Traffic Assignment Submodule of SMARTPLANS
The Traffic Assignment Submodule is the final stage of the Transportation Module. It is used by the program to assign the motorize trips predicted by the Modal Split Submodule on the provided road network of the study area. It should be noted that the user could combine two or more trip segments in the traffic assignment submodule to form a trip class. For example, if the user defined two trip segments like work trip and non-work trips in the trip generation, trip distribution and modal split submodules, then these two trip segments could be combined in the traffic assignment submodule to form a passenger vehicle trip class. On the other hand, a class could be the outcome of only one single segment (e.g., heavy trucks).
To configure the Traffic Assignment Submodule, characteristics of the links forming the urban transportation network, namely link’s design capacity (i.e., number of passenger car per hour per lane (pcphpl) x # of lanes), posted speed (kph), and length (km), must be defined for each modeled trip class. Since the link capacity is measured in terms of passenger car units (pcu), trips pertaining to other modeled classes (e.g., trucks) must be converted to their respective “Factor”. The program allows the user to input the pcu values for any of the modeled classes. However, the default Factor values are set to 1.
Additional information like background traffic and available toll cost can also be provided by the user. The user can also select a field which allows the program to exclude certain links for a certain vehicle classes. The field must be in Boolean format, such that it is set to 1 if the vehicle class in question is not allowed to use that link, 0 otherwise.
Since multiple vehicle classes can be defined, the traffic assignment submodule in SMARTPLANS is designed to perform a multi-class traffic assignment. If two or more vehicle classes are introduced, then the user will need to set the order (i.e., 1, 2, 3, …, K) of the assignment on the network. The program presents two types of multi-class traffic assignment models: (1) One Pass and (2) Multi-Pass. Both types are estimated by the program based on the following steps:
- Step 1: Set iteration r to 1. The link flows for the different vehicle classes k specified in the model interface are initially set to 0. That is, vk|r(a)=vk|1(a)=0 for all vehicle classes k and links a.
- Step 2: Starting with free-flow travel time tk|0(a), the model assigns the 1st vehicle class (i.e., Order=1) to the network. This will result in vk=1|1(a)≥0 for all a. It should be noted here that if the user have tolls taph_a (expressed in minutes) for link a, then the starting point will be the sum of the free-flow time tk|0 (a) and tolls taph_a
- Step 3: SMARTPLANS calculates trip travel time on a given link a based on the following Link Performance Function (LPF):
where tk|r (a) is travel time on link a due to traffic volume generated by vehicle class k (i.e., vk|r(a)) in iteration r. The two parameters alpha and beta in the LPF are constants set to 0.15 and 4, respectively. It should be noted that tk|0(a) represents the free-flow travel time on link a. Also, if the user has background traffic BV(a) (expressed in pcu), the volume to be used in the above LPF will be:
Likewise, if tolls tapha exist for link a, then the travel time used will be tk|r(a) + tapha.
- Step 4: The 2nd vehicle class (i.e., Order=2) is then assigned to the network. This will result in vk=2|1(a)≥0 for all a.
- Step 5: Repeat Steps 3 and 4 until all specified vehicle classes are assigned to the network.
One Pass Traffic Assignment Model stops here. However, if Multi-Pass Traffic Assignment Model is selected, the following extra steps are taken:
- Step 6: Once all vehicle classes are assigned to the network, iteration counter becomes r = r + 1, and then Steps 1 to 5 are repeated to generate vk|2(a) for all vehicle classes k and links a.
- Step 7: “Convergence Test” is performed using a pre-defined tolerance epsilon using the vehicle class with the highest order setting (e.g, Passenger), as follows:
Convergence tolerance epsilon is usually set to a small value like 3 minutes (i.e., program’s default value). Notice here that K pertains to the vehicle class with the largest order (i.e., the class that was assigned last to the network). If the convergence condition is met, then the Multi-Pass Traffic Assignment is completed; otherwise, the program repeats the calculation (i.e., increase r by 1) until convergence is met or the maximum number of iterations is reached. SMARTPLANS employ the Stochastic User Equilibrium (SUE) traffic assignment algorithm to simulate traffic flow on the road network.
Moreover, the SMARTPLANS program uses the “Method of Successive Averages (MSA)” algorithms to solve the stochastic user equilibrium traffic assignment problem.