Imagine a density function for the traffic of a circular city. With more urbanization in the inner city, traffic becomes more likely as you approach the middle of the city. The graph of the function would look like a three-dimensional bell curve, or a rounded hill. The road network will be built to accommodate this pattern of traffic, with more capacity as one approaches the city center.
But what happens when a destination on the city's periphery attracts a non-normal amount of traffic? Imagine the Southern California beach on a hot day. It attracts traffic in excess of the capacity to move that many people to the outer edge of the urban area. Using the circular city model, Los Angeles would not have a three-dimensional bell curve of traffic, but an off-center lump on the periphery. A road network built on the assumption of a normal traffic pattern will be unable to service these movements, resulting in traffic in non-centralized areas.