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Why do taillights usually use red glass?

And why are the indicator lights usually with yellow/orange glass?

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  • The use of red tail lights on horse carts was compulsory in London well before the invention of the internal combusion engine.
    – Chenmunka
    Nov 17 '15 at 11:36
  • @Chenmunka: Okay, I didn't know this. Though I was wondering if there is any technical reason behind it.
    – WedaPashi
    Nov 17 '15 at 11:37
  • 2
    Red is easy to see in inclement weather, like snow or rain.
    – vini_i
    Nov 17 '15 at 12:01
  • Because it's so anoying, I'll also point out that some other colors of lights are reserved and are illegal to have on a road vehicle. For instance, in the US, you cannot have a blue light anywhere on your vehicle, and you can't have red lights that face forward (reserved for police/emergency use).
    – JPhi1618
    Nov 17 '15 at 14:13
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Its a mixture is tradition and function.

Red has been used to signify "Stop" much longer and in many more applications than just motor vehicles. Stop and Do-Not-Enter signs are used everywhere. Possibly, although I can not confirm or cite a source, in history blood was used to mark doors and entryways that should not be entered and therefor the red color has always be associated with "stop". There are many other examples and demonstrations of red being used for indicate warning.

Functionally Red light has less effect on human low light vision than any other visible color. Red tail and brake lights are highly identifiable to the ROD cells in your eyes, while higher wavelength colors like green or blue are harder to distinguish.

All in All, many different factors may have been used to determine the standards used. Today the reason you have to have RED lights is due to the law. That's all the lawyers care about.

Red tail/brake lights serve as more than just indications of stopping. Tail Lights can inform you of another vehicle. By identifying Tail lights instead of headlights you can determine the vehicle is moving away from you. In the US there are limits to the width of separation of the two tail lights allowing you to be able to reliably determine distance. Other Features like clearance lights tell you how large a vehicle is. How long it is, How tall, etc.

US Clearance Light Requirements
US National Highway Transportation Safety Administration

As for the Yellow Turn Signals that is a European thing. Many US vehicles just flash the corresponding brake light to indicate a turn. The Yellow offers some differentiation to make it easier to identify a turning vehicle.

For some extra information on light based identification look into why boats and planes have red and green lights on the front. Runway landing lights have some interesting meaning and features as well.

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The Society of Automotive Engineers (SAE) does not define regulation, it reflects it.

In the United States the Code of Federal Regulations defines the rules for general safety in automobiles. In this case case, the DOT (Department of Transportation) has defined this framework and has published it in the CFR's.

Here is a link for the CFR (Code of Federal Regulations) as reflected in the FMCSA website

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  • CFR is "Code of Federal Regulations". I had to look it up...
    – JPhi1618
    Nov 17 '15 at 20:01
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In a nutshell, because regulations demand it.

For tail lamps:

  • ECE R7 (which covers all of Europe, Australia, New Zealand, Japan, South Africa) demands two red tail lamps.

  • Regulations defined in SAE J585e apply in the USA.

For turn signals:

  • ECE R6 demands two yellow lamps for front, rear and side indicators

  • SAE J588 permits red or yellow for the rear and side turn-signal lamps

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  • 1
    In addition, the lamps are often made from colored plastic rather than just having a colored bulb because there are also rules that specify that the vehicle must have red or amber reflectors in certain places.for visibility when parked. If the lights are already red plastic, making part of the plastic into a red reflector is that much easier.
    – JPhi1618
    Nov 17 '15 at 16:12

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