The LEDs on the I-35W Mississippi River bridge were before the cheap junk era and they've been going since 2008 without noticeably dimming out or any failures.

One thing is that MnDOT is learning that LEDs are vulnerable to surges, or over-voltage conditions if a drunk driver takes out a neutral. New installations have better fasteners on the neutral wires and surge protectors in the cabinets.

I'm pretty impressed with how long the I-35 lights have lasted. They aren't very bright  and have kind of an icky color but they are going on 12(?) years old this year and there hasn't been any big issues with them that I know of.

It is not that easy. The isolation transformers just provide circuitisolation and maybe current conversion, but still their output inherites the constant current nature of the main series system. And that does not work with the standard LED ballasts, you would need ones specifically desivned for that operation.
On the other hand it could be as simple as just a bridge rectifier and that isolation transformerbeing designed for the secondary current matching the LEDs, but it is definitely not a standard mains ballast plus a transformer.

In the site I linked you can see the specific isolation transformer is designed for exactly the purpose of allowing LED luminaires to operate on series systems. Of course any old isolation transformer won't work. Anyway I was more bringing up that there exists an off the shelf solution to run LED luminaires on a series system which Lumex120 was wondering about.

Then blame the municipalities for buying the cheapest products they can, and/or the state of your electrical network, and/or how the systems are used (e.g usage outside specified tolerances).

Yeah, the LEDs here that were installed in 2013-2015 still seem to be going strong aside from one or two early failures that got spot replaced. Same thing about the ones in another municipality that installed their LEDs in 2010. Of course they used mostly Philips luminaires which is one of the more decently made LED fixtures.

People in the traffic signal business are not impressed with Leotek LED modules either, preferring GE or Dialights.

Yeah, I believe GE and Dialight have much more of the market share of LED signal inserts too. I guess Leotek is more of a budget option just like their streetlights.

Caltrans had a big problem with the same Leotek fixtures yellowing and browning out. As a matter of fact they dumped Leotek. They now use a manufacturer called SolarMax, a local California manufacturer. They are yellowing somewhat after about two years but none of the problems the Leoteks have.

I've read, that not the LEDs and drivers are failing but the lenses are burnt reducing the light output extremely. Crazy  

Here it is:
https://www.gannett-cdn.com/presto/2019/05/06/PDTN/570da66a-bca2-4926-8ff3-893258dd3323-Burned-out-Lights-13.JPG?width=1080&quality=50

The lens burned out because of the huge amount of blue light emitted by the blue emitters of the LEDs. And LEDs are considered "No UV"...

The lens burned out because of the huge amount of blue light emitted by the blue emitters of the LEDs. And LEDs are considered "No UV"...

I doubt the blue photon has enough energy to cause the problem. I would rather guess the cause is a bit different:
The LEDs do not emit any UV at all.
So someone decided the bowl does not need any UV hardening anymore, so cut costs on UV hardening of the used materials (using cheaper material, without UV resistance).
They maybe backed that decision up by "lab tests" with just the lanterns burning, but just inhouse.
But the problem is, when outdoors, the sun delivers A LOT of natural UV with the daylight (it mostly gets diffused by the atmosphere, but the overall intensity is high).
When the lens were designed with the lamp UV in mind so using UV resistant material, the same UV hardening had protected it from the daylight as well.
When the UV hardening was not used "because LEDs do not emit UV", it wasn't then protected against the natural daylight UV anymore and so had yellowed over time.

About 10 years ago, Leotek came sniffing around our traffic signal shop trying to sell us on LED signals. We were already happy with GE and Dialight.
I had noticed, in my road trips, that certain failing signals in California and Arizona looked like Leotek, so I asked the rep if they sold to those areas, they proudly said they did then I let them know that it was obvious, as a visitor to those areas, that their product is cheap garbage.
Even though I didn't have the "authority" to make a decision on behalf of the city, I told them to basically go pound sand, and I lobbied our management to blacklist Leotek. Sometimes, a contractor will install Leoteks, and our inspectors will have them remove the Leoteks and replace with Dialight or GE.

I had no say what City Light does, and City Light did end up buying Leotek street lights, but that's a whole other can of worms

I would guess the main problem with Leotek would be their inexperience with any street lights (not much related to only LED).
They simply just not have the "grab on" how the streetlights are really used and what operating conditions to count on (voltage fluctuations, temperature profiles, typical installation defects and their effects,...), so have not much base to form a proper requirement specification for their engineers.

Then blame the municipalities for buying the cheapest products they can, and/or the state of your electrical network, and/or how the systems are used (e.g usage outside specified tolerances). The fact that the "Schreder Teceo 2 at HaDeshanim Road/Kiryat Ata Junction" (just one specific case, seriously?) fail so early in life (just one month after installation) is a clear indication of improper use of the equipment (voltage outside tolerances, faulty neutral or ground, spurious voltage spike in the network causing driver failure, etc...). In the very unlikely case of early failure due to improper product design (this must be a really bad design for failure to occur naturally just one month in), then if the luminaires are from serious and reputable companies, municipalities will get replacements as per the warranty, so no harm done.

It's not because the situation with LEDs is crappy in your country that it is the same everywhere, so please don't generalize like you always do, that only reveals your ignorance in this matter. Here is Northern Europe (except UK, apparently) the situation is far better with LEDs because we have the sense of installing proper quality luminaires (well, in most cases - there still are the occasional poorly-specified installation here and there; that was true back then, it is still true now). We know that it pays in the long term... going for the cheapest stuff always ends up costing you more in the long run, and Detroit is proof of that.

Spot on here Max, first of all, the Detroit situation, blame is with the Detroit officials, but more so with these idiots from Leotek, all they have done here was send their people shopping in China, to buy the cheapest crappiest garbage they could find, whilst keeping what was left over from Detroit’s money for themselves, then coming back and passing it off as their own product!, little wonder their now ignoring the after sales claims!

As for the UK, (not sure what you have heard here)? , but the only problem we have had in my town with LED lanterns was with Urbis’s Ampera, (filling with water, and diode boards going out within weeks), thankfully our street lighting guys have stopped using the Ampera now, (I’m not a fan of Urbis products anyway), and with DW Windsor’s Kirium, also not used anymore!

One of the first LED street lighting installations here was using Holophane’s Factor, (Holophane being a reputable manufacturer like you say), this installation is well over 6 years old now, and there hasn’t been ONE failure out of 50 odd lanterns, I know this as their on the road I drive to work on!

Our guys use three main types now, Philips’s UniStreet, Holophane’s V-Max and Lena Lighting’s Rubicon, all of which has proved very reliable in service  
It makes me laugh now how people still use that ‘LEDisease’ line on all lighting forums, it was funny at first, but it’s getting very old and boring now, HID street lighting has had its run, and its not coming back, no matter how much we miss it!

Why do people have to be so dumb

The Zilwaukee Bridge in Saginaw, Michigan went from HPS to LED also. The LEDs are not near as bright as HPS. I wished they would bring the HPS back because of its bright warm color. They are much easier on the eyes. 

They just need to use 3000k LEDs, no need to bring HPS back.  3000k will actually look much better and you'll like it vs the blah from HPS. 

I've seen 2700K LEDs with crap light quality and blue light glaring effect.

https://www.crainsdetroit.com/energy/detroit-lighting-authority-finishes-replacing-20000-faulty-streetlights

So now they have replaced 20,000 defective LED streetlights and replaced them with fixtures from a company called 'America's Green Line'.