While in theory expanding a highway to add lanes would ease traffic, in practice, it usually makes it worse. Engineers love the saying, 'build it and they will come,' because more lanes and less traffic will only invite more drivers to take the route, and thus ensuing a trap of continually widening highways.
It appears in Texas, they're experiencing the initial aftermath of widening their highway systems. The Dallas Morning News reported traffic started 'sailing' when figures released this month showing traffic improved after the state opened additional lanes during rush hour.
Aarian Marshall writes in this Wired article that officials cannot build their way of congestion. He provides two reasons this specific circumstance of widening the highway was successful: 1. it relieved bottlenecking where three-lane highways merge onto only two-lanes and 2. the traffic numbers aren't telling the whole truth. It's been six months since the highways released the data and they only included the data for the first few days the lanes were open. They don't account for any growth in traffic in the last six months.