Urban transportation is a highly dynamic system where one component impacts on others with retroactive (feedback) effects are also to be expected. Congestion is a classic example of a feedback loop. Pressures on the managers of transport infrastructure (usually the public sector) by different user groups being impacted by congestion may often result in the addition of new capacity such as new or wider roads. This new capacity often results in lower friction to mobility and this may impact on urban sprawl as people may trade more space for a similar amount of time. The outcome is likely to be an increase in trip lengths, more trips, and eventually more congestion. Consequently, users, through their modal choices, are recursively influencing the development of the urban transport system. Several North American cities found themselves in a vicious cycle that triggered an increasing reliance on road transportation and on the automobile. This vicious circle can be mitigated if urban population and economic growth stabilize, lifestyles and preferences change, or if alternatives such as public transit are more readily available. Irrespective of the location, this vicious cycle remains a challenge for urban planners across the world.