It's not just more roads, although more roads are one of the things it is possible to create.

You can also create more housing, so people are closer to their jobs and have to travel fewer miles. Manhattan has higher density than most places, but it also has more people, and would you be surprised to learn that the zoning in most of NYC no longer allows the buildings that are currently in Manhattan to be built almost anywhere? So as a result you can't create more of them and people who might like to live in Manhattan instead live in the suburbs around the city and drive into the city in a car.

You can also create things that aren't roads, like subways, which then allow you to remove cars and buses (and bus lanes) from the roads when it becomes viable for more people to take the subway, which reduces road congestion.