Transformers and Transfans answer the ageless question:
             *** WHY DID THE CHICKEN CROSS THE ROAD? ***

"The ZAM POW! chicken crossed the BLAM road to get to the ZOWIE! other 
side!" 
-- Warpath

"A full-spectral bioanalysis of the chicken's vector reveals that its 
forward momentum was stimulated by a transmission of neural impulses from 
its cerebral cortex to the nerve centers governing the reactions of 
muscular tissues within its bipedal locomotive appendages." 
 -- Perceptor

"WhyAreYouAskingMeAboutChickensCrossingRoads?IHardlyKnowAboutRoadsAndI
KnowEvenLessAboutChickensSoJustWhatExactlyAmIGoingToKnowAboutChickens
CrossingRoads?Nothing,That'sWhat!" 
 --Blurr

"Because mighty Galvatron commands it!" 
 -- Cyclonus

"You want to see a chicken cross the road??!? *I'LL* show you a chicken 
crossing the road!! [BLAM!] Muha ha ha ha ha!!!" 
 -- Galvatron

"Hmm... It looks like the chicken's crossing the road." 
 -- Ultra Magnus

"An empty stretch of roadway snakes its silent way through the harsh and 
unforgiving desert. Suddenly, through the midday haze, a white shape can 
be seen. With breath-taking swiftness, flashing feathers streak through 
the air, and razor-sharp wings strike with deadly accuracy as the 
Chickenator makes her lightening assault! No highway is too wide to slow 
the onslaught of this behemouth barnyard creature. Her mission complete, 
the Chickenator disappears into the distance, waiting for the next 
opportunity to cross the road!" 
 -- Beast Wars bio card

"Well, lookit, let's stop and think it through. We've got a chicken and a 
road. Obviously the chicken's on one side or the other. So she does what 
any good poultry animal would, and walks across. Only a car comes along, 
and she doesn't make it! Now all we have to do is figure out, who was 
driving the car?" 
 -- Nightbeat

"*Cross* the road? That's nothing! I'll show you some flying that'll make 
you forget all about the road!" 
 -- Powerglide

"Do not ask why, for it is the chicken's inherent right as a free being 
to cross the road as she wills. It is not our place to question her 
actions."  
 -- Optimus Prime

"She was brainwashed by Autobot dogma." 
 -- Raksha

"She isn't really a chicken; she's an archetypal figure symbolic of all 
road-crossers. She's part of a balance established throughout history, a 
balance of road-crossers and non-road crossers which can never really be 
tipped one way or the other." 
 -- Kendrick Kerwin Chua

"Poultrion: eject. Operation: road-traversion." 
 -- Soundwave

"Look, I've already explained to you why the chicken crossed the road in 
thorough detail. It's not the original series, this isn't the chicken we 
know from G1, and the road might not even be on Earth. If you're going to 
attack the chicken, please come up with some more intelligent and 
thought-out arguments than just 'the chicken sucks'." 
 -- Ben Yee

"No care about chicken-bot! Me Grimlock DINObot!" 

"Why the chicken crossed the road is not the issue! She had no choice! 
There was no other escape from the endless circle of death and 
destruction and hatred! She could not break the circle... but she could 
remove herself from it." 
 -- Fortress Maximus

"I don't know but as soon as I find another Krok I'll be able to finish 
my Action Master chicken!"  
 -- the Weeter

"I don't know, but if it's a Lehman chicken I wouldn't deal with her."  
--the Nixter

"Foolish chicken! To a master of Metallikato your clumsy road crossing is 
almost slow motion!"
 -- Bludgeon

"I just hope it doesn't leave any feathers on road... they could scratch 
up my base coat!" 
 -- Sunstreaker

"The chicken crossed the road because *Megatron* failed to stop it! 
Perhaps *I* should lead from now on." 
 -- Starscream

"With the chicken in our hands, the secrets of crossing the road will at 
last be ours, and we will crush the Autobots once and for all!" 
 -- Megatron

"Huh? When did this happen?" 
 -- Iggy Drouge

"Four out of five doctors advise that chickens crossing roads are in 
danger of being canceled or losing their time slot. Coming up the next 
Frugal Gourmet... cooking with road-kill chicken. Happy motoring, and a 
cock-a-doodle-doo morning to you!" 
 -- Wreck-gar

"Menasor... crush... chicken!"

"What was that?"
"I dunno, but it got feathers all over muh bumper!"
"Yeah, big mess! Heh-heh. I love a big mess!"    
 -- the Battlechargers

"Eh, a chicken couldn't cross the road; it'd never work!" 
 -- Huffer

"It's all explained in the new TMBG single. Go check it out." 
 -- M Sipher

"Death comes to he who crosses the road in front of me." 
 -- Dirge

"There are three thousand, four hundred and eighty-two recorded instances 
of chickens crossing roads, and in two thousand fifty one of these cases 
the chicken's motive is known too. In the first case..." 
 -- Rewind

"Darn it, we didn't even get the chicken here." 
 -- Darren/Dirge/Beachcomber

"Hey, guys! Come check out my new electro-powered auto-road-crossing 
mechachicken. It'll automatically cross any road in seconds!" 
 -- Wheeljack

"I'm the fastest road-crosser you ever saw!" 
 -- annonymous G2 bio card

"Probably the same reason I hadda jump over a thousand foot chasm in the 
wastelands of Joor -- it was the only way to escape from the hordes of 
Ooglians that was huntin' after my chassis!" 
 -- Kup

"Ooooo, fanfic fodder." 
 -- Robert Jung

"Hypothesis: This chicken's road crossing has provided us with the 
opportunity we need. Conclusion: We attack at once!" 
 -- Shockwave

"It does not matter.  We must deSTROY the road, NOW!  BeFORE the chicken 
has a chance to cross it."
 -- Dinobot

"Ehhh... She probably just had enough a' bein' around *you* guys."
 -- Rattrap

"A threat to the colony comes from across the road?  It will *burn!*"
 -- Inferno

"Ooooooo!  Chicken-bot crosses road.  Waspinator will destroy her!"
 -- Waspinator

"Look, we've said it before: we CAN NOT answer questions like 
that.  I can tell you, however, that IF and WHEN the chicken 
crosses the road it will NOT be for any of the reasons you're 
thinking of."
 -- Bob Forward

"what chicken why is everyone asking me about the chicken that was 
another chicken i have references"
 -- AfroJo

"Roads.  They exist, built by men, only to crumble hopelessly into dust
and ashes, their existance pointless and spent.  On one such road, a
chicken crosses.  It is a chicken that is young, and will not live to see
old age.  It knows not its fate.  But it will learn, for a car that is to
destroy its existance comes."
 -- HooksX

"It's hard to say, since in real life chickens do not instinctively cross
roads, at least not the species that this chicken is apparently supposed
to be."
 -- Thylacine 2000

"Ugg.  That.  The whole idea that the chicken would ever even come _near_
the road is simply preposterous.  And then, as if that wasn't enough, it
WALKS ACROSS THE ROAD.  This is sheer stupidity on the part of the chicken
and had me laughing so hard my stomach hurt."
 -- The Critic

"We join the chicken as it is crossing the road.  It crosses just like
it did in the show, only in this version all the chickens live and don't
die.  Next chapter, On the Other Side of the Road." 
 -- WARendfeld

"Huh.  Well.  Long as he's doin' his job, it's not really my business.  Good
work, chicken."
 -- Lexius

"I, er, I can percieve no viable reason for it to act in such a manner."
 -- Starblast

"Because it wanted to.  Isn't that reason enough?"
 -- Treadmark

"Because I am the Robot Train."
 -- Raiden (in the spirit of WWFF :]

"Don't know.  Did you see Rampage following it?"
 -- Depth Charge

"No!! The Matrix is mine! I will not be denied... (fires and destroys the 
chicken) ...by any creature!" 
 -- Thunderwing

*****************************
A little digging through the wonderful world of Deja News turned up 
some of the follow-ups to the original post; I've compiled some of the 
best ones here.  The authors are credited, but I've changed a few of 
the quotes around a bit.
*****************************

JOONA PALASTE:
"So it could face new, exciting dangers at the other side of the road and 
then kill them!"
 -- Sinnertwin


M SIPHER:
"That chicken isn't letting anybody dictate where it can go. She's made up
her own mind and isn't letting other chickens' ideas of what side of the 
road is the 'right' one to be on sway her opinions. *That's* why I prefer 
this chicken."
 -- Raksha

"Huh. Chicken'll probably get run over, and lay there in the hot sun, 
dying a slow, lingering death as the crows circle..."
 -- Dreadwind

"Good idea! Let's find somethin' to hit it with!"
 -- Darkwing

"Ah, friend chicken. A kindred soul, bound irrevocably to the Earth... I 
feel for you, friend chicken." 
 -- Lightspeed

"Datum: Average traveling speed of domesticated fowl *gallus gallus*. 
Datum: span of apshalt-covered motorway. Datum: cerebral and psychological
nature of chicken. Conclusion: probability that chicken crossing road was
completely random, 72.98443 percent."  
 -- Computron

"A chicken crossing the road?  Hey, that reminds me of a joke!  Why 
did... oh, wait.  Huh."
 -- Bluestreak


LIZARD:
"OK, we got chicken fricasee, fried chicken,
chicken-in-a-basket...lowest prices this side of Cybertron."
 --Swindle

"Well, that's where I dropped it, see?"
 -- Vortex

[Sqloosh] "Heh heh heh heh! Who cares!"
 -- Brawl

"Because the inferior flightless fowl could not bear to be in the
prescence of I, who can soar to the stars themselves!"
 -- Blast-Off

"It was executing manuever 65-Delta, as per my orders."
 -- Onslaught


DOUG DLIN:
"Um, I hate to burst your bubble, guys, but in the original 
Japanese version, it wasn't a chicken..."
 -- Doug Dlin


ZOBOVOR:
"Motives unknown.  Answer irrelevant."
 -- Omega Supreme

"Uhhhh... chicken... what?" 
 -- Sludge

"In fact, this creature's actions are not of interest to me, no.  But 
if it is, in fact, a Maximal, I *shall* destroy it, yes!"
 -- BW Megatron

"What do we care about some stupid Earth bird, who can't even fly?" 
 -- Slingshot

"The chicken has it in for me...  It always has...  Can't any of you see
this??  The chicken wants my job!!" 
 -- Red Alert

"Never fear, I shall swoop down with my superior design and discover the
solution to this perplexing puzzle as only I can." 
 -- Sky Lynx

"I don't know, but it's taking her forever to cross!  Hurry it up!" 
 --Windcharger

"The charge is jaywalking.  The verdict:  Innocent.  The sentence: death!"
 -- Quintessons

"Like, she probably just wanted to, you know, get somewhere.  Wow." 
 -- Beachcomber

"The chicken's made it past the crosswalk...  He's headed toward the
goal...  YES!  Touchdown!  What a play!" 
 -- Eject


RATBAT:
"To cross that road, the chicken will consume .54 units of
fuel... unacceptable for this mission. Return, chicken!"
 -- Ratbat

"I'll be putting the chicken question in the next FAQ when it comes
out..."
 -- Steve Stonebreaker


JONATHON MORIARITY:
Tracks: All right, all right, Raoul! I give up! Why?
Raoul: Ta get to the other side, ya doofus!
Raoul and Blaster: (Hysterical laughter)
Tracks: Seriously, I'm never going to understand human culture...

"Don't lookit me! I ain't goin' near any more birds!"
 -- Rumble


NITERUN:
"To die. Why else?" 
 -- Dead End

 
ASTROTRAIN:
"Chicken in the road?! Yeeeehaw! This I gotta see -- whooa! [SPLAT!] 
Woops! I guess he ain't crossin' anymore!"
 -- Wildrider


PLAYGROUND PSYCOTIC:
"There must be SOME connection between flightless birds and 
roads...(taps Teletran's keys like mad)"
 -- Wheeljack


PYYNK:
"*Why* did the chicken cross the road? <*sigh*> Prime, if you were here,
you would know the answer."
 -- Rodimus Prime


DAVID G TASHJIAN:
"Capsule: 'Chicken crossing road' bit is a little cliche, full articulation 
(for a chicken), moderately recommended." 
 -- Dave Van Domelen 

"It is fleeing in awe-struck terror from the fearsome power of the 
almighty Bee!"
 -- Jameel

"I _STILL_ hate Beast wars no matter how cute they become!"
 -- Trnsfrmr@aol

From: Carcharodon :
"So it wouldn't have to fly across."
 -- Silverbolt (G1)

"Because it is a good and brave chicken, crossing the
road of truth into the land of purity."
 -- Silverbolt (BW)

"It was following its natural beast instincts.  We should 
all learn from its example."
 -- Tigatron

"Because the primitive being acted foolishly and without
logic, it surrendered to a weak emotional impulse to cross the road -- an
illogical act that will seal its fate."
 -- Shockwave

Rumble:  "Lousy chicken, no crossing the road on the job!"
Frenzy:  "Yeah!"

Hook:  "The issue is not that the chickenbot crossed the road; 
        rather, we must focus on its clumsy walking
        style and flawed choice of pathway."

Galvatron:  "It crossed the road--against my will!  RAAARRRRGHHH!"


Dave Willis's post, which is just too good to edit:

> Picard42: "Because Ultra Magnus was gonna bonk him."
> 
> Carachodon: "I don't know if any of you guys know this, but the chicken
> was originally an organism called Chickdam ten years before in Japan...
> 
> Harleydo: "The chicken is just stupid, he is a disgrace to the original
> chickens.  I mean, the chicken is all wimpy flesh, and his head looks like a
> chewed caramel!  If you like this chicken, you are abandoning the original
> chickens!
> 
> PDilloway: "If you don't see that he's trying to get to the other side
> of the road, you are an idiot.  I have pictures of him on the other
> side to prove it!  I am 100% certain he is going to the other side!
> The chicken is NOT going to make a picture of himself on the other
> side of the road for the heck of it, people..."
> 
> Harleydo: "The chicken is just stupid, he is a disgrace to the original
> chickens.  I mean, the chicken is all wimpy flesh, and his head looks like a
> chewed caramel!  If you like this chicken, you are abandoning the original
> chickens!"
> 
> Robotech_Master: "[Review] Chicken Crossing (9 out of 10)"
> 
> QuakerX: "[SPOILERS] Why the Chicken crossed the road!"
> 
> Harleydo: "The chicken is just stupid, he is a disgrace to the original
> chickens.  I mean, the chicken is all wimpy flesh, and his head looks like a
> chewed caramel!  If you like this chicken, you are abandoning the original
> chickens!"
> 
> Harleydo: "The chicken is just stupid, he is a disgrace to the original
> chickens.  I mean, the chicken is all wimpy flesh, and his head looks like a
> chewed caramel!  If you like this chicken, you are abandoning the original
> chickens!"
> 
> Megatron33: "Because if he had the might and the ability to cross the
> road, why should he not if it is his destiny?"
> 
> Harleydo: "The chicken is just stupid, he is a disgrace to the original
> chickens.  I mean, the chicken is all wimpy flesh, and his head looks like a
> chewed caramel!  If you like this chicken, you are abandoning the original
> chickens!"
> 
> Dalmation: "[Fanfic]Galvatron & Cyclonus: 'Poulet Ponderings, part 11'"
> 
> TheNixtr: "Looking for Jenna Von Oy!"
> 
> --David "Someone do me!  Do me!" Willis :D

"Cause it had to poo!"
 -- Walky


PERCEPTOR :
"I have summoned him across for a purpose."
 -- Unicron


CORBETT OWEN:

World's Worst Fanfic:

[Chicken looks around cautiously as it steps into the middle of the road.]

Cheetor:  I like pie.

Chicken:  Oh, no, it is a bomb that is falling!

[A bomb that is falling falls down towards the chicken.  Strapped to it is
Wheelie and ATTers that the WWFF crew do not like.]

Wheelie and ATTers that the WWFF crew do not like:  Oh, no!  We are strapped
to a bomb and it is falling down towards a chicken!

[Scorponok dies.]

[The bomb lands on the chicken.]

Bomb:  EXPLODE!

[The bomb explodes and all life on Earth is destroyed except for the Predacons
and Bill Clinton.]

Megatron:  This is a new beginning for the Predacons.

Bill Clinton:  Shut up, you.  I am your new leader!

Blackarachnia:  Oy, I'm outta here.

 -- Corbett Owen (Ok, so maybe someone's done a few of these before, but hey,
        tenth time's a charm... :)