MOPAC (also “MoPac” or “Loop 1”) is a highway in west Austin. The highway was built along the Missouri Pacific Railroad, hence the “MOPAC” name.
 
For those who’ve experienced MOPAC during rush hour, the nicknames “More Packed” and “More Parked” and “SlowPac” have been applied.
 
           
Wikipedia: Texas State Highway Loop 1
State Highway Loop 1 is a controlled-access highway which provides access to the west side of Austin, Texas. It is named Mopac Expressway (or, according to some highway signs, Mopac Boulevard) after the Missouri Pacific Railroad (or “MoPac”), and sometimes jokingly referred to as “More Packed” by locals. The original section of the highway was built in the 1970s along the right-of-way of the Missouri Pacific Railroad (now owned by Union Pacific), with the railroad tracks running in the highway median between West 8th Street and Northland Drive. To the north, the tracks run along the east side of newer sections of the highway from Northland Drive to Braker Lane. 
(...)
Planned extensions
If and when SH 45 is completed to the south of Austin, Loop 1 will effectively serve as a true loop to the west of the city, being directly connected to SH 45, and indirectly to I-35, at both ends.
 
About.com: Austin, TX
Q. What is Mopac?
From Jacci Howard Bear,
You may have already traveled down Mopac and didn’t know it. Pronounced as mo-pack, it is also known as Loop 1.
A. MOPAC, Mopac, MoPac, or Mo-Pac, its official state name is Loop 1. MoPac Expressway is one of the major highways running from northwest to southwest Austin. MoPac is an acronym of Missouri-Pacific as in Missouri-Pacific Railroad, so named for the railroad tracks that run alongside a major portion of this roadway.
     
About.com: Austin, TX
Q. Why is MoPac Called Loop 1 When It’s Not a Loop?
From Jacci Howard Bear,
(...)
A. In Texas, the term Loop is often used regionally to refer to portions of some interstate highways, bypasses, and beltways. Additionally, certain short State Highways connections are officially designated as Loops. Loop 1 is one such highway. 
 
Google Groups: austin.general
Newsgroups: austin.general, austin.politics, alt.planning.urban, misc.transport.urban-transit
From: .(JavaScript must be enabled to view this email address) (Gene Crick)
Date: 1995/09/04
Subject: Re: 14-Lane Superhighway Planned for Oak Hill Main Street
 
BUT going the same 20 mile downs MoPark (the rush hour freeway) at snail pace takes MUCH, much longer. 
   
Google Groups: austin.food
Newsgroups: austin.food
From: Jose Cuervo
Date: 1998/01/14
Subject: Re: sushi, anyone?
 
Finally, there is Mushishino(sp?) over on the frontage road of Slowpac between Far West and Steck.
     
Austin Business Journal
Driving me to distraction
Austin Business Journal - September 17, 1999
by John Egan
(...)
Each weekday morning, I play Rush-Hour Roulette as I warily steer my Mazda along U.S. 183 and SloPac Expressway toward downtown. Is SloPac a highway masquerading as a parking lot? All those tail lights start looking like the bright lights of Las Vegas, but I certainly don’t feel like I’ve hit the jackpot.
     
Austin Chronicle (Letters, November 3, 2000)
When I moved to Austin 12 years ago, local inhabitants informed me that “MoPac” (or Loop 1) stood for the “Missouri Pacific” railroad. I now realize they were wrong. Actually, “MoPac” is an abbreviation for “More Packed.” I know this is true because each of the 12 years I have lived in Austin, Loop 1 has become more and more packed. 
   
Google Groups: austin.general
Newsgroups: austin.general
From: JETman

Date: Mon, 13 Aug 2001 00:15:22 GMT
Local: Sun, Aug 12 2001 8:15 pm
Subject: Re: Fry’s Electronics
 
> Anyone know why it’s call ‘Mopac’?
> -sw
 
I usually refer to it as “more packed,” the road that begins in nowhere and goes nowhere…..
     
Work In Progress
Wednesday, August 24, 2005
The One with an Austin Favorite Part 3
(...)
After sitting in traffic on SLOPAC, we finally arrived at the Veloway. The Veloway is a very well known 3 mi loop off MOPAC in South Austin that is just for cyclists and rollerbladers.
   
2007: The Year of Fun
29 January 2007
Observation:
Since moving to Austin, I have spent 94.4% of my waking hours in dead-stop traffic on Mopac (“slopac” or “mopark” according to Truecraig.)
 
Austin American-Statesman
Want to get away from it all?
Try the Austin City Store
By John Kelso
Tuesday, August 14, 2007
(...)
If you put a cluster of expensive lofts in the snow globe around the Capitol so you couldn’t see the Capitol when you looked into the snow globe, it would be more like Austin today. Also, how about placing a tiny MoPac Boulevard (some call it SlowPac) traffic jam in there with cars with California plates stacked up all the way from U.S. 183 up north to the Fifth Street exit toward downtown?