FDs on the ALLEGRO network

One way to obtain the relationship between speed and density is to classify the traffic flow, then to divide each class into two subclasses (one congested and the other not) and to assign a subclass to each 6-minute period according to its traffic flow. The FD speed is the average of speeds by subclass. Congestion is selected when the speed is lower (or density greater) than the critical speed (or density). These critical speeds and density can be  

Here this method has been used, while forcing the same critical value on the three lanes (the value 35 vehieles/lane per kilometer has been obtained). 

Unlike usual traffic simulation, no global form of a speed-density relationship is required. The main constraint - a decrease in speed when density increases - is not always satisfied with empirical data, and it is necessaiy to process monotonically the obtained average speeds by class. We have developed a heuristic which does so. 

In the middle and median lanes, capacity is the same whatever the speed limit in the slow lane, capacity is the same at 110 and 130 km/h, but decreases at 90 km/h. As the sections used for determining the FDs were different, the FD modification carmot be completely attributed to the change in speed limits, and part of it is probably due to local geometric or traffic characteristics which are not independent of the speed limit decisions. 

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