Traffic lights: double red I have seen several times bike traffic lights like these:



This one is from Vienna, Austria. One can also find them in Bordeaux, France and in Paris if I remember well.

What purpose do these serve ? There are a few possiblities, as discussed in this blog post for pedestrian lights, but I don't find any of them very convincing.


Redundancy, in case one fails. But people tend not to consider an 'off' traffic light as green, and it's not used for car traffic lights, for which it would make much more sense. Also, that's +50% power consumption.
Colorblindness. This makes it possible to distinguish between red and green light other than by color alone. As a partially colorbind person, I have doubts. I can normally distinguish between red and green, but in some situations, no (think bright sunlight hitting the light), then I can not even tell if one of the two lights is on, so a double red wouldn't help me. For truely colorblind people, that might be different.
Be more assertive, i.e. discourage people from crossing at a red light, which works according to a Danish study, according to the post. I cannot find it through &lt;search engine&gt;, but that may be the best explanation. Note that in Vienna, these double red lights are far from the rule, and I haven't seen them for pedestrians.


Edit: here is a poll, so you can signal if such lights are used in your area.

I wrote to authorities in Austria that might be able to shed some light on this question, in terms similar to those above. Here are the answers.

TL;DR : Redundancy in case of failure*.

Anwser #1 (Austrian ministry of transportation)

In short, they don't know, citing Section 38.8 of the Traffic Code, which states that special lights can be used for some specific groups of users, like pedestrians, taxis, cyclists, etc. without any specifics. They also tell me to ask the local (Viennese) authorities, which I did, see answer #2.

Original text, for the brave:


  Nach Rücksprache mit der entsprechenden Fachabteilung übermitteln wir
  Ihnen gerne die folgende Auskunft:
  
  Gemäß § 38 Abs. 8 der Straßenverkehrsordnung 1960 (StVO) „Bedeutung
  der Lichtzeichen“ gilt, dass zur gesonderten Regelung des Verkehrs auf
  einzelnen Fahrstreifen oder für bestimmte Gruppen von
  StraßenbenützerInnen, wie etwa FußgängerInnen, RadfahrerInnen oder
  Fahrzeuge des Kraftfahrlinienverkehrs sowie Taxifahrzeuge,
  unterschiedliche leicht erkennbare Lichtzeichen verwendet werden
  dürfen.
  
  Das bmvit kann Ihnen jedoch keine Auskunft darüber geben, wieso an der
  von Ihnen angesprochenen Kreuzung eine solche Ampel zur Regelung des
  Verkehrs herangezogen wurde. Wir ersuchen Sie, sich diesbezüglich an
  das zuständige Magistrat der Stadt Wien zu wenden, dessen Kontaktdaten
  wir Ihnen gerne übermitteln: Verkehrsorganisation und technische Verkehrsangelegenheiten (MA 46) [...]


Anwser #2 (Wien MA 33, Öffentliche Beleuchtung und Verkehrslichtsignale)

The first paragraph emphasizes the fact that for this specific location (photo), the bike lane goes against traffic in a one way street.

The second paragraph explains that this additional red light is meant to avoid switching to the blinking yellow light in case of failure.

Original text:


  Bei der Verkehrslichtsignalanlage (VLSA / Ampel) Burggasse #
  Schottenfeldgasse wurde für den Radverkehr in der Schottenfeldgasse
  ein separates Vierkammerradsignal montiert, weil die RadfahrerInnen
  gegen die Einbahnrichtung der Schottenfeldgasse unterwegs sind.
  
  Da für die Überwachung des Rotlichts zwei Signale pro Fahrtrichtung
  erforderlich sind, damit die Anlage bei Ausfall eines Rotlichts nicht
  sofort in den Gelbblinkbetrieb übergeht, wurden zwei Rotlichtsignale
  übereinander montiert.


*My german deciphering skills are pretty limited so please correct me if I'm wrong.
After reading that blog post and some forum threads about this issue, the cause seems pretty obvious to me.

At the time, the local traffic light crew had 4-light groups on hand, and didn't have 3-light groups.

This 4-light group was probably originally designed for car traffic, with one of the lights being a turn light.

If having an extra red was an improvement of any kind, it would be much more common (and likely mandatory).