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Florida Department of Transportation (FDOT) Express Toll Lane Modeling Workshop Peer Review Report

5.0 Issues and Key Concepts for Express Toll Lane

The afternoon discussion with the panel members in the planning and demand forecasting session was used to address many of the questions listed in Section 4.0 of this report. The three areas of discussion included: Policy Guidance, Model Sensitivities, and Model Design and Application.

5.1 Policy Guidance

The responses from the panelists regarding policy guidance for express lanes centered on two topic areas: guidance on identifying and screening potential express lane projects and guidance for defining a consistent set of MOEs.

Identifying potential express lane projects is the first and highest level of screening analysis in the project development process. The panel encouraged the use of the regional planning process to identify potential express lane corridors, making this a part of each region's LRTP process. They also suggested the use of available sketch-plan models that might offer a way to prioritize and/or screen potential projects, without investing the time and effort required to conduct a more formal analysis with a regional model.

In terms of how to rank projects, a quick measure of Density, Distance of congestion, and Duration of delay (termed "3-D" measures) was suggested by an attendee and was well received by the panel. Another measure that was discussed was the evaluation of the ability to cover O&M costs of the proposed project, a potential minimum-threshold criterion. Other related criteria might be the ability to cover all toll-related infrastructure costs, or even the ability to cover overall maintenance or some portion of the general purpose-related construction costs. Finally, given the policy minimum toll and assuming reasonable value-of-time and speed advantage, a minimum feasible express lane length can be established.

The second main topic was identifying a consistent set of MOEs for evaluation and comparison of competing express lane projects. The panel felt that defining benefits and costs even at the early stages of a project was important in order to maintain consistency of evaluation and clarify to decision-makers early on what criteria will be used. User benefits minus generalized costs for users based on time and monetary costs should be used in some form in the evaluation, as this value provides a very basic measure of public benefit. Both toll and non-toll users should be evaluated when assessing total social/user benefits. Travel time savings and reliability are also important metrics with which to measure the effectiveness of a proposed express lane. In any measure, a risk analysis should be conducted to give proper perspective of the relative merits of each proposed project.

5.2 Model Sensitivities

The panel also discussed key features of managed lane models that they felt were necessary for proper and effective traffic forecasting. In the initial discussion, the panel recognized the importance of several state-of-the-practice model features that should be included to support managed lane modeling activities. These included:

  1. Use of feedback in the model - to ensure that the level of service, time cost, and convenience-related variables are consistent between those generated from assignment and those used for distribution and mode choice. This practice takes on an added importance when considering variable tolling and the time-toll cost influences on demand.
  2. Use of a destination-choice model formulation for trip distributions - to allow a multi-modal consideration of accessibility which, in the case of toll facilities, may be used to influence trip table (O/D) patterns.
  3. Time-of-day stratification - important when toll rates change throughout the day, affecting travel patterns in terms of O/D pairs, trip purposes, and travel times.
  4. Market stratification - which may be employed by trip purpose, socioeconomic measures (income groups), and/or vehicle occupancy type, all of which can affect toll eligibility and toll rates.
  5. Multi-class assignment -to identify tolled versus non-tolled paths and demand. Classes may also be established by trip purpose and/or traveler income for integrated toll choice and assignment methods.
  6. Rigorous assignment closure criteria - in order for proper comparison of alternatives, the equilibrium assignments must be executed to satisfy tight closure criteria. One panelist suggested a gap criterion of at least 0.0001.

All these features were identified as necessary but not sufficient conditions for proper express lane evaluation. They are included in the FSUTMS Transit Model Update recommendations.

Beyond these model features, the panel suggested several other model sensitivities critical to effectively model priced managed lane operations. Of these, reliability measures are considered very important, and they can be estimated even within a static assignment framework.

Studies have shown that reliability may represent up to 40 percent of the attractiveness of a managed lane. Shifts in travel by time-of-day are also a common response to tolling, so a time-of-day choice model may be a desirable feature in a managed lane model. It was agreed that the use of a range or distribution of values of travel time, in mode choice and/or assignment, is necessary to more closely represent actual behavior.

A specific algorithm to balance toll level and demand was also suggested. This balance is particularly important when modeling variable tolls and is an issue not found in other costs in a model, which are typically not demand-based.

Finally, the use of smaller time slices for trip assignment was suggested to provide for greater sensitivity to changes in both demand and toll rates throughout the day. Greater time-of-day detail will also aid in providing time-specific flows for mesoscale simulation models.

5.3 Model Design and Application

The third major topic was basic model design and application. In this discussion, the focus was on how the analyst should make use of the model most effectively. The panel had several important points on this topic.

First, the panel encouraged early evaluation of data and survey requirements. Though they may not be needed in the early screening phases, surveys and other data collection activities take time. Analysts should anticipate data collection needs and plan accordingly so that the necessary data are available when the more advanced modeling is required.

Similarly, a continuity of model design or specification should be maintained throughout the development process. While different models of various levels of detail may be used, these models can still share common parameters such as values-of-time, capacities, and operation policies. The panel felt that we should see the model use through project development as a continuum, rather than a series of distinct model applications.

In response to a question from an attendee, the panel affirmed that transit demand was important in evaluation of express lanes. The models used should be sensitive to changing transit levels of service as a result of an express lane project.

Finally, the question of whether toll choice should be implemented in the mode choice stage or integrated with assignment was discussed. Both methods have advantages, and subsequently, the panel did not reach a consensus regarding a clear preference to either design.

5.4 Summary

The following bullet points summarize additional discussion points either presented or elicited by the afternoon session on demand forecasting:

Updated: 03/28/2014
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