FHA Signal Timing On A Shoestring

proportional to the size of the two bodies (traffic volume) and inversely proportional
to the distance squared. In equation form, the Coupling Index is:
Where:
CI = V / D 2
CI = Coupling Index
V = 2-way total traffic volume peak hour / (1000 vph)
D = Distance between signals (miles)
There are several variations on this approach. The “Linking Factor” as used by Computran
in Winston Salem, NC, and the “Offset Benefit” as described in NCHRP Report 3-18 (3) two
examples of different similar techniques that have been used to determine signal group
boundaries
A recent review 4 and analysis of these grouping methods by Hook and Albers concluded
that there is no absolute best method to use for determining where system breaks should
occur. The authors further concluded that each method gives about the same result and the
simpler methods are just as valid as the complicated methods. In general they suggested
that the following criteria be used:
1. Group all intersections that are within 2,500 feet of one another.
2. Use all links that are 5,000 feet or more in length as boundary links.
3. Calculate the Coupling Index for all links between 2,500 feet and 5,000 feet in
length and link all intersections that have a value greater than 50, consider linking
intersections that have a value of 1 to 50, and do not link intersections that have a
value of less than 1.
The following process is suggested to be used with any of the Index procedures. The
first step in this process is to determine which sections of roadways are to be
analyzed. These links are then drawn on a map which may be distorted to provide
space to display information related to each link.
Various traffic data can be superimposed over the roadway network to determine
applicable traffic volumes for the particular segment being registered. Some links
may not have any corresponding traffic data. In which case, the segment is still
registered, but with a zero value given for the traffic volume, which in turn results
in a coupling index of zero.
The next step is to calculate the indices for all of the registered links. The final step
is to identify signal groups by linking together intersections with high index values
and identifying group boundaries using links with low index values.
4 “Comparison of Alternative Methodologies to Determine Breakpoints in Signal Progression” TRB
Paper by David Hook and Allen Albers, 2002.
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