I-35 in Austin has already been mentioned once but I have to second it.
they had a pretty good idea splitting the highway and making an expressway, but they messed up b/c it merges four lanes to 3 right at downtown if you are going south, and does nothing to ease congestion going north into downtown, not to mention merging from 4 lanes to 3 right where the two NE, SW highways cross 35 in North Central Austin. You might think that Austin is not that big of a city (only 650,000 residents). However there are no major east/west highways here so everything is built right along I-35 causing a major traffic problem. You would be considered in the "Austin metro area" for almost 40 miles of interstate.....
To make things even better they put in a toll road to the Austin airport, it runs almost 15 miles east of town, and is out of the way unless you live in the northern most suburbs of Austin.
not to mention all the truck traffic that travels from Laredo, all the way up I-35 To DFW.
To be honest... almost all of the major interstates in Texas are overcrowded... I think Houston is the only city with an 8-lane span (3rd largest city in the US) and most of the highways are only 4 lanes between the major metro areas.... SA-Houston, SA-Dallas, Houston-Dallas.