Most people will think traffic behave like water that you need to send through a network of pipes. It is not, traffic is made of humans and humans reactions will make traffic behave wildly differently than waters in pipes.

• Some people and businesses will move next to the new highway for its supposed ease of access, creating traffic
• some people might change their habits and go shopping to this place instead of that place, or getting a job far away from their home (or a home far away from home)

The exact reasons for the increase in traffic is complex and my example could be totally off. But we don't need to know the exact reason for the increase in traffic, we know it happens because it has been observed on every road enlargement projects in the last decades.