# Speed and I are related, although distantly

I am one, my twin is another
Moving along, you might see either of us
Although which one you see, depends on where you look
Only at night are we truly very vivid

Speed and I are related, although distantly
But not as distant as you think, as you see me often enough
My twin can show you the way, but I certainly cannot.
For every which way you look, I face the other way

I am a something you must name specifically,
And my twin is another you must name too

What am I? What about my twin?

I'm guessing, that you are:

I am one, my twin is another
Moving along, you might see either of us
Although which one you see, depends on where you look

Depending on whether you look at the cars ahead of you, or behind you, or in the opposite lane, you'll see either headlights or taillights

Only at night are we truly very vivid

Although they can be seen in daytime as well, they truly 'light up' at night

Speed and I are related, although distantly
But not as distant as you think, as you see me often enough

You can gauge the speed of a vehicle at night based on its lights
The vehicle's speed is also determined at night by the throw of its headlights (i.e. don't drive faster than you can see)
On bicycles with dynamo-driven headlights, the brightness of the headlight is proportionate to the speed of the bicycle
Further, as pointed out by NL628 via a comment (thanks NL628!), if the car is braking, the taillight would be red, so if you brake, the red color appears.

My twin can show you the way, but I certainly cannot.
For every which way you look, I face the other way

Headlights are used to illuminate the path ahead, i.e. the path of travel
Taillights illuminate you for the cars behind (i.e. the other way)

I am a something you must name specifically,
And my twin is another you must name too

Hence my answer that you're the headlights and taillights on a vehicle.

• Wow nice I was thinking red and yellow b/c of what you said, but this is basically the same! May 17, 2018 at 2:49
• For the speed and I are related part I was thinking gung vs gur pne vf oenxvat, gur gnvyyvtug jbhyq or erq, fb vs lbh oenxr, gur erq pbybe nccrnef. May 17, 2018 at 2:51
• Yes @Nl628 that is closer to my original thinking, but Phylyp is essentially correct. May 17, 2018 at 2:52
• Nice one @NL628 , let me add that in. Thank you, Lifesaving Linen! May 17, 2018 at 3:00