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SONG: Simulator of Network Growth 1.0






User Instructions

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SONG 1.0 is a simulator designed for simulating the processof transportation network growth. The growth or decline of transportationnetworks is normally treated as the result of top-down decision making in thelong-range planning efforts of metropolitan planning organizations (MPOs).However, changes to transportationnetworks are essentially the results of numerous small decisions by propertyowners, firms, developers, towns, cities, counties, state department oftransportation districts, MPOs, and states in response to market conditions andpolicy initiatives. These system behaviors demonstrate the characteristics of decentralizedsystems, where organized patterns and structures can emerge not because ofcentralized control, but because of the interactions among decentralized systemcomponents. This phenomenon ofdecentralization and self-organization occurs in many circumstances, such asbird flocks, ant colonies, life evolutions, neural networks. Increasingly researchis applying decentralized models in different domains as a new way of viewingthe world.

In SONG 1.0, transportation networks are treated asdecentralized systems that demonstrate self-organizing patterns. By design, thesimulator models behavior of individual system components and small decisions, andthen demonstrates the patterns resulting from interactions of component models.The idea is that it is the actionsof the components, not the whole that are controlled by the designer; theresulting patterns or structures, which often behave very differently than theelements that compose them, are not designed. › webstore › detailAPK Downloader - Chrome Web Store - Google Chrome - Download ...

It is worth to notice that SONG 1.0 shows a different kindof simulation, called 'soft simulation', which 'provides qualitativeunderstanding of a complex system by constructing a simple one that shares thesame principle (Papert, 1992).' By developingthe simulator, the focus is not on perfect reproductions of the real world, butrather to help people explore the 'microworld' of transportation networksystems and to stimulate new ways of thinking about the network growth process.

Simulation Models


SONG 1.0 simulates behaviors of individual links to explorethe patterns of resulting network forms. In particular, each link is treated asautonomous agent and the simulation models individual link behaviors based oncertain rules that determine link revenue, costs and investment decision. Themodel flowchart in Figure 1 provides a global view of the model structure;detailed descriptions of components models are followed after the flowchart.

Figure 1: Simulation flowchart

1. Land Use and Network Settings

The land use and network are treated as initial conditionsinput into the model. The land use is modeled as a layer of a grid of landblocks called land use cells. Transportation networks are modeled as directedgraphs overlaying a land user layer.

The main objective of setting initial land use is to provideinput to transportation models. Information contained in the land use layerincludes: location, population density, and market density. It is assumed inthe simulation model that trips produced from a land use cell are directlyproportional to population density and trips attracted to a land use cell aredirectly proportional to market density.The model considers three land use patterns:(1) random initial land use, where tripsproduced and attracted are randomly distributed; (2) uniform initial land use,where trips are uniformed distributed, and it is a conceptual conditiondesigned to control the effect of land use on the formation of network; and (3)bell-shaped land use, which is designed to replicate the distribution of landuse patterns with a Central Business District (CBD).

The transportation network is represented as a directedgraph that connects nodes with directional arcs, links. The simulator applies agrid network structure.

2. Travel Demand Model

The travel demand model converts land use information(population and employment data) into link flows on a given transportationnetwork through trip generation, trip distribution and route assignment models.

The trip generation model divides geographical area intozones centered on network nodes and converts land use properties of the zonesinto trips produced and attracted. The land use layer is modeled as a square,and it is assumed that the land use layer is static throughout thesimulation.

The trip distribution model matches the generated trips bypurpose between origin (O) and destination (D). Trips produced are differentiatedaccording to destination; trips attracted to a zone are differentiatedaccording to origins.

Thetraffic assignment model predicts the travelers’ choices of routes by therelationship between flow and travel time. In particular, it converts the ODmatrix, from the trip distribution step, into flows on the links depending onthe way travelers choose routes.Byassumption, every traveler tries to optimize his travel time by choosing thequickest route to reach the destination.

3. Revenue Model

The network revenue model determines the revenue roadproviders can collect by providing the road services, depending on link speed,flow, and length. (From travelers’ perspective, it is the toll they pay forusing the road.) The revenue modelis a link-based model that calculates revenue for each link.Revenue is calculated by multiplying thetoll and flow. Therefore, the higher the flow on the link, the higher is therevenue.

4. Cost Model

Thismodel calculates the cost that is required to keep a link in its present usablecondition depending on the flow, speed, and length.

where,Ciais the cost of maintaining theroad at its present condition,

mis the (annual) unit cost of maintenance for a link,

a1, a2,a3 arecoefficients indicating economies or diseconomies of scale

5. Investment Model

Depending on the available revenue, andmaintenance costs link changes its speed.If the revenue generated by a link is insufficient to meet its maintenancerequirements its speed drops.Ifthe link has revenue left after maintenance it invests that remaining amount incapital improvements, increasing its speed. A major assumption in this model isthat a link uses all the available revenue in a time step without saving forthe next time step.

With the new speed on the links the travel time changes andthe whole process from the travel demand model is repeated to grow thetransportation network.



As shown in Figure 2, theInterface of SONG 1.0 contains three portions of information: the parameterpanel, the visualization panel, and the output panel.

Figure 2: Interface ofSONG 1.0


1. Parameter panel