This Eyewitness News article reports on a rather unusual reason for a big-city traffic jam:

Drivers on the city’s south side found themselves caught up in a very unusual traffic tie up overnight.
Officers are used to pulling over drivers, but a bull on the Beltway proved a much greater challenge. Authorities did finally catch the bull, but not before the animal ran loose for about 30 minutes.
The bull originally got loose at about 11pm, and started blocking the Beltway for drivers. It was spotted first headed east on the South Belt near Sabo.
At one point, someone had a rope around the bull, but that person was dragged a little bit and the bull got loose again. The bull jumped the median and started heading west, finally exiting at the Pearland Parkway, and U-turning through the underpass.
Officers from the Houston Police Department and the Constable’s office finally managed to round the bull up and tie him to a fence.