Lec 27: Ch3.(T&LD): Traffic Analysis –
Directional distribution of site traffic
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Notice the difference in the trip distribution
process between urban transportation planning
and site planning
Understand several factors affect directional
distribution of site-generated traffic
Be familiar with methods used to distribute site
traffic to surrounding roads
Directional distribution of site traffic
The assumptions and method used in estimating the direction
in which traffic will approach and depart the site will vary with
several location-specific conditions, such as;
 Size of the proposed development
 Type of development (e.g. industrial, commercial, residential,
etc.)
 Prevailing conditions on the existing street systems
 Available data base
Avoid evaluating TIA based on
“mechanical method”  A very good
warning from the authors.  Choose a
method appropriate for the situation
rather than using a canned procedure.
(P.49)
Primary market
Analogy
OD (easy said, hard
done)
Gravity method (we did
this already.)
Primary market
The primary market (area-of-influence) method utilizes a
unique trade area or area of influence. It is the geographical
area from which a high percentage (80%) of the sitegenerated traffic will be “drawn” to the site.
The boundary of the trade area might be identified by one of the
following techniques:
Method 1: Delineate the trade area as a regular geometric
shape (a circle or square with the site at the center) with a
radius of travel appropriate for the type of development
Method 2: Delineate the trade area by establishing the most
distant points that can be reached within some selected travel
time over the specific street system
Method 3: Use Reilly’s Law of Retail Gravitation to establish
the boundary between competing centers
Maximum travel times (how far people travel?)
Travel time (not distance) and trip
purposes are the determinants of trip
length.
Community shopping
center: 15-20 min
Neighborhood shopping: 10
min
Industrial park: 30 min
Residential: peak-hour = 30
min; Other = 20 min
Method 1: Defining the study area
Step 1: Select the
appropriate max trip
length for the study site
Step 2: Draw the selected
geometric shape (a circle
or a rectangle) with the
center located over the
site
(Steps 2 & 3)
Method 1: Defining the study area (cont)
Step 3: Divide the trade
area into sections (zones)
and determine the amount
of activity in each.
Step 4: Calculate the
proportion of each section
(zone) as a percentage of
the total primary market
area
(Step 4)
Method 1: Defining the study area (cont)
Step 5: Identify the most
logical route from the
centroid of each section
(zone) to the site.
Step 6: Calculate the
directional distribution by
assigning the percentage
from step 4 to the
minimum paths identified
in step 5.
34%, not 14%
(Step 5)
Method 2
This method uses the
travel distance calculated
along each route using the
appropriate maximum
travel time and the travel
speed(s) on each street.
Results in an irregular
geometric shape. Steps 1
and 2. Travel time = say 10
min. Speeds on routes are
different  resulting in
different distances.
Steps 3 through 6 are
the same as Steps 3
through 6 of Method 1.
(Hah!? No shape shown!)
Method 3: Reilly’s Law of Retail Gravitation
(used when competing markets exist)
Reilly’s law:
PA
A
d AB
dA 
PB
1
PA
Pi: size of center I,
commonly
measured in 1000
sq. ft. of gross
leasable area
PB
dA
dAB
A’s market boundary
B
Method 3: Reilly’s
method (used when
competing markets
exist) (cont)
Once the boundary
is defined, the rest
is like step 3 though
6 of Method 1.
14 min
11 min
15 min
(See page 53 for an
example)
Analogy (p.54)
Identify existing situations which have the “same”
characteristics as expected of the subject. A traffic survey of a
similar development in close proximity to the subject site
follows this methodology.
In many cases, a manual count of turn movements is
sufficient to obtain the necessary data for analysis of the
direction of approach. With a large development where there
are several approach streets, a license-plate survey might be
used (find out how customers drive through the network to
get to the site).
Similar site
Subject site
(See p.54 for possible applications)
Analogy (p.54) (cont): Example in p.54
(Turn movements used to determine
traffic distribution)
(Assumed site-traffic distribution)
Find out %
distribution:
from north or
from south
Note that these two sites
face the same Main St.