Exactly, the specification are quite tight, therefore imposing even further restriction mean quite severe difficulties, usually yielding faster degradation, so at the end shorter service life.
The robustness against "ghost lights" from the sun reflections is required the same, regardless of the technology used, I'm not aware about any exewmption...
The recent development allow the brightness to be adaptable to the surrounding illumination, but the link (output lumens vs light level) to the surrounding light level is very tight as well. This then serve quite well as the prevention from phantom lights: With the direct sun the lamps are so intense, they are clearly visible. But that mean during e.g. night the lamps are dimmed so, they operate at really low duty ratio, causing the stroboscopic effect to be visible when someone look for it...

And by the way, very similar requirements are in place for on-vehicle lights (exact color coordinates, lumen output, ghost lights,...)...

The LED's are usually configured in series strings, each lamp consisting of multiple (3..6) such strings.
Each string usually has it's own current limiter, so if one of them get interrupted, the main controller does notice the lower current.
That work for the sudden electrical failures, but not for the gradual output degradation. But as the gradual degradation is rather slow process, a scheduled inspection, when the lights are optically measured (and their brightness recalibrated) is sufficient to keep the performance till next service period.

But the snow cover buildup is something, what simply can not be addressed in time by neither controller diagnostic, nor regular inspections. Only the link to a camera may allow the event to be spotted in time, but that make the system very complex and unreliable (once the camera fail during the winter, you have to shut down the signal and quickly get it fixed), so not any big win.

So with all these problems, the flicker is really no issue at all...

Although I find that led flickering is very annoying, there are some cases in which I can deal with it:

- when there is no room in the socket for a decent driver, e.g. in GU10 led bulbs (that replaces halogen reflector bulbs) or in my A-shape Archipelago bulb (with the same clear bulb as an incancescent)

- when the purpose is not general lighting, e.g. car tail lamps or Christmas cheapo lights

- on vintage fluorescent fixtures with their original magnetic ballast (I also try to use tubes that don't flicker as much as vintage deluxe halophosphate tubes or modern "full spectrum" tubes)

So I hate headlamps or flashlights that flicker with PWM (there are several great drivers that allow real firefly modes without any noticeable flickering: HDS, Zebralight, Armytek,...), as well as PWM backlights in cars.

It would be nice to have all lights 100% flicker-free, but except for some really special cases it is not real necessity. So compromising other parameters by just having the light 100% flicker free without real need is a nonsense. And the cost is one of those parameters: With given budget per lamp, I would prefer the money to be spent on a more robust and longer lasting design than on reducing the flicker. Even with LED's, there is no free lunch...

It is true, sometimes the PWM frequency is too low without any real reason, e.g. ~100Hz even with quite a large duty ratio (with ratios down to 0.1% I may understand the frequency going such low, but not anymore at 30% levels).
But when the frequency is sufficient, the flicker could be visible really only with fast movements as a slight stroboscopic effect (and only when you focus on that). It is the way, how most "flicker-free" drivers do the dimming: Just use high enough frequency.

With compact lamps (so the driver is integrated into a small assembly) the problem is the need for an electrolytic capacitor to filter the 100/120Hz. Such capacitors will limit the life quite a lot (the best capacitors I've seen are rated for 10k hours at 105degC, while the desired LED lamp life is 25k+ hours). So the drivers for such lamps are made without any such capacitor. That mean there is no such life limiting component, but the consequence, there is nothing to filter the 100/120Hz.

Other aspect is the requirement for phase cut controlled dimming: Phase cut dimmers require high power factor load, so the simple diode bridge + filter won't work there. So there is necessary a PFC stage at the input. So either the driver would have to be of two stage design (PFC and then LED supply converter), so will be large, complex and quite lossy, or the PFC functionality could be integrated into the main converter. But as this have to follow the mains voltage, there is no 100/120Hz filter in front of the LED's anymore, so the LED's get a 100/120Hz "1+sin" current.
And as above ~15W the power factor control is mandatory by regulation, while the lanterns are required to be compact, this "elyt-free" concept is frequently used even for the higher power levels as well.

The problem with dimming by just DC current (so really without any flicker) is the color shift and mainly the high leakage of LED's when heated up (it takes current, but make no light at all) and mainly as the LED's age. The PWM makes sure, the condition on the LED is constant when lit, so the color and brightness properties are constant as well.
So the dimming range would be limited to minimum ~10..20% (and even that will reduce the useful LED life - quite a problem of many drivers with poor current regulation, when they are mostly operated at full power, but the dimming is sometimes needed). For deeper dimming there is no other method than the PWM.

I haven't seen cheep prices on LED bulbs. But I like LED bulbs better on the Christmas tree better than the incandescent bulbs on the Christmas tree. The LED bulbs can last much longer and I don't have to worry about them getting hot on the tree branches. As for the LEDs on the traffic signals, they are more energy efficient than the incandescent traffic signals. I'm a person who prefers to try to save energy and energy costs. I currently use CFL 6500K bulbs in my regular light fixtures and I use the LEDs on the chandalier fixtures. I hope to look into 7000K LED bulbs. I will see if they would be a menace or not. I personally think my electric bill will go lower when I have all of my light fixtures using LEDs. I understand that many of you prefers incandescent and so did I. I do love LEDs as they have nice lighting and I love saving energy costs. If the traffic signals do get covered with snow, I am sure firetrucks with ladders can clean the snow off of signals. It never hurts to try new technology. I never experienced any flickering LED in my area. I believe my Electric Company (First Energy: Ohio Edison) uses the right tools supply power that feeds the products around here on days weather doesn't damage power lines.