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 include network building and calculating zone-to- zone travel times. Trip Generation Trip Assignment Mode Split Trip Distribution

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 Special Generators 2 xxx xx xxx xx 3 xxx xx xxx xx SPECGEN Generation TAZ PRODS ATTRS 1 xxxx xxxx 2 xxxx xxxx 3 xxxx xxxx 4 xxxx xxxx FSUTMS Trip Generation

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.

Distribution PRODS ATTRS Person-Trip Table T O ______ xx xx xx xx 2 xx xx xx xx 3 xx xx xx xx 4 xx xx xx xx F R O M __ FSUTMS Trip Distribution

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.

Person- Trip Table Mode Split Vehicle-Trip Table T O ______ xx xx xx xx 2 xx xx xx xx 3 xx xx xx xx 4 xx xx xx xx F R O M __ 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).

Vehicle- Trip Table Assignments xxx Network Link Volumes From To Node Node Volume xxxx xxxx xxxx FSUTMS Highway Assignment

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 Disney 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 Execute full FSUTMS model run Employees per 1,000 sq ft–Typical Rates Office: 4 Industrial: 2 Hospital: 3 Warehousing: 1 Retail <200KSF: 3Hotel: 1 per room Large-scale retail: 2 Source: ITE, ULI

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

External ITE = 15,000 15,000 x 30% 4,500 15,000 x 70% 10,500 3,000 7,000 10,000 FSUTMS Output (Development Trips) 30% 70% Distribution Factors (manually calculated) ITE-Generated Development Trip Loadings Link Distribution Percentages Method

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 K 100 = 100 th 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

“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.” Selected Zone Versus “With & Without”

“With & Without” Method FSUTMS Total Trips (With Development) 50,000 45,000 10,000 FSUTMS Total Trips (Without Development) 48,000 0 “Net” Impact??? 50,000 45, , ,000 2, ,000 or Ø 10,000

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: DRI and DRI, Jan 14, 1991.

“With & Without” Method FSUTMS Total Trips (With Development) 50,000 45,000 10,000 FSUTMS Total Trips (Without Development) 48,000 0 “Net” Impact??? 50,000 45, , ,000 2, ,000 or Ø 10,000 UNACCEPTABLE

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 –Model Task Force –FSUTMS training workshops –Modeling newsletter –Documentation –Technical support