in 14 years, or seven years for new installations.
A parallel trial with 70W ceramic metal halide lamps (S/P=1.6) also resulted in energy savings and good visibility. However, short bulb (20,000 hours compared with 60,000 for induction) meant reduced energy cost would never cover increased maintenance cost.
The trial report recommends HPS lamps be replaced with sources offing high S/P ratio, a correlated colour temperature around 6,500K, and 65 to 70 lm/W (photopic) efficacy.
It goes on to say: "White LEDs should be considered as replacements for HPS street lighting in about three years when their efficacy is higher and their cost has reached reasonable levels to be economically viable."