Methodology
The ICU methodology requires a specific set of data to be collected. The data needs to include volumes, number of lanes, saturated flow rates, signal timings, reference cycle length, and lost time for an intersection. The method sums the amount of time required to serve all movements at saturation for a given cycle length and divides by that reference cycle length. This method is similar to taking a sum of critical volume to saturation flow ratios (v/s), yet allows minimum timings to be considered. The ICU method uses the concept of the Level Of Service (LOS) where LOS reports on the amount of reserve capacity or capacity deficit. In order to calculate the LOS for the ICU method, the ICU for an intersection must be computed first. ICU can be computed by:
ICU = (max (tMin, v/si) * CL + tLi) / CL = Intersection Capacity Utilization
CL = Reference Cycle Length
tLi = Lost time for critical movement i
v/si = volume to saturation flow rate, critical movement i
tMin = minimum green time, critical movement i
Once the ICU is fully calculated for an intersection, the ICU LOS for that intersection can be calculated based on the following criteria:
LOS
A: If ICU is less than or equal to 55%
B: If ICU is greater than 55% but less than 64%
C: If ICU is greater than 64% but less than 73%
D: If ICU is greater than 73% but less than 82%
E: If ICU is greater than 82% but less than 91%
F: If ICU is greater than 91% but less than 100%
G: If ICU is greater than 100% but less than 109%
H: If ICU is greater than 109%
This grading criteria shows some specific details about the specific intersection:
A: Intersection has no congestion
B: Intersection has very little congestion
C: Intersection has no major congestion
D: Intersection normally has no congestion
E: Intersection is on the verge of congested conditions
F: Intersection is over capacity and likely experiences congestion periods of 15 to 60 consecutive minutes.
G: Intersection is 9% over capacity and experiences congestion periods of 60 to 120 consecutive minutes.
H: The intersection is 9% or greater over capacity and could experience congestion periods of over 120 minutes per day.
Most industry standards require the ICU LOS to be E or better. This is not always easy to achieve and therefore much care is given to the signal timings and geometry in order to get the LOS to be better than E.
Read more about this topic: Intersection Capacity Utilization
Famous quotes containing the word methodology:
“One might get the impression that I recommend a new methodology which replaces induction by counterinduction and uses a multiplicity of theories, metaphysical views, fairy tales, instead of the customary pair theory/observation. This impression would certainly be mistaken. My intention is not to replace one set of general rules by another such set: my intention is rather to convince the reader that all methodologies, even the most obvious ones, have their limits.”
—Paul Feyerabend (1924–1994)