driving at night can be dangerous especially when the spectrum that lights the road is of improper spectrum,although mercury vapor has disadvantages,its advantages such as high contrast far surpass that of high pressure sodium,when i take family road trips that envolve leaving very early(4:00am),or very late(8:30pm)i always look at the streetlights to pass the time,i often notice that towns lit with 400 watt mercury vapor lamps at night seem brighter than towns lit with 200-250 watt high pressure,even though they both produce about 22,000 lumens,this is due to the fact that the human eye is more sensitive to green than yellow,meaning that it takes twice as many lumens from a high pressure sodium lamp to produce the same amount of visibility & contrast as a mercury vapor lamp,i observe this in my own back yard,while the backyard is adequetly lit with a 175 watt mercury vapor lamp,the same cannot be said for a 100 watt high pressure sodium lamp,it takes a 150 watt high pressure sodium lamp which is twice as bright(16,000 lumens)to produce the same visibility as a 175 watt mercury vapor lamp so all in all high pressure sodium lamps do offer energy savings but at the expense of night vision and contrast,when ceramic metal halide lamps come down in price,they might be a future option.

LED Traffic light is a fine example of that. The Red or Yellow ones are always dim while the green one is extremely bright even though the're the same wattage. (For those unfortunate to live in an all-HPS town.)

There are many arguments about using lights that is more sensitive to human eyes while others like Dark Sky says its better to use ones that humans are not sensitive to......so they make up that older people with weaker eyes are more dangerous to have whiter lights....but I call that a big excuse because there are other ways....and car headlights are worse......

Also Dark Sky is always missing the fact that high pressure sodium has red in it and has longest wavelegth so they travel farther....why do you think you more often see orange skies and not as much on white (except with stadium ones with 1000 watt MH or 1500 watt MHs....)

i heard mercury vapor lamps were also banned because of the hazzards of mercury...that's just a stupid reason to ban the lamp,in fact more mercury is produced by burning coal which has some mercury in it..HPS has mercury and so does MH,which can have more in it!

i heard mercury vapor lamps were also banned because of the hazards of mercury...that's just a stupid reason to ban the lamp,in fact more mercury is produced by burning coal which has some mercury in it..HPS has mercury and so does MH,which can have more in it!

Really not. It's, because at first, they are coated, so yielding poor optical control, at second people didn't learned, then it is not good idea to keep an old lamp burning past it's lifetime, so the efficacy of average-in-service bulb is way too poor.
And unlike to Europe, electricity in US is cheap, so there is no economic pressure to manage the lighting towards efficiency, so not to waste power and emit mercury from power plants...

And back to topic:
At first: The eye is more sensitive to longer wavelengths at high light levels and more to shorter at low level.
At second: For visibility is not as important the overall light level, as contrast on objects to be viewed.
At third, the view should not be disturbed by some intense bright object (as the streetlight itself)
The light source, as it's the source, is a high brightness object
The illuminated scene is lower brightness "object"
The sodium light lies mostly in wavelengths longer then eye peak sensitivity.
Mercury light lies mostly in wavelengths shorter then eye peak sensitivity.

So even if the sodium-illuminated scene is brighter, the higher sensitivity to bright light source makes (any) sodium light rather a glare-bomb then effective device for nighttime visibility. So very, very good optical control (or more often much larger overall illumination level) is necessary to supress this effect.
In contrast MV's (or other cool-white sources) cause the eye's sensitivity increase for the darker scene and decrease for the glary light source, so even with lower lighting level, the visibility is still good.

I have noticed the same thing about color and glare.  At night, large arrays of bright green traffic lights make seeing thru the intersection difficult because of the glare.
My old car stereo had a color-changeable back light for the display.  Because it's mounted high in the Cherokee, I always had to keep it set to Red because that color produced the least glare.

about bad relamping services in USA.....its not really "specifically" USA...its also depending on electric company...ive noticed it varies on electric companies....for example in MD, you'll see a lot more dimmed out mercs never relamped until completely burnt out and lots of cycling HPS in PEPCO.....

In BGE, mercs are relamped regularly...and is very well bright.....the areas thats more often not regularly serviced is BGE owned MV lighting private areas and alleys but still, BGE has seemed to have a lot better lighting servicing than PEPCO....

If you google "PEPCO Streetlights" I had a little hint reading that linemen there "thinking of ways" is "if it still lights, don't relamp it"