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Simulation Loop


11 March 2018

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The Simulation Loop

A typical simulation of traffic network operations is illustrated in the middle column of Figure 2. Basically, a time-step mesoscopic traffic model proceeds in two phases: the update phase and the advance phase.

  • 1) Update Phase: Update phase updates traffic dynamics at the link level (density, speed, queue, etc.) which are used in the current interval.
  • 2) Advance Phase: Advance phase advances vehicles along their selected path to the new positions at the end of the simulation interval. The time step is set to a small enough value so that vehicle can just move along a link or is transfered from one link to the next. Vehicle cannot traverse more than two links within one simulation time-step. When a vehicle moves along the link where it is currently located at, the vehicle movement follows traffic dynamics calculated in the update phase.

However, when a vehicle is in the queuing part at the end of a link, it can only move to next link if three conditions are fulfilled:

  • 1) The current link has enough output capacity (which is updated by downstream end node)
  • 2) The next link has enough input capacity (which is updated by upstream end node)
  • 3) The next link has available empty space (which is updated by both upstream and downstream end nodes).

The first two conditions are local to each node, however the third one creates upstream-downstream data dependency because the available empty space of each link depends on both the upstream and the downstream nodes.

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