Stochastic assignment procedures assume that traffic participants in principle select the best route, but evaluate the individual routes differently due to incomplete and different information.
In addition, in a stochastic PrT assignment the demand is distributed to the found routes as for a PuT assignment using a distribution model (e.g. Logit, Kirchhoff, Box-Cox, Lohse or Lohse with variable beta) (Distribution models in the assignment).
In order to take the spatial similarities of the routes into account during the distribution, a similarity measure is determined from overlapping routes (analogous to independence during timetable-based PuT assignment) – it is called the Commonality Factor (⇒ “C-Logit“) – or the independence of each route (according to Ben Akiva) is determined.
This results in the following sequence:
1. Route search for all traffic cells for current impedance.
2. Commonality Factor or independence calculated from overlapping of all routes of an origin/destination pair.
3. Distribution of demand to the routes of each OD pair, taking the Commonality Factor or independence into account.
4. Repeat from step 3 until demand for all OD pairs is in equilibrium.
5. Repeat steps 1 – 4 until no new routes are found or until the change in the link volumes between two iteration steps is very small.
During the route search, the number of possible routes can be increased in that it is not just the shortest route that is found, but a number of alternatives are found using a multiple best path search and a variation in the link impedances.
