Editing Route assignment (section)

===Heuristic procedures===
To take account of the effect of traffic loading on travel times and traffic equilibria, several [[heuristic]] calculation procedures were developed.  One heuristic proceeds incrementally.  The traffic to be assigned is divided into parts (usually 4).  Assign the first part of the traffic.  Compute new travel times and assign the next part of the traffic.  The last step is repeated until all the traffic is assigned.  The CATS used a variation on this; it assigned row by row in the O-D table.

The heuristic included in the [[Federal Highway Administration|FHWA]] collection of computer programs proceeds another way.
*0. Start by loading all traffic using an all or nothing procedure.
*1. Compute the resulting travel times and reassign traffic.
*2. Now, begin to reassign using weights.  Compute the weighted travel times in the previous two loadings and use those for the next assignment.  The latest iteration gets a weight of 0.25 and the previous gets a weight of 0.75.
*3. Continue.

These procedures seem to work "pretty well," but they are not exact.