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Sub-problem 3a - Page 2 of 9

ID# C103A02

Sub-problem 3a: Oversaturated Intersection Analysis

Now let's review each of these questions, and how they are important to this analysis.

What is the difference between volume and demand, and why is it important to distinguish these two terms? Most field studies at intersections include the measurement of volume. Volume, or sometimes service volume, is the traffic flow at the point where vehicles are entering the intersection just past the stop-line. Demand (or sometimes demand volume), by contrast, is the traffic flow desiring to enter the intersection. When we measure demand in the field, we must be at a point upstream of any queues that form at the intersection. When we conduct analyses using the Highway Capacity Manual, we must always use demand volumes and not service volumes. If demand is less than capacity, then demand volume equals service volume, and we can collect stop-line counts for use with the HCM. However, if demand exceeds capacity (as evidenced by continuing queues that don't dissipate at the end of each cycle), then we must collect traffic flow data upstream of the intersection to account for all vehicles desiring to use the intersection during a given time period. Special care must be taken when collecting turning movements when queues extend upstream of the intersection as it is sometimes difficult to see the final direction that a given vehicle follows when the observation is conducted upstream. Videotaping might be considered as an aid in the data collection process when these conditions occur.

Can the intersection operate at level of service F when demand is less than capacity? Level of service for a signalized intersection is defined by average control delay; and while it is somewhat dependent on capacity, it is often more dependent on other factors such as arrival type. So it is possible for an intersection to operate at level of service F (when delay exceeds 80 seconds per vehicle) while demand is less than capacity. The reverse is also true: lane groups, approaches, and intersections can sometimes be found to operate at levels-of-service better than F, even while the computed v/c ratio is greater than 1.0, especially in situations where the cycle length is short and/or progression is very good. The point to remember from all this is that LOS is not, by itself, a sufficient measure of the operating status of the lane group, approach, or intersection: other factors like v/c ratio, queue length, and cycle length must also be considered when forming an overall judgment.

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