If you study the highway map, you will see the capacity of the highway system to the south is lower than any other sector. This puts more traffic on the pike. The planned northeast corridor route would have balanced this a little better...but here we are. Google maps with the traffic layer on illustrates this status on a daily basis. The big dig fixed a ton of access issues around the city. There has been capacity issues on 93 south for decades prior to the project and they continue to worsen.