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Accessibility is one of the main features of an urban area. Land rent and housing price largely depend on it. In turn, accessibility depends on the location of the attraction centers (CBD and working places, supermarkets, public facilities such as schools etc.) and on the roads and public transit network. This last is a crucial factor in an urban area in that it changes the metric of the space, making easy the movements along the network's links. The model calculates the accessibility map, given a set of centers in which some facilities are located, and the roads network which is utilized in order to travel from the origin to the centers. Costumers or users of the facilities located in the centers, are distributed uniformly on the surface. They utilize the roads networks in order to easily travel. In fact the distance covered out of the roads network, is evaluated the double of the real distance.
The calculation of the distance plays a crucial role in the model. The distance between the centers and the patches in which consumers are located is composed of three parts. The distance between the patch and the nearest node of the road network is the first part of the total distance. The second is the distance calculated with the minimum path method (Djistra) , along the roads network, from this nearest node to the nearest node to the destination center. The third part is the distance from this last nearest node to the center. The first and third part are multiplied by a parameter equal 2, which represents the difficulty to travel out of the roads network. Once the distance between patches and centers has been established, the accessibility in each patch is calculated with the classic negative exponential function of the distance (exp (-beta*distance)). This function is calculated for each center. The result for each center is summed up. The final addition is the accessibility value of the patch which is plotted with a corresponding green tone.
First click on SETUP. Later you have to draw the roads network and to locate the centers. Click on the button DRAW-ROADS, and by using the mouse you may design the roads network. Please care that the network is connected. The connection is automatically established among each cell and the eight bordering cells. Later click on the button LOCATE-CENTERS, and by using the same method, you may locate a maximum of 20 centers. No problem in case of a mistake: click on CANCEL-CENTERS or on CANCEL-ROADS and by using the mouse you will be able to cancel the chosen road of center.
Once the centers and the roads networks are published, click on the button CALCULATE-ACCESSIBILITY and the value of the accessibility is represented with green tones.
Once the accessibility map has been calculated you can makes some interesting variations to your initial spatial pattern, changing both the roads network and the location and number of the centers. Click again on CALCULATE-ACCESSIBILITY and the new accessibility map will appear. By clicking on the button ACCESSIBILITY-DIFFERENCES the differences between the previous and the actual map, will be represented with green tones.
Comments are welcome, as well as suggestions to upgrade the model. Please send you comments and suggestions to semboloniATunifi.itCopyright 2011 Ferdinando Semboloni. All rights reserved.