stions identifies and explains six types of situations that often occur within cities; the author states that these types of congestions may appear as such or in combination amongst themselves. This specification is extremely useful in today's society and consequently urbanity, which changes on daily basis, generating new causes and effects. The congestion types identified by Vickrey are: "simple interaction, multiple interaction, bottleneck, triggerneck, network and control, and general density" (Vickrey, 1969). Single interaction occurs in light traffic when two vehicles are in immediate vicinity and one has to modify its behavior in order to avoid collision or other undesired events. Multiple interactions occur in more heavy traffic when the speed in traffic is directly proportionate to the flow. The bottleneck occurs when a particular road sector is narrower than the previous and next segments. Triggerneck happens when bottleneck expands to other road segments and affects those that do not wish to use the narrower passage. Network and control congestions occur at high hours when the streets become extremely agglomerated raising as such the need for increased control. General density congestions mean that "congestion costs in the long run are a function of the overall density of transportation flows in a given area for all models combined with and over all routes" (Vickrey, 1969).
             In Accidents as a Cause of Congestion Vickrey looks at the matter of congestion from a different perspective as compared to other scholars. In this particular meaning, he identifies two forces that had been overlooked in analyzing the costs of congestions: delays and accidents. With the aid of a given example on the highways and vehicles in California, Vickrey concludes that there is:
             "a favorable effect on accidents of building roads of the same type to more ample dimensions and greater capacity, and
             an adverse effect on...