update on developing evacuation model using dynamic traffic assignment chiping lam,...
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Update on Developing Evacuation Model using Dynamic
Traffic AssignmentChiPing Lam, Houston-Galveston Area CouncilMatthew Martimo, Citilabs
Review last Presentation
During Rita Evacuation, evacuation routes were very congested. “Crawling parking lot.”
H-GAC was asked to develop a tool for evacuation planning.
Challenges
Large network and demands Long trip length and travel time Interaction between evacuation and non-
evacuation traffic Network changes during evacuation period
(eg: contraflow, HOV and toll open to public)
Goal of this model
Re-generate the Rita evacuations Provide evacuation demands Estimate traffic volumes and delays Sensitive to various scenarios and plans Apply to non-evacuation planning
(corridor, sub-area, ITS, etc)
H-GAC’s Expectation
Validation – Normal Day Traffic– Rita– Year 2010 Scenario
Able to adjust evacuation trip tables for different situations
Sensitive to policy factors Allow road changes within evacuation
Review – Why DTA?
Why NOT use traditional (Static) assignment?– No impact of queues– No ability to deal with upstream impacts– Links do not directly affect each other– Not conducive to time-series analysis
Why NOT use traffic micro-simulation?– Study area of interest too large and complex– Too much data and memory required– Too many uncertainties to model accurately
Cube Avenue Technical Facts
Unit of travel is the “packet”– Represents some number of vehicles traveling from
same Origin to same Destination
Link travel time/speed is a function of– Link capacity– Queue storage capacity– Whether downstream links “block back” their queues
Link volumes are counted in the time period when a packet leaves the link
Progress on Last Presentation
Based on TXDOT survey, develop trip generation model
Using a simplified and relax gravity model to assign evacuation demands
Develop hourly factors for evacuation traffic and normal traffic reduction
Progress on Last Presentation(2)
Ramp Storage Adjusted to account for storage lane and through lane on freeway, to avoid over-estimate backup
Network simplification to save memory Single class assignment 72 1-hour assignment to account for
network changes
Computer Limitations
32 bit computing (Windows XP) limits how much computer memory can be accessed by a single process to 2GB.
Initially the problem size was requiring more than 2GB of memory and was failing altogether.
Previous suggestion: Simplified Network to reduce memory requirement
Overview for this presentation
Problem Size – Greater Houston-Galveston Metropolitan Area– 72 hour simulation of evacuating vehicles
Initially strained the available computing resources
Mesoscopic modeling versus standard Macroscopic Travel Demand Modeling
Simplified Network Abandon
Only Major arterials, highways, and freeways remained in the simplified network.
In retrospect, this was a VERY bad idea… because of the nature of Mesoscopic Simulation… This will be described in a few minutes.
In fact, the more detail available in the network, the better. We are now modeling with the full travel demand modeling network.
Multi-Class Assignment Single class assignment remove some of the
ability of the model to properly replicate flows seen on the roadways
Making calibration more difficult. Now model multi-class assignment similar to the
static model, each with their own path sets. Drive alone free (No HOV, Toll, HOT) Drive alone pay (No Toll) 2 person free (No Toll, HOT) 3+ person free (No Toll) Share ride pay (allow everything)
Increase Number of Iterations
Originally zero to 1 iteration (similar to AON assignment)
Vehicles jam to the AON route, cause extremely long travel time and consume more computer memory
Ill-conceived as with each subsequent iteration, the vehicles learn more about possible routes and their environment.
With each subsequent iteration, the model is more stable, reliable, and easier to calibrate.
Number of Iteration vs Travel time for Single hour assignment
Packets Network are simulated in packets. A group of trips with same origin, destination,
and start time. Treated as if a single unit Each packet can hold any number of trips. Tracking and simulating these individual
packets is what consumes the memory. 2GB can simulate more than Six Million packets at anyone time.
Limit the Size of Packets
Originally, the maximum size of packet is ten vehicles or less
Large size is to reduce number of packets; to consume less memory
With software upgrade and increase iteration, now is one vehicle trip per packet
Reduce number of non-integer trips
Non-integer Trips
Example: Drive Alone Free Trip Table
10 million tripsDue to non-integer trips, the number of packets ends up being MUCH larger.
Reduce Number of Non-Integer Trips (1)
Alternative 1: traditional bucket rounding for each hourly demand
Add fraction trips across column, and assign a trip when the sum of fraction equals to or exceeds 1
Does not reserve column (destination) total, which is bad as evacuation traffic is concentrated on a few external destinations
Reduce Number of Non-Integer Trips (2)
Alternative 2: Cross-time bucket rounding Summing across time rather than column,
hence preserve origin-destination total Too little traffic on early hours because for
many origin-destination, sum of early hour trips is less than 1 (no packet assigned)
Probabilistic Integerization (1)
For each origin-destination pair, produce probability distribution based on hourly demands
Simulate integer trip based on probability Sum of Daily Trips for each origin-
destination reserves, and early-hours are assigned with adequate traffic
Probabilistic Integerization(2)
Changes to the Software
To properly simulate network changes, such as reversible HOV facilities, contra flow lanes and etc, the following changes were made to the software: Ability to turn facilities on and off during the
simulation Ability change the capacity of facilities during
the simulation. Ability to animate packet during the simulation
Changes to the Methodology
Previously, break down the 72-hours evacuation into 72 single hour assignments to allow network changes
Now simulate the entire 72 hours of evacuation in one long simulation, and turn on contraflow lane or reversible HOV in the middle of simulation
Reduces run time from 3 days to half days
Cluster
Speed up the simulation by distributing the work to more than one processors
Now groups of computers can work on finding the best path for each packet (one major task).
While others work on simulating the packets as they become available (the other major task).
Volume Delay Curves
In macroscopic assignment, assigned volume can exceed capacity.
The Volume-Delay curves were adjusted to limit the ability of the model to assign more trips than the available capacity.
The speed is too high comparing to reality
Example: Freeway curve
Volume Delay Curves(2)
On contrast, DTA does not allow volume to exceed capacity.
Therefore, speed should decrease sharply when volume approaches capacity
Standard speed-capacity curve from Highway Capacity Manual replaces the volume delay curve in regional demand model
Mesoscopic Simulation
When Compared with Macroscopic Assignment:– Vehicles take up space and progress through
the network.– Capacity strictly limits the rate at which
vehicles progress.– Available Storage strictly limits the number of
vehicles that can occupy a link.– If vehicles cannot progress they must wait. – A full link blocks ‘back’ and will impact
upstream links
Theorem of One Bad Link
In static assignment, volume on one link may over capacity and does not impact adjoining roadways.
In the mesoscopic simulation, when a link is over capacity, incoming vehicles must queue on upstream links to wait for their turn
A link with extremely high v/c ratio could cause serious congestion on adjacent links
Impacts on Mesoscopic Assignment
Example of a centroid connector between a mall (represented by a TAZ) and a frontage road … It is the only centroid connector of that TAZ.
Frontage road has capacity of 1444 vph , but than 6000 trip demands during 8am…
tens of thousands of trips sitting on the upstream links blocking all the roadways.
Solution: adding more centroid connectors
Network Clean up
Incorrect Network coding may cause illogical path. Its impact could be very severe in mesoscopic assignment
Missing turn prohibition Incorrect distance coded Lazy coding: one coded link to substitute
many links in real world
Impact of Incorrect Distance
The Frontage road coded as 0.2 miles instead of 1.1 miles
Freeway through traffic diverts to frontage road
Subsequent time slices showing illogical backup on other links
Example of Lazy codingOne link to represent all direct ramps
Detail CodingLazy Coding
Calibration Now in Calibration Phase of a normal day
assignment Identify (and fix) problem spots in the
network using two approaches:1.A static assignment to check for areas
were Volume greatly exceeds capacity2.Run DTA on sub-areas for faster run time
and easier problem identification, particularly network problem.
Conclusion - Discovery
Sufficient number of iterations is required to eliminate long travel time and nonsense backup
Clean network is necessary High V/C ratio link in static model will
cause severe congestion on adjoining links in DTA assignment
HCM curve is more suitable for DTA than volume delay curve for regional model
Conclusion - Progress
Develop probabilistic distribute to aggregate and to simulate fraction trips to integer trips
Replaces the “simplified” network with full network
Multi-class assignment adopted A single 72-hours simulation substitute 72
one-hour assignment, saving run time
Continuing Challenges
Calibrate the normal day scenario Mesh evacuation traffic with non-
evacuation traffic, as these two types of traffic behave very different.
Code traffic signals More network cleanup may be necessary Trip Table adjustment?