Sub-problem 3a - Page 4 of 5**Sub-problem 3a: Actuated Signal Detection and Signal
Timing**

If we look back at the analysis
completed in Sub-problem 1b, page2, we assumed the default value of unit extension
(3.0 seconds) and a g/c ratio of 0.33 for Reitz Union. However, from the
discussion on the last two pages, we found that the phase length varies as a
function of the signal timing parameters and thus, depending on the
parameters the g/c ratio, may change accordingly. Essentially, the phase
length is a function of the unit extension; once traffic is served, the
controller times the gap setting as extra time looking for an additional
vehicle to cross the detector.

*Step 2. Results*

*How do signal timing
parameters affect the phase length and the volume to capacity ratio in the
HCM methodology?* Let's
assume our minimum green
time is 5 seconds, our maximum green time is 40 seconds (as was used in the
HCS analysis from Sub-problem 1b, page2), and our unit
extension is three seconds. If we reduce the unit extension, we would expect
the actuated side street movement (Reitz Union) to receive less green
time. HCS calculates that a shorter unit extension reduces the delay for the
entire intersection because there is potentially less time wasted at the
intersections.

If we make this change in our
HCS file, now assuming that the unit extension is one second, (because we
have detection on the north and south approaches that allows shorter unit
extension time) (Reitz3a4 - ask PJK) we find that changing the unit
extension does not change the g/c ratio assumed in the analysis for the
north/south approach. This is due to the procedures within HCS represent
analyses of fixed lengths of intervals and sequences of phases. In other
words, there is no direct link between the g/c ratio assumed and signal
timing parameters such as unit extension. The reduced unit extension does
result in a reduction in delay for the intersection and each individual
movement in the calculation.

*Is this representative of
what happens in the field?* This may not be representative of what occurs
in the field and for a detailed analysis, measured green time for each
approach may be used to refine the analysis. Reducing the unit
extension would reduce the g/c ratio for the north/south movement. The
coordinated phase, Museum Road through (east/west) would receive the
additional time that is unused from the Reitz Union approaches.

Report results from analysis here...

*Discussion:*

Take
a few minutes to consider how much more the phase times could vary if we
assumed equally spaced random arrivals at the intersection. What conditions would make
the 50th-percentile phase length the same as the average phase length?
How significant is this difference in phase length?