Once you have determined the number of trips that will enter and leave each zone, as well as the transportation modes that the travelers will use, you can identify the exact roadways or routes that will be selected for each trip. Trip assignment involves assigning traffic to a transportation network such as roads and streets or a transit network.
Traffic is assigned to available transit or roadway routes using a mathematical algorithm that determines the amount of traffic as a function of time, volume, capacity, or impedance factor. There are three common methods for trip assignment: all or nothing, diversion, and capacity restraint.
All-or-nothing is often referred to as the minimum path algorithm. The minimum path, or tree, represents the minimum time path between two zone centroids and is assigned all of the traffic volume between the zones in question. As volumes and travel times increase, the results of this method become more unreliable.
As an example of this method, imagine that zones A and B are connected by ten separate routes. Route 3.0 has the shortest travel time which means that, according to this model, all trips from A to B will use route 3.0.
Diversion is the allocation of trips to two or more possible routes in a designated proportion that depends on some specified criterion. In most cases the criterion that is used is time, although some also use distance and generalized cost. Diversion is very similar to the all-or-nothing method, except that portions of the total number of trips are allocated to different routes, with fewer trips being given to those routes with longer travel times.
Many different capacity restraint equations have been developed and tested and are available for use. There are two basic characteristics common to capacity restraint models; (i) they are non-linear relationships and (ii) they use the volume-capacity ratio or v/c as a common factor. The underlying premise of a capacity restraint model is that the travel time on any link is related to the traffic volume on that link. This is analogous to the level of service (LOS) criterion, where LOS A corresponds to a low v/c and a higher vehicle speed. LOS E and the corresponding v/c = 1 represents capacity.
Capacity restraint models assign traffic to possible routes in an iterative manner:
The capacity restraint model used by FHWA is applied in an iterative manner. The adjusted link speed and/or its associated travel impedance is computed using the following capacity restraint function: