As a road widening project is completed, traffic is alleviated for a short amount of time. Then as time passes word spreads, or more people move to the city, or kids get older and get their driver's licences. More and more people know this widened road is the fastest route, so more people take it, thus undoing the improvement. Then the cycle starts again - either with the same road being widened again, or another one a block over, on and on until the world is covered in asphalt.
The solution is to make alternative transit more appealing than cars. Bikes and public transit already have significant financial benefits, but lack infrastructure to make it more viable in North America. Busses get stuck in traffic, bikes are forced to share lane space with cars or sidewalks with pedestrians.