**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.