Congestion reaches a point at which it constrains further growth in peak-period trips.
If road capacity increases, the number of peak-period trips also increases until congestion again limits further traffic growth. Research indicates that generated traffic often fills a significant portion of capacity added to congested urban road. Generated traffic has three implications for transport planning. First, it reduces predicted congestion reduction benefits of road capacity expansion, particularly over the long term. Second, it increases many external costs, particularly over the longer term due to changes in transportation choices and land use patterns.