They're actually very illegal. And really stupid; they emit less light than
non-blue bulbs.

As will high powered filament lights of a similar output. HIDs are far
more common than aftermarket filament bulbs of the same light output -
most light fittings and wiring can't handle 150 watt bulbs.

That is not necessarily the case. Remember how much you liked your
yellow headlamps in bad weather, before you took them out of the car?
Well, it doesn't matter where such filtration is applied (at the bulb,
at the lens, at the windshield, at the driver's eyes), the effect is
the same.

For any given intensity, the higher the blue content of the SPD, the
greater the discomfort glare -- *WITHOUT* any significant corresponding
increase in seeing performance. In the case of automotive headlamps,
HID vs. halogen, the discomfort glare ratio with intensity held fixed
has been found to approximate 1.46 (i.e., for any given intensity
level, the HID headlamp produces 46% more discomfort glare than the
halogen headlamp). There has been some discussion of using this effect
in inverse fashion to optimize the effectiveness of emergent LED
headlamps (i.e., tweak their SPD to contain less blue, permitting
higher actual intensities without increased discomfort glare), but it
has been drowned out by marketeers who demand that each new generation
of headlamps be bluer and bluer and ever bluer because that's what
they're geared up to sell.

Just as the population at large is split roughly down the middle into
glare-sensitive and glare-nonsensitive individuals, so too is it split
roughly down the middle into blue-sensitive and blue-nonsensitive
individuals (not the same 50/50 split; i.e., there are individuals who
are glare-sensitive, blue-sensitive, both and neither). The obvious
upshot is that any given HID headlamp will strike some people as more
glaring (and some people as MUCH more glaring) than it will strike
others.

Me, I happen to be somewhat blue-sensitive, so cutting the blue out of
the light reaching my eyes at night is beneficial. Most commercial
glasses sold for the purpose are grossly improper for the task; they
have amber or orange or honey-brown lenses that block much too much
useful light. You want to cut ONLY the blue. There are all kinds of
fancy ads and competing claims for Blu-Blockers, Serengettis, etc. Most
of it is BS. Your eye does not know whose name is on your lenses. Your
eye also does not know how any given spectral gamut is achieved. The
chief trait of an effective set of night-driving glasses is that they
strongly attenuate the blue wavelengths so they don't reach your eye.
When you look at a white light through this type of lens, the light
will appear yellow (not orange, not amber, not brown).  Interestingly,
the human visual system -- with all its foibles -- does an excellent
job of color correction; even when wearing glasses such as these it is
not difficult to discern yellow from white road lines, for instance.

While cutting the blue and violet out of the spectrum has been shown to
give some contrast-enhancement and glare-reduction effects during
nighttime tasks under mesopic conditions (e.g., driving at night), it's
very easy to overdo it, at which point the safety benefits of the
reduced
glare and enhanced contrast are overbalanced by the safety hazard
caused by the absolute reduction in light reaching your eyes. IOW, it
doesn't matter if a pedestrian's red shirt looks redder if you don't
see him in the first place!

To help avoid an overly large absolute reduction in light, the lenses
should be more towards selective yellow, and should NOT appear notably
brown, orange and/or amber (which would indicate excessive attenuation
of greens and yellows, which are of prime importance for human vision
while
driving at night). If you notice me repeating myself on this point,
it's because it's so important.

Me, I had my night-driving spectacles made to my own specifications. An
outfit known as "Calichrome" makes the correct-hue yellow dye. I picked
a large, sturdy, inexpensive plastic frameset by Rodenstock(!), so as
to handle as much of my peripheral vision as possible. Since I'm
nearsighted, and nearsightedness tends to increase with fatigue *and*
with dark (both of which conditions tend to exist at night), I had the
lenses ground 1/4 diopter stronger than my normal glasses and my
sunglasses. The lenses were then put through the Calichrome dye process
to
the correct-depth selective-yellow tint. I love them; they're perfect.

Your franchised "one hour glasses" type place probably won't know what
the hell you're talking about and won't grind lenses to any but the
prescription you hand them (those glasses-in-a-hurry places can't be
relied on to adhere strictly to a prescription anyway...) so you'll
have to patronize a good, competent independent optician. I had my
current set made in Michigan when I lived there.

An excellent guide for the hue and tint is a "K2" type camera lens
filter, available at any photo supply store.

DS

PS, remember too, the HID light source produces roughly 3x the light
flux of the filament light source. This can be used by engineers for
good (keep the optics the same size, increase total beam flux) or by
stylists for evil (reduce the optics' size, keep beam flux "legally
high"). When optical size is reduced, surface luminance increases,
which creates a more glaring visual impression ("signal image"). And
that's assuming quality optics; poor optics are another whole source of
glare.

I'm not sure what this has to do with "you turn the switch on and the
road it lit up".

Uh yeah, a little.

Who'd have ever thought you need more than 640K ?

K is a measure of light color, not the amount of luminous flux; it's a
qualitative, not quantitice measurement. HIDs do draw less power, significantly
so. They do not put out more light than thr Cibie CSR's, which are halogen,
in my 633.

The color is the worst problem. But as long as I can get more light for 1/4 the
cost I see no point in HID lamps to say nothing of the fact it's easy to install
or adapt good halogens, but retroftting HIDs to any car is simply a fantasy.

It's a great technology to be sure, but its young and has a way to go.

Foreseeable future != forever.

DS

Both. New cars will migrate away but it is also true that
filament lamps will always be around for older cars.