(COntinuous Network Traffic Assignment Model) CONTRAM 5 finds wide application in scheme assessment and research as the only traffic assignment model producing (approximately) Dynamic User Equilibrium (DUE) loadings. It achieves this by assigning a stream of small flow packets to minimum cost routes, taking into account traffic interactions and anticipated network conditions. Any capacity or delay function may be used, so realism is not sacrificed to mathematical convenience. However, convergence is difficult to define, and scope for parallel or distributed computing is limited. CONTRAM 6 overcomes this by assigning time-dependent origin-destination movements independently and adjusting "splitting rates" to alternative routes (the first known practical implementation of this method). It drops fixed-column data for more flexible space-delimited relational format, better suited to graphics and model integration, with fewer restrictions on network size and value ranges. It updates networks only after assigning all o-ds, unlike CONTRAM 5 which updates after each packet, so initially large fluctuations in loading can occur. To control convergence, each o-d effectively has its own "step length", adjusted according to how path costs change. Relative duality gap (DUE "error") and total journey time are displayed graphically as the program runs. Typically, convergence to a few percent gap needs 15-30 iterations. After 30 iterations, 15 of 16 networks tested (up to 1663 links) produce a gap of 3% or less. Total journey times and other results are similar to CONTRAM 5. Run times are comparable with CONTRAM 5, but the inherently parallel, scalable logic of CONTRAM 6 offers the prospect of substantially reduced run times using parallel or distributed computation. (A)
Samenvatting