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Section 3: Travel Demand Modeling Process

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Overview

The Texas Package modeling process (see Figure 2-1) involves:

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Traffic Analysis Zones, Districts, and Sectors

The area MPO and TxDOT cooperatively develop traffic analysis zones (TAZs). TAZs are modified prior to each decennial census and may be modified during each travel model update when a new base year is established. Zones also may be modified between the base and the forecast years if anticipated development warrants change to the base year zone structure.

Subsequent to development of traffic analysis zones, TPP aggregates zones into districts and combines the districts into sectors. TPP geocodes and maps zones, districts, and sectors using TransCAD and provides maps to the MPO and local district office.

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Network Development

The development of the network involves identifying major roadways to be included, mapping those roadways, and collecting and developing data required to code the network. A base year and a forecast year network are required. The base year network includes roadways and conditions as they exist in the base year. The forecast year network will include the existing base year network plus committed network improvements. In subsequent model runs other network changes may be included for alternative analysis.

Network Preparation. The computerized network of the regional roadway system, zone centroids, and centroid connectors is developed cooperatively by TPP, the MPO, and local district office. TPP develops the computerized network from county road maps drawn from a United States Geological Survey (USGS) map and provides copies of the network maps to the MPO and local district office.

Network Data Collection. The MPO conducts a roadway inventory and collects the following data for each link in the roadway network:

  • number of lanes
  • posted speed limit
  • one-way or two-way facility
  • divided or undivided facility
  • legal parking conditions
  • 24-hour non-directional traffic volumes.

TPP develops additional roadway descriptive characteristics for each network link:

  • Link speed and capacity – The traffic assignment process uses roadway link speeds and capacity. TPP develops speed/capacity tables for network roadways based on area type, functional classification, and facility types.
  • Area types - Area type is one characteristic used to assign network speeds and capacities to individual roadway links. The coded network speeds and capacities within a given roadway functional classification vary by area type.

    TxDOT computes area types for each traffic analysis zone based on a density measure (see Figure 2-2) calculated on the zone population, employment, and area in acres provided by the MPO. Typically, five area types are used: CBD (Central Business District), Urban, Urban Fringe, Suburban, and Rural. In some areas a six area types are used, typically CBD, CBD Fringe, Urban, Suburban, Suburban Fringe, and Rural.

    Density Measure for Area Type Determination (click in image to see full-size image) Anchor: #i999861

    Figure 2-2. Density Measure for Area Type Determination

  • Functional classification and facility types - Each link of network roadways is identified by a functional class. The functional classification used for travel demand forecasting is not always consistent with the functional classification used by FHWA of funding allocation. Typical functional classifications (see Table 2-2) include Interstate highway, freeway, expressway, major and minor arterial, collector, local, ramp, and frontage road. Facility type such as divided or undivided roadway further subdivides functional classes. Functional and facility type classification schemes vary among urban areas.
  • Distance - The length of each network link is required in determining link travel time. TPP calculates distance during the process of digitizing the network from USGS maps.
  • Capacity and speed - The capacity assigned to each network link is cross-classified by functional classification, facility type, and area type. The capacities typically used are based on a service volume at level of service C and are derived from the Transportation Research Board Highway Capacity Manual. In some less congested urban areas, a threshold capacity level of service A or B may be used in the modeling process.

    The trip distribution process uses network link distance and speed to produce an estimate of link travel time, from which minimum path routes between all traffic analysis zones are developed. TPP develops a speed/capacity look-up table (see Table 2-3) for each urban area modeled. TPP derives the speeds found in this table from two sources: speed surveys of observed travel speeds and speed and typical roadway speed for facilities based on functional classification and level of service as found in the Highway Capacity Manual.

Anchor: #BGBECHCHTable 2-2. Two Typical Functional Classification Schemes

TxDOT Functional Class Code

Typical Facility Description

Alternate Facility Description

0

Centroid Connector

Interstate

1

Interstate, Expressway or Freeway

Major Arterial, 4 Lanes Divided

2

Multi-lane Highway or Rural Highway

Major Arterial, 4 Lanes Undivided

3

Divided Principal Arterial

Major Arterial, 2 Lanes

4

Undivided Principal Arterial

Minor arterial, 6 Lanes Divided

5

Divided Minor Arterial

Minor Arterial, 4 Lanes Divided

6

Undivided Minor Arterial

Minor Arterial, 4 Lanes Undivided

7

Collector

Minor Arterial, 2 Lanes Undivided

8

Frontage Road

Collector, 2 Lanes

9

Ramp

Local

A

None

Frontage Road

B

None

Ramp

C

None

Centroid Connector


Anchor: #BGBHJADJTable 2-3. Typical Speed/Capacity Look-up Table

Description

Lanes

Functional Class

CBD

Urban

Suburban

Suburban Fringe

Rural

Interstate

4D

0

45

103500

50

75000

55

49700

55

28400

60

23600

Major

Arterial

4D

4U

2U

1

2

3

40

N/A

40

N/A

40

N/A

45

N/A

45

N/A

45

N/A

50

25600

50

22800

50

10600

50

25600

50

22800

50

5800

55

28400

55

27000

55

5200

Minor

Arterial

6D

4D

4U

2U

4

5

6

7

37

64200

36

42800

33

3800

30

15800

42

44700

41

29800

38

26500

35

11000

47

28300

46

18900

43

16800

40

6900

47

17700

46

11800

43

10300

40

4200

52

N/A

51

N/A

48

N/A

45

N/A

Collector

2U

8

25

14100

30

9700

35

5900

35

3500

40

2300

Local

2U

9

20

14100

25

9700

30

5900

30

3500

35

2300

Frontage

2U

A

35

16900

40

12100

45

10600

45

5800

50

5200

Ramps

2U

B

35

16900

40

12100

45

10600

45

5800

50

5200

Centroid

Connector

C

15

20

25

25

30


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Trip Generation Data Development

Trip generation requires two types of data: socioeconomic data and travel behavior data.

Socioeconomic Data. The MPO, in cooperation with TPP, develops the following socioeconomic data for each traffic analysis zone in the urban area for the base year and the forecast year:

  • Total Population - The total population for each traffic analysis zone in the urban area for the base year and the forecast year.
  • Number of Households - The total number of households for each traffic analysis zone in the urban area for the base year and the forecast year.
  • Average Household Size - The average household size for each traffic analysis zone in the urban area for the base year and the forecast year.
  • Total Employment - The total employment for each traffic analysis zone in the urban area for the base year and the forecast year.
  • Basic Employment – The total employment in standard industrial classification (SIC) categories of mining, construction, manufacturing, wholesale trade, transportation, communication, and public utility groups.
  • Retail Employment – The total employment in SIC retail industry groups.
  • Service Employment – The total employment in SIC financial, insurance, real estate, education, government, and service industry groups.
  • Median Household Income – Median household income not adjusted for inflation.
  • Regional or Zone Distribution of Households by Household Size and Median Household Income – The number of households in each size and income category.
  • Special generator data (see Table 2-1) - The specific information needed for identified special generators.

Travel Behavior Data. TxDOT obtains information on travel behavior from travel surveys and uses it to develop trip generation and trip attraction rates for each urban area. A typical travel survey in Texas contains a household survey, a workplace survey, external station survey, commercial vehicle survey, special generator survey, and, where needed, an on-board public transit survey. In some non-attainment or near non-attainment areas an air quality survey may be conducted.

  • Household Survey – In households selected at random throughout the urban area, interviewers ask people who agree to participate to record in a diary the activities and travel by each person over the age of five years during a 24-hour period. For each trip, interviewers ask participants to record the time, activity, place the trip began and ended, mode of travel, number of passengers, purpose of the trip, and other descriptive information. In addition to the data on travel, participants record characteristics of the household such as number and age of persons in the household, number of household members employed, household income, and number of vehicles available. TPP uses data from the household survey to develop trip production rates for the urban area.
  • Workplace Survey - A workplace survey collects information on travel at the destination end of trips. This type of survey consists of two primary parts: one part designed to collect travel information and household characteristics (such as income, vehicle availability by type/make of vehicle, household size, etc.) of employees at the work sites, and one designed to collect travel data on visitors (non-employees) traveling to/from the workplace during the day.

    For the workplace survey, TPP cross-classifies employment establishments by industry type (basic, service, or retail), area type, and freestanding or non-freestanding business. Interviewers provide employees at participating workplaces with a survey and ask them to record all of their trips on a specified day. Data include the origin and destination for each trip, arrival and departure times, travel mode, reason for trip (trip purpose), vehicle occupancy, vehicle make/model/year, and transit and parking costs. Interviews with randomly selected non-employees arriving at the workplace during that day ask them to identify trip origin, trip purpose, mode of travel, vehicle occupancy, and arrival/departure times. TPP uses the data collected from the workplace surveys in conjunction with traffic or person counts at the specific workplace to develop attraction rates for basic, service, and retail employment for each area type.

  • External Station Survey - At each external station location, surveyors randomly select vehicles and interview the drivers to determine information on the trip purpose, the trip origin and destination, and the vehicle occupancy. TPP uses this information to estimate the number of trips originating outside the study area and traveling to a point inside the area, the number of trips beginning and ending at a point outside the study area (through trips), and trips originating inside the study area and destined to a point outside the study area.
  • Commercial Vehicle Survey – In some areas, TxDOT conducts a separate commercial truck survey to develop a more comprehensive database of travel patterns, vehicle weights and fuel types for commercial trucks operating in the urban area. TPP uses information collected from this survey to develop truck trip rates for trip production and in modeling for air quality conformity analysis. Participating firms provide data for each trip taken during one day’s travel. Information collected includes departure and arrival times, an address for each destination, truck types, truck weight, fuel type, and truck routes traveled.
  • Special Generator Survey - Special generator surveys collect information on travel patterns for employees and visitors at sites that exhibit special trip generating characteristics. TPP uses data obtained in the surveys to develop trip attraction rates by trip purpose for each site.
  • On-Board Public Transit Survey - An on-board survey of bus passengers collects information on current bus rider characteristics and to provide data to develop a representative origin-destination trip table for use in the travel demand models. Data collected include trip origins and destinations, mode of travel to/from bus stop, trip purpose, bus routes taken for trip, ridership frequency, fare paid and method of payment, vehicle availability, household size, and income.
  • Air Quality Survey – In some non-attainment or near non-attainment areas, TxDOT conducts air quality travel surveys to collect speed, delay and vehicle classification. TPP uses the data from such surveys to validate the travel model trip assignment speeds used in air quality conformity analysis.

TxDOT conducts travel surveys in cooperation with MPOs. TxDOT analyzes data from the surveys to develop trip production and attraction rates used in trip generation. The conduct of travel surveys is determined by TxDOT on an annual basis and is dependent on need and funding. Travel surveys have been completed in 12 urban areas (see Table 2-4) since 1990.

Anchor: #i1009923Table 2-4. Travel Surveys Completed Since 1990

Urban

Area

Survey

Year

House-

hold

Work-

place

External

Commercial

Vehicle

Air

Quality

Data

On-Board

Transit

Special

Generator

Amarillo

1990

X

X

X

X

X

Austin

1997/98

X

X

X

X

X

X

X

Port Arthur

Beaumont/

1993

X

X

X

X

X

X

Brownsville

1991

X

X

X

X

X

Corpus

Christi

1996

X

X

X

X

X

Dallas

1994/95

X

X

X

El Paso

1994

X

X

X

X

X

X

X

Houston

1994

X

X

X

X

X

X

X

San Antonio

1990

X

X

X

X

X

X

Sherman/

Denison

1991

X

X

X

X

X

Tyler

1990/91

X

X

X

X

X

Victoria

1995/96

X



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Trip Distribution Data Development

In addition to the network data, trip distribution data require: trip length frequency distribution curves for each trip purpose, a zone radii parameter that provides the distance in minutes of travel time from the center point of the zone centroid to the nearest network point on the perimeter of the zone, and estimates of bias factors by trip purpose.

  • Trip Length Frequency Distribution - A trip length frequency distribution (TLFD) model is available as a part of the trip distribution model set. Input needed for the TLFD includes an estimate of the mean trip length and the maximum trip length.
  • Zone Radii (Centroid Distance Parameter) - This measures the distance in minutes of travel time from the center point of the zone centroid to the nearest point on the perimeter of the zone.
  • Bias Factors - Certain social and economic factors other than travel time affect travel patterns within individual urban areas. TPP develops these factors for each trip purpose as needed based on data from survey results and gravity model results.
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Model Calibration

Model calibration is an attempt to duplicate travel for the year in which the field data (i.e. travel survey, network, and other data) are collected. In urban areas without recent travel surveys, TPP estimates values for trip rates, bias factors, mean trip lengths, and trip length frequency distribution. The calibration process includes intuitive tests of the models to see if the variables and coefficients are reasonable. TPP compares the output of each model individually to base year data.

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Model Validation

Validation is the process of applying the calibrated models sequentially in the base year. TPP validates a model for the base year to determine its predictive ability to replicate observed traffic counts using the trip rates, bias factors, and other variables estimated in calibration. If the series of models cannot produce traffic volumes similar to what is observed in the base year, then TPP re-evaluates the models and makes appropriate adjustments until validation is accomplished.

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