I'm valuable to many, especially to those who need me

Beware if you see me, great horrors may await.
Over where I run free, in me life runs straight.

Nick me open, a city's worth of traffic passes by.
Dressed in poems, especially when one is about to die.

So by me, men are made mortal.

What am I?

• "I'm valuable to many, especially to those who need me" so, every object in the universe. Next! – Quintec Sep 27 '18 at 1:48

Are you

Blood

Beware if you see me, great horrors may await.

if you see blood, in real life or in horror movies, horrors may await

Over where I run free, in me life runs straight.

Well, blood vessels are kinda straight. kinda.

Nick me open, a city's worth of traffic passes by.

Cut someone and a lot of blood cells will pass by (perhaps a city's worth)

Dressed in poems, especially when one is about to die.

many "dressed up" poetic references to death and blood. (thanks OP!)

So by me, men are made mortal.

indeed, without blood, death is there

Title

Blood is very valuable to those who need transfusions.

Acrostic

Bonds. Refers to the bond of family (blood) or a bond as strong (blood brothers)

• This is the answer I was looking for. As for the second to last line, rot13(Cbrzf qerff hc oybbq naq qrngu jvgu qrfpevcgvbaf.) – Joe-You-Know Sep 26 '18 at 16:10
• I did not see that coming lol! +1 – PerpetualJ Sep 26 '18 at 16:28

Are you a

Police

Beware if you see me, great horrors may await.

If they are there it means that there is trouble somewhere.

Over where I run free, in me life runs straight.

On the highway?

Nick me open, a city's worth of traffic passes by.

They can stop road traffic and then there will be a build up.

Dressed in poems, especially when one is about to die.

They have a law?

So by me, men are made mortal.

They fight against criminals.

Acrostic

Jail bonds?

• In reference to your poems portion, Miranda Rights fit well (but not the dying part, but this could be a metaphor). +1 Great answer! – PerpetualJ Sep 26 '18 at 15:50
• Wow, another really interesting take on this puzzle. +1 These are way different than what I was thinking of. – Joe-You-Know Sep 26 '18 at 15:51

Would you happen to be:

The hangman's noose?

Beware if you see me, great horrors may await.

People are terrified of the noose.

Over where I run free, in me life runs straight.

After flailing a bit, life leaves the body and it becomes limp and straight.

Nick me open, a city's worth of traffic passes by.

When the noose was traditionally brought out, city-wide gatherings would occur.

Dressed in poems, especially when one is about to die.

Most were asked if they had final words, this could be paying homage to that.

So by me, men are made mortal.

Men are definitely proven mortal by the noose.

Hidden Acrostic:

BONDS $$\leftarrow$$ My first thought was to bail bonds (due to the title), but it didn't fit. However, most victims of the noose are bound in chains; which in older times were referred to as bonds.

• I'm really curious to see your reasoning behind this answer, especially concerning the third line. – Joe-You-Know Sep 26 '18 at 15:45
• Well it is now available lol – PerpetualJ Sep 26 '18 at 15:48
• Nice answer! $(+1)$ :D – Feeds Sep 27 '18 at 7:54

I'm not quite sure but there are a few lines in particular that give me a guess.

Are you ......

A Bridge

Beware if you see me, great horrors may await.

Going over a bridge means you're passing over something dangerous (a river, gorge, or maybe even a troll), or may be entering an unfamiliar place where you should be cautious.

Over where I run free, in me life runs straight.

A bridge that runs into free space could end a life, but bridges are generally quite straight

Nick me open, a city's worth of traffic passes by.

As soon as a bridge is opened everyone tries to use it as it's generally the best available route.

Dressed in poems, especially when one is about to die.

Bridges attract graffiti and other art, especially when old and out of repair.

So by me, men are made mortal.

Bridges are a well known way for people to end their lives.

Hidden Acrostic:

BONDS ← Bridges bond otherwise inaccessible places.

I'm not very confident but wanted to guess.

Thanks!

The answer has been found already, but let me give it a shot.

Are you

Time

Beware if you see me, great horrors may await.

When you see your last hours come along, you'll die soon.

Over where I run free, in me life runs straight.

Time can neither be stopped or reverted and keeps ticking straight.

Nick me open, a city's worth of traffic passes by.

Lot of traffic in cities, speccialy at rush hours.

Dressed in poems, especially when one is about to die.

Many "dressed up" poetic references to death and time. (Stole this one from @SteveV)

So by me, men are made mortal.

When someone's time has come for him to go.

Title

"Time is a valuable thing watch it fly by as the pendulum swings." (In the end - Linkin Park)

Acrostic

Bonds. Investments to make profit over time.