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					Trip Distribution
         and Assignment



             Modeling Methods
Traffic Impact Analysis
Modeling 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
Modeling Methods
         • FSUTMS–Florida Standard Urban
           Transportation Model Structure
            – Florida’s 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)
Travel Demand Modeling
  • Four-Step Process
                           – Other auxiliary modules
    Trip Generation          include network building
                             and calculating zone-to-
                             zone travel times.


       Trip Distribution

         Mode Split

           Trip Assignment
FSUTMS Trip Generation
     SFDU MFDU Hotel      Employment
    1 xxx xxx   xxx       xxx xxx xxx
    2 xxx xxx   xxx       xxx xxx xxx
    3 xxx xxx   xxx       xxx xxx xxx
    4 xxx xxx   xxx       xxx xxx xxx
            ZONEDATA

                                   Generation
     Special Generators
     2 xxx xx xxx xx
     3 xxx xx xxx xx

       SPECGEN                          TAZ   PRODS   ATTRS
                                         1     xxxx     xxxx
                                         2     xxxx     xxxx
                                         3     xxxx     xxxx
                                         4     xxxx     xxxx
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 A’s
         (usually downward) until number of P’s and A’s in
         the entire MPO are equal.
FSUTMS Trip Distribution
  PRODS


           Distribution

  ATTRS




                                      Person-Trip Table
                                                T O______
                                           1   2 3 4




                          F R O M__
                                      1   xx   xx xx xx
                                      2   xx   xx xx xx
                                      3   xx   xx xx xx
                                      4   xx   xx xx xx
Trip Distribution
• Inputs:
  – P’s and A’s 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.
FSUTMS Mode Split


    Person-
      Trip    Mode Split
     Table




                                       Vehicle-Trip Table
                                                  T O______
                                             1   2 3 4




                           F R O M__
                                        1   xx   xx xx xx
                                        2   xx   xx xx xx
                                        3   xx   xx xx xx
                                        4   xx   xx xx xx
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).
 FSUTMS Highway Assignment

                                       Vehicle-
                                         Trip                                               Assignments
                                        Table


                                             xxx
                               xxx




                                                                          xxx
                                       xxx
                                                           xxx                   xxx
                   xxx
xxx    xxx
                                             xxx




                                                                          xxx
                         xxx
             xxx
                                                                                xxx
                                       xxx               xxx
                                                                                                       Network Link Volumes
                                 xxx



                                             xxx




                   xxx                 xxx         xxx
                                                                                                     From    To
                                             xxx
                                 xxx




                                                                                xxx
                                                                 xxx




 xxx

                                       xxx           xxx          xxx                                Node Node       Volume
                                                                                      xxx
                                 xxx




                                                                        xxx
                                                                                                     1001   1002       xxxx
                                             xxx




                                                                                                     1001   1003       xxxx
                                                                          xxx




       xxx               xxx           xxx
                                             xxx




                                                     xxx                                             1002   1004       xxxx
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.
FSUTMS vs. ITE Trip Gen
Which 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
FSUTMS vs. ITE Trip Gen
Which 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.
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
Replicating ITE Results in FSUTMS
(For the proposed development’s TAZs)
   • Two alternative methods:
       – Special Generator method
           • uses model’s 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
    Special Generator Method
      • Uses SPECGEN input file
      • Requires several assumptions to factor the
        development’s 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



ey Logo / Arena in Miami
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
      development’s ITE external trip generation
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
             Employees per 1,000 sq ft–Typical Rates
          Office: 4                   Industrial: 2
          Hospital: 3                 Warehousing: 1
          Retail <200KSF: 3           Hotel: 1 per room
          Large-scale retail: 2                 Source: ITE, ULI




 • Execute full FSUTMS model run
Link Distribution Percentages Method

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



    Apply ITE total external generation
      to link percentages
       Development trips for each roadway link =
                 Link % x Total External ITE Generation
Link Distribution Percentages Method
                                  ITE-Generated
                            Development Trip Loadings
     FSUTMS Output
    (Development Trips)
                                      External
                                    ITE = 15,000

               10,000


       3,000    7,000                                15,000
        30%     70%

        Distribution
          Factors               15,000      15,000
    (manually calculated)       x 30%       x 70%
                                 4,500      10,500
Link Distribution Percentages Method Benefits

     • Easier application
     • Generally similar results as SPECGEN
       method
     • If development’s land use quantities
       change slightly, no need to rerun model
        – CAUTION: Significant land use changes
          may affect distribution patterns, requiring
          a new model run
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
FSUTMS Site Impact Analysis
HIGHWAY 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
Selected Zone Analysis Procedures
  • Input proposed development’s 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
Using MOCF
Model 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).
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
Internal Trips & FSUTMS
  Q .   Is FSUTMS the best method for
        estimating the number of internal trips?
  A .   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.
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
Selected Zone
Versus “With & Without”
 • “With & Without” Methodology
    – Two separate model runs, one with proposed
      development in place, the other with development’s
      zonal data zeroed out
    – Link volumes for “without” run subtracted from “with”
      run, yielding net impact of development on each
      roadway link
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.”
“With & Without” Method
    FSUTMS Total Trips
    (With Development)
                            “Net” Impact???
               10,000


                                       10,000
     50,000    45,000

                              50,000    45,000
     FSUTMS Total Trips     - 48,000   -48,000
    (Without Development)      2,000    - 3,000
                                           or
                                           Ø
                0


      48,000     48,000
Selected Zone Versus
“With & Without”
 • Court’s 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.
“With & Without” Method
   FSUTMS Total Trips
   (With Development)
                            “Net” Impact???
               10,000


                                       10,000
     50,000    45,000

                              50,000    45,000
     FSUTMS Total Trips     - 48,000   -48,000
    (Without Development)      2,000    - 3,000
                                          or
                                          Ø
                0


      48,000     48,000
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
FSUTMS Resources
• www.fsutmsonline.net
  – Model Task Force
  – FSUTMS training workshops
  – Modeling newsletter
  – Documentation
  – Technical support

				
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posted:8/31/2011
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