KHOU mentioned highway 6, 35 and 71 out of Houston. They made the same point as you did - that often these roads are much less crowded. A current traffic report on those routes is coming up.
Just announced - Mandatory evacuation called for Liberty County. Unfortunately, they are almost out of gas in the area and there is a 15 mile back up into Liberty County coming from the South. A local trooper said that in the next 2 hours the situation could become desperate as thousands of cars run out of gas in traffic because there is simply no gas around.