Sub-problem 3a: Actuated Signal Detection and Signal Timing

Unlike with unsignalized intersections, the volume to capacity ratio for a signalized intersection will vary depending on the signal timing parameters. If an intersection approach operates over-capacity, adjustments in the green allocation could be made to alter the operational characteristics of the approach.

As previously mentioned, Museum Road will be coordinated with development of the signal at Reitz Union Drive. This will be necessary not only to decrease corridor delay, but, due to their close spacing, also to prevent queues from extending through adjacent intersections. Because we have a fixed cycle length, the intersection will operate as if it were semi-actuated, with additional time from the minor street phases reassigned to the major through street phases.

The jurisdiction may also require the intersection operate at a Level of Service "E" or better, asking that a volume to capacity ratio less than 0.80 be maintained.

What does it mean for the minor street approach to operate at a v/c ratio of 0.80? On Reitz Union, we will assume that the queue accumulates during the red time, and that once the phase is green, saturated flow will occur on the through movement. During each phase, the volume to capacity ratio is 1.0 as long as saturation flow is achieved and maintained. As the green per cycle ratio is increased for an intersection, the v/c ratio is reduced assuming all other things being equal. Thus, as time is provided to a particular movement that is unutilized, the volume to capacity ratio is reduced.

As an example, let's assume we had sixteen vehicles in queue at the start of the green phase and based on our saturation flow rate we assume the intersection would need 30 seconds for queue clearance. Once the phase gapped out 3 seconds later at 33 seconds,  (due to a 3 second unit extension), the approach would be operating at a v/c ratio of 0.91, adding 3 seconds of wasted delay to conflicting movements. If we were to maintain a v/c ratio of 0.8 at this approach, this phase would be 30/0.8 seconds, or 37.5 seconds. This would equate to 7.5 seconds of wasted time being added to the delay of conflicting movements. What this suggests is that there is a difference between the volume to capacity ratio reported for planning purposes and the way these intersections operate in the field. Analyses that require a future year volume to capacity ratio of 0.80 may suggest a planning method that is insensitive to detector operations being used.

Discussion:
Take a few minutes to consider why the v/c ratio for side streets may be greater than 1.0. If you were to hold the g/c ratio constant, what could you do to reduce the v/c ratio for the side street? How would this affect delay?