Describe and illustrate cross-classification procedures for (a) trip production and (b) trip attraction.
What will be an ideal response?
The FHWA method for estimating trip productions and attractions is based
on the use of cross classification. Cross classification is a method used to
determine the number of trips that begin or end at the home. Relationships are
developed based on socioeconomic data (typically income data) and origindestination
surveys. Determining the number of productions and attractions
involves a process of 5 steps which determine the percentage of trips by purpose,
i.e. the number of trips that are home-based work (HBW), home-based other
(HBO), and non-home based (NHB). From these percentages the number of
productions and attractions can be determined.
Step 1: Determine the percentage of households in each economic category (low,
medium, high). Develop a plot of average zonal income versus income
distribution.
Step 2: Determine the distribution of auto ownership per household for each
category. Draw a curve showing percent of households, at each income level, that
owns x autos.
Step 3: Determine the number of trips per household for each income-auto
ownership category. Draw a curve showing the relationship between trips per
household, household income and ownership.
Step 4: Calculate the total number of trips generated in the zone. This is done by
computing the number of households in each income-auto ownership category
and multiplying this result by the number of trips per household, as determined in
step 3, and summing the results. Thus,
Ti = (HH) × (Ii) × [?(Aij) × (T / HH)ij]
Where:
Ti = trips generated by income group i
HH = number of households in the zone
Ii = percentage of households in the zone with income level i
Aij = percentage of households in income level i with j autos per
household
(T / HH)ij = number of trips produced in a household at income level i and
auto ownership j.
Total trips generated: TT = ?Ti
Step 5: Determine the percentage of trips by purpose.
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