Stochastic Route Choice and Dynamic User Equilibrium

Definitions of these two key assignment methods follow.

Stochastic Route Choice (SRC)

Stochastic route choice (SRC) is a non-iterative dynamic assignment method. It aims to reproduce the perception errors in drivers' understanding of a network. Such perception errors lead drivers to try to minimize the perceived costs of a network rather than its actual costs. In an SRC assignment, a vehicle is randomly assigned to a path based on a probability distribution.

At each set interval the costs are updated and some vehicles might then choose to change their path en route to their destination. However, the model does not learn from these experiences and a specified number of separate replications are required to generate comparable outputs.

Note that during an SRC assignment, simulated vehicles can be observed and their statistics studied.

Dynamic User Equilibrium (DUE)

Dynamic user equilibrium (DUE) is an iterative assignment method that aims to achieve user equilibrium according to J.G. Wardrop's first principle: "the journey times on all routes actually used are equal and are not greater than those which would be experienced by a single vehicle on any unused route." Vehicles choose an initial path and with each iteration they learn more about the network and aim to improve their path until all iterated paths converge.

Such convergence occurs if, for each OD pair at each instant of time, the actual travel times experienced by vehicles departing at the same time are equal and minimal: at this point the dynamic traffic flow over the network is in a travel-time-based state of dynamic equilibrium.

Note that, unlike in SRC assignments vehicles cannot be observed in a DUE assignment.