1. Norman Washington Garrick CE 2710 Spring 2016 Lecture 07
Transportation Forecasting
Trip Generation
Norman Washington Garrick CE Spring Lecture 07

2. The Four Step Model Trip Generation
Estimates the number of trips from a given origin and the attractiveness of a given destination
Trip Distribution
Determines the destination for each trip from a given origin
Mode Choice
Determines the mode choice for each trip
Route Assignment
Determines the specific route for each trip

3. Trip Generation
Trip Generation model is used to estimate the number of person-trips for a given traffic analysis zone (TAZ). These are referred to as trip production for trips starting in that TAZ and trip attraction for trips going to that TAZ 1 2 3 4 5 6 8 7
The unit of analysis for traffic generation is the TAZ

4. Trip Generation Developing and Using the Model
Survey Base Year
Socio-economic, land use
and
Trip making
Estimated
Target year
socio-economic,
land use
characteristics
Calibrated
Model
Relating trip making
to socio-economic
and land use
characteristics
Predict
Target year
No. of Trips

5. Trip Generation Travel Survey
Trip Generation models are often developed from travel surveys.
These surveys are used to determine the trip making pattern for a sampling of households in the area.
This trip making pattern is then related to land use and socioeconomic factors that are considered to affect travel patterns
Common socioeconomic factors considered include household size, income, and auto ownership rates

6. Trip Generation Trip Purpose
Often separate predictions are mode for different type of trips since travel behavior depends on trip purpose
In other words different models must be developed for each trip type
The category of trip types commonly used include
Work trips
School trips
Shopping trips
Recreational trips

7. Trip Generation Form of the Model
The trip generation model typically can take the form of
No. of trips = Function (population, income, auto ownership rates)
The model is developed and calibrated using the BASE year data

8. Trip Generation Example of a Trip Generation Model
One way of presenting the trip generation model developed from a survey is as a cross-classification table

9. Table 1 Trip Rates Cross-classification Table
Home-Based-Non-Work Trip Rates
Persons per Household
Type of Area
Vehicles per HH 1
2,3 4 5+
High Density
0.6
2.1
4.6
7.0
1.5
3.0
5.5
7.9 2+
1.8
3.4
5.9
8.3
Low Density
1.0
2.5
5.0
7.4
1.9
3.5
6.0
8.4
2.3
3.9
6.4
9.0
Total Home-Based-Non-Work Trip Rates
Persons per Household
Type of Area
Vehicles per HH 1
2,3 4 5+
High Density 2+
Low Density

10. Trip Generation Estimating Target Year Data
Calibrated
Model
Relating trip making
to socio-economic
and land use data
Estimated
Target year
socio-economic,
land use data

11. How Many Home-Based-Non-Work Trips are made from TAZ 5 in 2020?1 2 3 4 5 6 8 7
If we assume that TAZ 5 is a high density area and that
the trip rate model in Table 1 applies to this TAZ then…
We will need to get for TAZ 5 the number of households in each of the
12 categories for number of vehicles and number of persons
For example, how many HH in TAZ 5 will have 0 cars and 1 person?

12. Table 2 2020 Demographic Data for TAZ 5
Number of Households in 2020
Persons per Household
Vehicles per HH 1
2,3 4 5+
100
200
300 2+
Number of Households in 2020
Persons per Household
Vehicles per HH 1
2,3 4 5+ 2+

13. Table 3 Trip Rates for TAZ 5
Trip Rates in 2020 for TAZ 5
Persons per Household
Vehicles per HH 1
2,3 4 5+
0.6
2.1
4.6
7.0
1.5
3.0
5.5
7.9 2+
1.8
3.4
5.9
8.3
Trip Rates in 2020 for TAZ 5
Persons per Household
Vehicles per HH 1
2,3 4 5+ 2+
We use the values from Table 1 for high density areas

14. Trip Generation Predicting Number of Trips
Calibrated
Model
Relating trip making
to socio-economic
and land use data
Estimated
Target year
socio-economic,
land use data
Predict
Target year
No. of Trips

15. Table 4 Predicted Number of HBNW Trips for TAZ 5
HBNW in 2020 for TAZ 5
Persons per Household
Vehicles per HH 1
2,3 4 5+ 2+
HBNW in 2020 for TAZ 5
Persons per Household
Vehicles per HH 1
2,3 4 5+ 60 2+
Number of Trips = trip rate*no. of HH = 0.6 * 100 = 60

16. Table 4 Predicted Number of HBNW Trips for TAZ 5
HBNW in 2020 for TAZ 5
Persons per Household
Vehicles per HH 1
2,3 4 5+ 60
420
460
700
1640
300
900
1100
790
3090 2+
180
680
590
1660
3110
Total
540
2000
2150
3150
7840
Total Number of HBNW Trips from TAZ 5 is 7840

17. Trip Generation Planning for the Future and Uncertainties
Earlier we talked about the uncertainties associated with making prediction for the future and the importance of not treating predictions as if they are set in stone but rather as a guide to help in decision making
In considering the ‘trip generation’ process it is important to understand some potential sources of uncertainties

18. Trip Generation Sources of Uncertainties in Predicting Number of Trips
Significant errors can creep into the trip generation process in a number of places including
Errors in the survey
Who is surveyed, how well was the survey constructed, did we consider all important parameters?
Errors in the prediction of future demographics
Will the population grew, what will be the make up of the population
Errors in how well the model can actually reflect the future
Will the land use change, will transportation change, will technology change, will peoples attitudes change

19. Trip Generation Effect of Changes in Land Use
Changes in Land use and the type of transportation provided can have a huge impact on travel
But the trip generation process typically assume that this factor is constant over the period of the study

20. Trip Generation Demographics and Trip Making Factors affected by Land Use
The land use pattern and transportation type may affect
Car ownership rates
Household size and composition
Number of daily trips
Mode of trips
Length of trips