From cnn.com:
Houston resident Tim Conklin told CNN that he had been in bumper-to-bumper traffic for 13 hours and had only gotten about 48 miles. He said the drive to Dallas, where his father-in-law lives, usually only takes about four hours.
On Highway 290, the main road between Houston and Austin, people were pushing their cars and minivans to save gas -- and were moving just as fast as the vehicles that were driving. Others were stopped on the side of the highway after breaking down or running out of gas.
48 miles in 13 hours = 3.7 mph.