Lec 26: Ch3.(T&LD): Traffic Analysis –
Trip generation
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Know the definition of site-oriented and nonsite traffic
Understand the phases of site impact traffic
evaluation (S.I.T.E.)
Learn how to estimate trips generated by a
development
Know how to use ITE Trip Generation Handbook
Site-oriented traffic vs. Non-site traffic
Site-oriented
traffic (trips
having an OD
on the site
With the
development
(new or
existing)
Non-site traffic (no
OD on the site)
Without the
development
“SITE” Methodology
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Phase I: Establish existing traffic conditions
without the site (proposed)
Phase II: Forecast future traffic conditions
without the site
Phase III: Forecast future site-only traffic
conditions
Phase IV: Determine total future traffic
conditions (II + III)
Phase V: Evaluate alternative improvements
Phase VI: Negotiations
Phase VII: Implementation
Flow chart of SITE process
Phase I
Phase II
Input from
MPO and
local/state
agencies
Existing Background
Traffic
Calculate
LOS
Traffic growth
factors
Future background
traffic w/o site
Forecast traffic
for other new
adjacent sites
Combine
Calculate
LOS
Compare
Flow chart of SITE process (cont)
Phase III
Forecast traffic for site
development
Phase IV
Combine
Total traffic
Calculate
LOS
Compare
Flow chart of SITE process (cont)
Phase V
Develop alternative
solutions
Evaluate
solutions
Document
findings
Feedback
Phase VI
Begin negotiations between
local officials and developers
Perform supplemental
technical analyses
Feedback
Phase VII
Assist in preparing
construction plans
Implementation
Site-oriented traffic (site-generated trips): much
easier to estimate because…
The land use activities are known
The number of trips expected to be generated by the
proposed development can be determined using the ITE Trip
Generation Handbook
The present and future directional distribution can be
estimated
5 Primary
characteristics
 The total number of trips generated
 The number of trips to and from the site during the
peak hours of the adjacent street
 The number of trips to and from the site during the
peak(s) of the generator
 Daily variation
 Monthly variation
Unit used as the base variable for tripgeneration rates
Be functionally related to the volume of traffic generated
(e.g. 48 trip ends per 1000 sq.ft floor area, 5.7 trip ends per
occupied dwelling unit)
Be relatively easy to establish/measure (meaning easy to
measure the size, like size of an household, size of a store)
Provide consistent and transferable rates (Although
variations exist, establishments of similar characteristics have
similar trip generation rates.
(See the handout)
Issues (pp. 29, 39, 40)
Age of data base – Not much difference between old and new data
Daily variations – Peaking of traffic varies depending on the type of
development
Location variation – ITE trip generation data came from suburban areas
(for vehicular trips)
Pass-by traffic – Some generators attract traffic from the passing traffic
stream
Mixed-use development – Mixed-use land may produce less trips than
the sum total of individual land uses (many rates are derived from standalone developments)
Vehicular trips – The generation rates are vehicular trips for “average”
vehicle occupancy
Variability in rates – variation in data
Sample size – Some have only a small number of samples
Pass-by Traffic
Land use activities such as offices, industrial parks,
hotels, high-quality restaurants, and residential
development generate traffic which intends to have a trip
end at the particular site and therefore constitute new
traffic on the street system.
However, a sizable portion of the traffic generated by
fast-food restaurants and gas stations is already on the
adjacent street and merely stops at the establishment in
passing by  I.e. pass-by traffic. They are NOT new
traffic. They were there but because a new establishment
was developed, they just decided to make a detour.
See Table 3-4 for variation of the
amount of pass-by traffic by
generator type and size, and Table 35 for daily variation, and Table 3-6 for
weekly variation
Mixed-use development
When an activity center is developed with a mix of different land
uses (e.g., retail and office), there are fewer trips on and off the
site than if the retail and office activities are developed on
separate sites. In a mixed-use development, some individuals will
complete two or more purposes without leaving the site.
Shopping center
(See Table 3-7 and 3-8)
Satellite development
Regional shopping centers attract other commercial retail
developments to property surrounding the center. The
interaction (shared trips) may be 10% or more of the traffic
generated by a satellite development. The number of
shared trips will be subtracted when it is loaded to the
external roadway. Note: this is not a chained-trip. A
chained trip is a trip that combines different trip purposes
(work, shop, day-care drop off, etc.).