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I have seen several times bike traffic lights like these:

enter image description here

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 <search engine>, 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.

  • Did you check the drivers ed manual for the area? – Batman May 1 '16 at 13:21
  • I have a french driving license and haven't been taught anything about that (over 10 years ag). I will ask the local authorities. – G.J May 1 '16 at 13:36
  • 1
    There are also such traffic lights in Berlin. Beside the redundancy (which I find plausible) I always thought that the doubled red light improves the perception, especially with harsh sun light. Because most (all?) of them have a very small/short light screen which does not provide enough protection compared to the car's traffic light. (While car drivers look in a much wider angle onto the lights, pedestrians and cyclists look in a very narrow angle on it.) I'm not color blind, so I don't understand whether doubling really helps (but it does not really convice me too). – try-catch-finally May 1 '16 at 14:54
  • I have seen those mostly on tram/streetcar crossings. There, the meaning seems to be: "Yes it's a really bad idea to cross now! Believe me or you are likely to die!" – linac May 1 '16 at 14:59
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    In Germany they don't have a special meaning. I thought it's redundancy and perhaps a bit more emphasis when a road is really dangerous. Car traffic lights are bigger and have two or more bulbs behind the same glass, so they still work if a bulb blows, but the smaller cycle/ped lights can only contain one incandescent bulb without overheating. Now with LED this is no longer an issue. Are there any LED double red lights? – Stephan Matthiesen May 2 '16 at 16:17
3

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).

  • 1
    That would explain why a few would be sporting 4 lights, but in Vienna it seems that most (all?) of them are like that, though that needs checking. – G.J May 1 '16 at 15:59
  • It's also possible the brackets on the pole were already setup for a 4-light fixture and using a 3-light fixture would require drilling and tapping new holes, so they just went with the easier solution. – Carey Gregory May 1 '16 at 21:27

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