ML063490287
| ML063490287 | |
| Person / Time | |
|---|---|
| Site: | Indian Point  |
| Issue date: | 12/12/2006 |
| From: | KLD Associates |
| To: | NRC/FSME |
| References | |
| FOIA/PA-2006-0299 | |
| Download: ML063490287 (6) | |
Text
APPENDIX B Traffic Assignment Model information in this. record was deleted in accordance with tYe Freedom of Informatiort Act, exemptions 7"
APPENDIX B: TRAFFIC ASSIGNMENT MODEL This section describes the integrated trip assignment and distribution model named TRAD that is expressly designed for use in analyzing evacuation scenarios. This model employs equilibrium traffic assignment principles and is one of the models of the IDYNEV System.
To apply TRAD, the analyst must specify the highway network, the volume of traffic generated at all origin centroids, a set of candidate destination nodes on the periphery of the EPZ for each origin and the capacity (i.e., "attraction") of each destination node. TRAD calculates the optimal trip distribution and the optimal trip assignment (i.e., routing) of the traffic generated at each origin node to the associated set of candidate destination nodes, so as to minimize evacuee travel times.
Overview of Integrated Distribution and Assignment Model The underlying premise is that the selection of destinations and routes is intrinsically coupled in an evacuation scenario. That is, people in vehicles seek to travel out of an area of potential risk as rapidly as possible by selecting the "best" route. The model is designed to identify these "best" routes in a manner that distributes vehicles from origins to destinations and routes them over the highway network, in a consistent and optimal manner.
The approach we adopt is to extend the basic equilibrium assignment methodology to embrace the distribution process, as well. That is, the selection of destination nodes by travelers from each origin node, and the selection of the connecting paths of travel, are both determined by the integrated mode.
This determination is subject to specified capacity constraints, so as to satisfy the stated objective function. This objective function is the statement of the User Optimization Principle by Wardrop.
To accomplish this integration, we leave the equilibrium assignment model intact, changing onlythe form of the objective function. It will also be necessary to create a "fictional" augmentation of the highway network. This augmentation will consist of Pseudo-Links and Pseudo-Nodes, so configured as to embed an equilibrium Distribution Model within the fabric of the Assignment Model.
Specification of TRAD Model Inputs The user must specify, for each origin node, the average hourly traffic volume generated, as well as a set of candidate. There are no destination nodes. The number of trips generated at the origin node, that are distributed to each destination node within this set, is determined by the model in such a way as to satisfy the network-wide objective function (Wardrop's Principle).
Indian Point Energy Center B-1 KLD Associates, Inc.
Evacuation Time Estimate Rev. 1
The user must also specify the total number of trips which can be accommodated by each destination node. We call this number of trips, the "attraction" of the destination node, consistent with conventional practice. Clearly, we require that the total number of trips traveling to a destination, j, from all origin nodes, i, cannot exceed the attraction of destination node, j. By summing over all destination nodes, this constraint also states that the total trips generated at all origin nodes must not exceed the total capacity to accommodate trips at all destinations.
In summary, the user must specify the total trips generated at each of the origin nodes, the maximum number of trips that can be accommodated by each of the specified destination nodes and the highway network attributes which include the traffic control tactics. The TRAD model includes a ftmction which expresses travel time on each network link in terms of traffic volume and link capacity. This function drives the underlying trip distribution and trip assignment decision-making process. Thus, the TRAD satisfies the objectives of evacuees to select destination nodes and of travel paths to minimize evacuation travel time. As such, this integrated model is classified as a behavioral model.
At the outset, it may appear that we~have an intractable problem:
If TRAD retains the basic assignment algorithm, it must be provided a Trip Table as input.
- On the other hand, if the distribution model is embedded within the assignment model, rather than preceding it, a Trip Table is not available as input.
The resolution of this problem is as follows:
- 1. We construct an "augmentation" network that allows the user to specify only the volume for each origin node. The allocation of trips from the origin node to each candidate destination node, is not specified and will be determined internally by the model.
- 2. We construct pseudo-links which enforce the specified values of attraction, Aj, for all destination nodes, j, by suitably calibrating the relationship of the travel time vs.
volume and capacity.
This augmented network is comprised of three subnetworks:
- 1. The highway subnetwork, which consists of "Class I' Links and Nodes.
- 2. A subnetwork of "Class IH"Pseudo-Links which acts as an interface between the highway subnetwork and the network augmentation.
- 3. The subnetwork of "Class 111" Pseudo-Links and Nodes which comprises the network augmentation described above.
Indian Point Energy Center B-2 KLD Associates, Inc.
Evacuation Time Estimate Rev. 1
The need for these Class II links will become clear later. The classifications are described below:
Class I Links and Nodes These links and nodes represent the physical highway network: sections of highway and intersections. Trips generated at each Origin [Centroid] Node are assigned to a specified Class I link via a "connector" link. These connector links are transparent to the user and offer no impedance to the traveler; they represent the aggregation of local streets which service the centroidal generated trips and feed them onto the highway network. The real-world destination nodes are part of this network. The immediate approaches to these destination nodes are Class I links.
Class IHLinks These pseudo-links are constructed so as to connect each specified destination node with its Class III Pseudo-Node (P-N) counterpart on a one-to-one basis.
Class III Links and Nodes Class III links and nodes forn the augmentation to the basic network. These Pseudo-Links provide paths from the Class II links servicing traffic traveling from the specified [real] destination nodes, to the Super-Nodes which represent the user-specified set of destination nodes associated with each origin node.
Each Class of links provides a different function:
- Class I links represent the physical highway network. As such, each link has a finite capacity, a finite length and an estimated travel time for free-flowing vehicles. The nodes generally represent intersections, interchanges and, possibly, changes in link geometry. The topology of the Class I network represents that of the physical highway system.
The Class II links represent the interface between the real highway subnetwork and the augmentation subnetwork. These pseudo-links are needed to represent the specified "attractions" of each destination node, i.e., the maximum number of vehicles that can be accommodated by each destination node. Instead of explicitly assigning a capacity limitation to the destination nodes, we assign this capacity limitation of the Class II Pseudo-Links. This approach is much more suitable, computationally.
The topology of the network augmentation (i.e., Class III Links and Nodes) is designed so that all traffic from an origin node can only travel to the single "Super-Node" by flowing through its set of real destination nodes.
Indian Point Energy Center B-3 KLD Associates, Inc.
Evacuation Time Estimate Rev. I
The Class II Pseudo-Links and the network augmentation of Class III Pseudo-Nodes and Links represent logical constructs of fictitious links created internally by the model allows the user to specify the identity of all destination nodes in each origin-based set, without specifying the distribution of traffic volumes from the origin to each destination node in that set.
Calculation of Capacities and Impedances Each class of links exhibits different properties. Specifically, the relationship between travel impedance (which is expressed in terms of travel time) and both volume and capacity will differ:
For Class I links, the capacity represents the physical limitation of the highway sections. Travel impedance is ftinctionally expressed. by relating travel time with respect to the traffic volume-link capacity relationship.
For Class II links, link capacity represents the maximum number vehicles that can be accommodated at the [real] destination nodes which form the upstream nodes of each Class II link. Since Class II links are Pseudo-Links, there should be virtually no difference in impedance to traffic along Class II links when the assigned traffic volume on these links is below their respective capacities. That is, the assignment of traffic should not be influenced by differences in travel impedance on those Class II links where the assigned volumes do not exceed their respective capacities.
For Class III links, both capacity and impedance have no meaning. Since the Class HIlinks limit the number of vehicles entering the Class IR subnetwork at all entry points (i.e., at the Class II Pseudo-Nodes) and since all these links are Pseudo-Links, it follows that the Class III network is definitionally an uncapacitated network.
Specification of the Objective Function It is computationally convenient to be able to specify a single impedance (or "cost") function relating the travel time on a link, to its capacity and assigned traffic volume, for all classes of links. To achieve this, we will adopt the following form based on the original "BPR Formula":
Indian Point Energy Center B-4 KLD Associates, Inc.
Evacuation Time Estimate Rev. 1
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iI Of course, it is still possible for the assignment algorithm within TRAD to distribute more traffic to a destination node than that node can accommodate. For emergency planning purposes, this is a desirable model feature. Such a result will be flagged by the model to alert the user to the fact that some factor is strongly motivating travelers to move to that destination node, despite its capacity limitations. This factor can take many forms: inadequate highway capacity to other destinations, improper specification of candidate destinations for some of the origins, or some other design inadequacy. The planner can respond by modifying the control tactics, changing the origin-destination distribution patter, providing more capacity at the overloaded destinations, etc.
Indian Point Energy Center B-5 KLD Associates, Inc.
Evacuation Time Estimate Rev. 1