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Update on Developing Evacuation Model using Dynamic Traffic Assignment. ChiPing Lam, Houston-Galveston Area Council Matthew Martimo, Citilabs. Review last Presentation. During Rita Evacuation, evacuation routes were very congested. “Crawling parking lot.”
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ChiPing Lam, Houston-Galveston Area Council
Matthew Martimo, Citilabs
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
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.
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.
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.
Example: Drive Alone Free Trip Table
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
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).
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
When Compared with Macroscopic Assignment:
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
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
Now in Calibration Phase of a normal day assignment
Identify (and fix) problem spots in the network using two approaches:
A static assignment to check for areas were Volume greatly exceeds capacity
Run DTA on sub-areas for faster run time and easier problem identification, particularly network problem.