Highway Assignment

This page provides an overview of the highway skimming and assignment procedures. For more information on the inputs, outputs, and parameters of the ABM components, please see the Highway Inputs and Highway Networks and Skims pages.

Description

Highway assignment is a multi-class static user equilibrium assignment with the following default user classes:

• Drive Alone (free),
• Drive Alone (pay),
• Shared Ride (free/HOV),
• Shared Ride (pay),
• TNC-Alone: transportation network company - single passenger with direct service,
• TNC-Shared: transportation network company (TNC) - multi-passenger with grouped service,
• Large Trucks, and
• Externals.

The solution to the traffic assignment problem is found using the Frank-Wolfe algorithm. The convergence criterion is a relative gap of 0.0001 achieved on three consecutive equilibrium iterations. A generalized cost function that includes travel time and toll cost is used to find the least cost paths at each user equilibrium iteration. The generalized cost is based on a $15 / hour peak value of time (VOT) and $12 / hour off-peak VOT. Distance / operating costs are not considered in the assignment procedure and the VOT is consistent across occupancy rates, i.e. the toll cost is not assumed to be shared amongst all passengers.

Managed Lane Toll Cost

To Be Determined - May be simplified or removed

Volume-Delay Functions

The highway assignment procedure is applied in an iterative fashion, where travel times are updated after each iteration to reflect congestion occurring on the network. These updates to travel time are based on a volume-delay function for each link. The existing volume-delay functions are the modified versions of the standard BPR functions with parameters varying by facility types, which will be reviewed and modified if necessary during validation. The free-flow time is based initially on the network data provided for each link and then updated in each iteration to represent the travel time resulting from the assigned traffic volumes from the last iteration.

Turn Penalties

Turn penalties are included in the trip assignment model to either prohibit certain turn movements or to penalize certain turn movements. These are included in the model by identifying specific turn movements by their node numbers, and then coding the penalty function that will apply to these turn movements.

Speed Feedback Averaging

The demand forecast procedure starts with seed highway travel time skim matrices, which are used to run the activity based model (ABM) and non-ABM models. The resulting vehicle trips are assigned to the highway network to obtain link level vehicle flows, V, and “congested” or loaded travel times. Since these travel times can be different from the times in the original seed skims, a new travel time is computed and the demand model is run again. This means of successive averaging (MSA) process ensures that the speed feedback iterations will converge by dampening the iteration to iteration changes. Link volumes from the current speed feedback iteration are averaged with the averaged results of previous iterations. Volumes are averaged by applying 1/iteration of the difference between the last MSA volumes with the current volumes. This process is repeated until equilibration is achieved.

Equilibration

The statistic used to establish that equilibrium between supply and demand has been achieved is the %RMSE of the AM and PM drive-alone travel time, computed over the full travel time matrix. The %RMSE is calculed in a demand weighted fashion, such that the OD travel time error is weighted proportionally to the number of trips. This statistic ensures that errors on frequently observed OD pairs are given more importance than errors on infrequently used OD pairs. When both the AM and PM weighted %RMSE are less than 1%, the model is considered converged and exists the speed feedback loop.