Assignment

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Trip Distribution and Assignment
Modeling Methods
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Traffic Impact AnalysisModeling Methods

• Why
• Most manual distribution and assignment
  techniques include numerous subjective inputs
• Models offer an MPO-adopted tool to aid in
  distributing and assigning traffic

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Modeling Methods

• FSUTMSFlorida Standard Urban Transportation
  Model Structure
• Floridas standard model used in all 26 MPOs
• FSUTMS uses Cube Voyager software engine
• Effective for site impact analysis when combined
  with manual techniques
• At a minimum, FSUTMS trip generation outputs must
  be verified with manual analysis (ITE)

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Travel Demand Modeling

• Four-Step Process

• Other auxiliary modules include network building
  and calculating zone-to-zone travel times.

Trip Generation
Trip Distribution
Mode Split
Trip Assignment
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FSUTMS Trip Generation
Generation
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Trip Generation

• Inputs
• Number of homes and employees (ZONEDATA file) in
  each traffic analysis zone (TAZ)
• Outputs
• Number of Production trip ends (generated by
  homes) and Attraction trip ends (generated by
  employment sites) for each zone

Note The model adjusts the number of As
(usually downward) until number of Ps and As in
the entire MPO are equal.
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FSUTMS Trip Distribution
PRODS
Distribution
ATTRS
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Trip Distribution

• Inputs
• Ps and As calculated in trip generation
• Outputs
• The number of person-trips traveling between each
  zone pair in the MPO area
• Production trip ends are matched up with
  attraction trip ends from throughout the network
  to form person-trips, using a gravity model.

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FSUTMS Mode Split
Mode Split
Person- Trip Table
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FSUTMS Mode Split

• Inputs
• Person-trip table from trip distribution
• Outputs
• Vehicle-trip table
• Vehicle occupancy factors or transit usage
  equations are applied to convert person-trips to
  vehicle-trips
• Vehicle occupancies differ by trip purpose
  (e.g., work trips generally have fewer passengers
  than recreational trips).

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FSUTMS Highway Assignment
Assignments
Vehicle- Trip Table
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Highway Assignment

• Inputs
• Vehicle-trip table
• Outputs
• Number of trips upon each roadway link
• Vehicles are routed along specific roadways to
  arrive at the zone they desire to reach.

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FSUTMS vs. ITE Trip GenWhich is better for Site
Impact Analysis?

• Intended Purpose
• FSUTMS
• Designed to replicate MPO areawide travel
  patterns
• Calibration Avg Link Volume-To-Count
  comparisons
• NOT calibrated to zone-specific traffic volumes
• ITE
• Designed to find correlation between land use
  quantities and number of trips entering and
  exiting specific types of developments

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FSUTMS vs. ITE Trip GenWhich is better for Site
Impact Analysis?

• Application
• FSUTMS
• Does not calculate Trips for employment sites.
  Rather, the model calculates Attractions, which
  are only measures of relative attractiveness to
  draw Productions to the zone.
• ITE
• Calculates the average number of trips generated
  by varying quantities of specified land uses.

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So what is FSUTMS good for?

• Key strengths
• Trip distribution and assignment
• Non-development trips
• Therefore
• The best modeling methodology for traffic impact
  analysis
• Uses the gravity model and capacity-restrained
  assignment
• But, calculates the number of development trips
  consistent with ITE

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Replicating ITE Results in FSUTMS(For the
proposed developments TAZs)

• Two alternative methods
• Special Generator method
• uses models SPECGEN input file
• requires running the model and adjusting several
  times
• Link Distribution Percentages method
• uses conventional ZONEDATA input file
• single model run indicates percentage of total
  external development traffic traveling on each
  link

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Special Generator Method

• Uses SPECGEN input file
• Requires several assumptions to factor the
  developments ITE-generated trips
• Vehicle-trips to person-trips (using vehicle
  occupancy factors)
• Percent trips by purpose (work, shopping,
  social-recreational, non-home-based, etc)
• Percent productions versus attractions

Disney Logo / Arena in Miami
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Special Generator Method

• Requires trial-and-error attempts to match FSUTMS
  to ITE
• Check final trip table after FSUTMS trip
  attraction balancing and vehicle occupancy
  factoring
• Compare to ITE calculations
• Adjust SPECGEN inputs and rerun model until
  traffic loadings to/from development zones match
  developments ITE external trip generation

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Link Distribution Percentages Method

• Insert dwelling unit and employment estimates
  into conventional ZONEDATA input file
• Requires converting square feet to industrial,
  commercial, and service employment
• Execute full FSUTMS model run

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Link Distribution Percentages Method

• For each roadway segment, calculate development
  traffic percentage
• Development traffic on link
  .
• Total external generation of site (modeled)

Link
? Apply ITE total external generation to
link percentages Development trips for each
roadway link Link x Total External ITE
Generation
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Link Distribution Percentages Method
ITE-Generated Development Trip Loadings
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Link Distribution Percentages Method Benefits

• Easier application
• Generally similar results as SPECGEN method
• If developments land use quantities change
  slightly, no need to rerun model
• CAUTION Significant land use changes may affect
  distribution patterns, requiring a new model run

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FSUTMS Highway Assignment

• Based on a capacity-restrained, equilibrium
  assignment routine
• Capacity restraint accounts for route diversion
  that occurs when drivers encounter congestion
• FSUTMS decreases speeds on congested roadways
  after each iteration of the assignment, until
• Equilibrium is achieved when all trips in the MPO
  area have found the least congested,
  shortest-time path to their destination

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FSUTMS Site Impact AnalysisHIGHWAY ASSIGNMENT

• Uses the Selected Zone analysis tool
• Single assignment tracks total trips and
  development trips throughout the
  capacity-restrained roadway network
• Background traffic Total minus development trips

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Selected Zone Analysis Procedures

• Input proposed developments land use into zonal
  data
• Run FSUTMS
• Display traffic that enters/exits development
  zone(s) on the loaded network using the traffic
  assignment path file
• Save development traffic as a new link attribute
  for further analysis
• Check for reasonableness

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Using MOCFModel Output Conversion Factor

• Theory behind use
• Land use inputs reflect peak-season occupancy
  rates
• Base-year traffic counts used during validation
  represent peak-season volumes
• Therefore, traffic volumes calculated by the
  model reflect peak-season weekday average daily
  traffic (PSWADT).

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MOCF

• AADT needed to calculate peak hour traffic
  volume
• AADT x K100 100th Highest Hourly Volume
• MOCF converts PSWADT to AADT (always reduces
  model output volumes)
• MOCF source Florida Traffic Information disc
  Peak Season Factor reports
• CAUTION Use MOCF for appropriate part of
  county (e.g., tourist area vs. rural) do not
  reduce development trips

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Internal Trips FSUTMS

• . Is FSUTMS the best method for estimating the
  number of internal trips?
• . FSUTMS is only a tool that may help in a manual
  determination of internal trips.

Caution Size (land area) of TAZs and length of
centroid connectors are the prime determinants of
intrazonal trips in FSUTMS. (Longer centroid link
more intrazonal trips.) The model does not
consider whether the specific land use types in
the proposed development are compatible and thus,
conducive to internal trip making.
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Selected Zone Versus With Without

• Selected Zone Analysis
• Single model run with total trips and development
  trips
• Development trips are tracked within the total
  trip assignment paths
• Retains the capacity-restraint trip diversion
  that occurs in the assignment of total trips

Show with
Show without
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Selected Zone Versus With Without

• With Without Methodology
• Two separate model runs, one with proposed
  development in place, the other with
  developments zonal data zeroed out
• Link volumes for without run subtracted from
  with run, yielding net impact of development on
  each roadway link

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Selected Zone Versus With Without

• With Without methodology presents a problem
• Equilibrium highway assignment capacity-restraint
  equation diverts trips, often resulting in
  virtually no change in traffic volumes
• Developer So, what! Diversion occurs in the
  real world. I should only be required to
  mitigate for net impacts of the development.

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With Without Method
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Selected Zone Versus With Without

• Courts Ruling
• DRI process requires accounting for ALL trips
  caused by development, NOT net impact resulting
  from displacing existing trips to other roadways
• Rationale if all developers used the argument
  that trips are diverted and net impact is
  negligible, no developer would be responsible for
  mitigation

Westinghouse Gateway Communities, et al. v. Lee
County Board of County Commissioners Case Nos
90-2636DRI and 90-2638DRI, Jan 14, 1991.
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With Without Method
UNACCEPTABLE
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Distribution/Assignment Summary

• Definition Identifying traffic volumes on each
  roadway link
• Total traffic
• Development traffic
• At future analysis years
• Next step after assignment Determine if any
  links are
• Operating at adverse LOS
• AND
• Carrying a significant volume of development
  traffic

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FSUTMS Resources

• www.fsutmsonline.net
• Model Task Force
• FSUTMS training workshops
• Modeling newsletter
• Documentation
• Technical support