There is no especial night vision problem caused by amber rear
directional signals, per se. The glare issue with vehicle rear lights
is related to intensity and luminance, not colour. The US standards
allow (but do not require) both red and amber rear lights (brake and
signal) to produce light at levels that can cause significant glare.
On the other hand, the ECE standards allow (but do not require) both
red and amber rear lights to produce light at levels that can be
insufficient for good conspicuity under certain conditions, e.g. sun
shining directly on the lighting device. IOW, the US standard allows
rear lamps to be too bright, while the ECE standard allows them to be
too dim. Neither standard controls for relative intensities of
adjacent devices -- that is, it's legal to have a directional signal
near the low end of the intensity spec right next to a brake lamp near
the high end of the intensity spec -- which can cause masking effects.
ECE R48 at least avoids brake lamp masking by stipulating a minimum
100mm separation between adjacent lit edges of the brake and rear fog
lamp functions. Such a provision in US regulations, applied to
separate red brake and turn signals, would go quite a long way towards
alleviating one of the biggest issues with red rear turn signals.

The existing window of overlap in intensity range for the brake lamp
and rear directional signal functions is sufficiently large, but could
be shifted and tweaked a little for optimisation. If any of the
world's numerous expert regulators were interested seriously (and not
just nominally) in a worldwide lighting standard, it would take about
half an hour to come up with an intensity range - and perhaps some
specs for min and max luminance and relative intensities of adjacent
functions - that would eliminate the too-dim and too-bright issues and
control adjacent-device masking, too, without increasing automakers'
production costs. This may yet eventually happen; it's the direction
both regulations have been moving -- very slowly -- for years.

It is interesting to remember the Australians' clever solution in the
'50s through '80s to the conflicting demands of amber rear turn
signals, cost control, and compatibility with US-based vehicle designs
that had space for two colours, not three, in the rear lamp
assemblies. They stipulated red brake, red tail, amber rear
directional, and white *or amber* reversing lamps. Many vehicles had
red brake/tail and amber turn/reverse lamps. Signal for a turn, and
the applicable rear amber lamp blinked. Shift into reverse, and both
amber rear lights lit up steadily. Signal for a turn while in reverse
(as when parallel parking), and the applicable amber rear lamp blinked
while the other one remained steadily lit. In function and electrical
hookup, it was very similar to US combination brake/turn lamps, only
it was superior in that a full (left + right) brake indication was
always given, even when signalling for a turn. The amber colour worked
fine for the reverse lamp function. Eventually Australia adopted ECE
regulations and began requiring a white reversing lamp. The amber turn/
reverse setup  would work fine in North America, where we permit
steady-burning amber or white light to give the message "Here's the
front of the car" (parking lamps, DRLs). When you're reversing, the
rear of the car becomes its effective front, so there you are. Such a
setup would also probably discourage the trend towards reversing lamps
that don't serve to light your way rearward effectively.

DS (Yes, I'd rather have red brake, amber turn, and white reverse, but
I would gladly settle for red brake and amber turn/reverse as a close
second preference).