  Applications such as megaregional planning require efficient methods for
solving traffic assignment problems (TAPs) on large-scale networks. We propose
a decomposition heuristic that generates approximate TAP solutions by
partitioning the complete network into subnetworks which are solved in parallel
and use an iterative-refinement algorithm for improving the network partitions.
A novel network transformation and three-stage algorithm are also proposed to
solve a constrained shortest path problem as a subproblem of the heuristic.
Experiments on various networks show that the heuristic can generate 15.1-67.8%
computational savings in finding solutions with initial relative gap of 0.02.
The performance benefits of the proposed heuristic when warmstarting standard
TAP algorithms are demonstrated with an average computational savings of 10-35%
over a TAP solver without warmstarting.
