ITE Journal - January 2020 - 36

```The method is wrongly applied in a few ways:
1. A single period analysis is applied when a multiple period
analysis should be used; or
2. Traffic demand is underestimated.

Single Period vs. Multiple Period Analyses
Most of the procedures in the HCM are based on the peak
15-minute rate of flow, which is then converted to an equivalent
hourly rate using the peak hour factor (PHF), where:
Hourly Volume
PHF ------------------
Peak rate of flow within the hour

and acceleration). Most importantly, delay was defined to include
delays incurred beyond the analysis period when the lane group is
oversaturated. The d3 term was defined as, "residual demand delay
to account for oversaturation queues that may have existed before
the analysis period." A method for estimation of the d3 initial queue
delay term was included in an appendix to the method.
The method was clearly improved in its ability to address
oversaturated conditions. This improvement, however, raised
questions in its application:
 If multiple 15-minute periods within a peak hour are oversaturated, which one should be the focus of the analysis?
 What if the entire peak hour is oversaturated?

For a 15-minute peak flow rate, this equation becomes:
V
PHF ----
4xV15
where V is the hourly volume (in vehicles per hour) and V15
is the volume during the peak 15 minutes of flow. As part of the
analysis, the demand volume is divided by the PHF to represent
an equivalent hourly volume for the peak 15-minute period of the
hour. In other words, the HCM method is an analysis of the heaviest
or worst 15 minutes during the peak hour.
What happens if there are multiple periods when traffic
conditions are bad? Furthermore, how might congested traffic
conditions in one 15-minute period affect traffic conditions in an
adjacent, subsequent period? The HCM Signalized Intersection
delay equation (delay is the performance measure upon which intersection level of service is based) was modified in the 1997 update to
the HCM to account for this. In the 1985 HCM, the delay for each
lane group was expressed:
d = d1 + d2
Where d is the average stopped delay (in seconds) per vehicle for
the lane group, d1 is the uniform delay that occurs if arrival demand
in the subject lane group is uniformly distributed over time, and d2
is the incremental delay of random arrivals over uniform arrivals
and for the additional delay due to cycle failures. Both terms where
multiplied by a progression adjustment factor (PF) in computing
average stopped delay and corresponding level of service.
The method was modified with the 1997 update to the HCM
(published in 1998).2 The progression factor, PF, was applied to the
uniform delay (d1) component only and a third term was introduced:
d = d1 PF + d2 + d3
The delay definition was changed from stopped delay (i.e. delay
from being stopped at an intersection) to control delay (total delay
including stopped delay plus delays incurred during deceleration
36

J a n uar y 2020

i te j o u rn al

Beginning with the HCM in 2000,3 specific guidance was given
directing the analyst to study the entire period during which
volumes approach and exceed capacity, even if the duration of the
period was greater than one hour. Furthermore, lane group volumes
should reflect the actual demand and not a measured or counted
volume, as the demand is not entirely served during periods of
oversaturation. A greater emphasis was placed on computing
the initial queue delay (d3) as the procedure was extended to
analyze delay over multiple time periods. As stated in the current
HCM, 6th Edition:4
"If the analysis period's demand volume exceeds capacity, then
a multiple time-period analysis should be undertaken when the
study period includes an initial analysis period with no initial
queue and a final analysis period with no residual queue. ...
This approach provides a more accurate estimate of the delay
associated with the congestion."
What are the results of performing a single-period analysis
when conditions are oversaturated?
1. The estimate of delay associated with congestion will be less
accurate, much more inaccurate as demands increase.
2. The estimate of delay will be less than the delay computed from
a multiple-period analysis where the initial queue is computed
for each individual analysis period.
3. Resulting selected mitigation measures may not be sufficient due
to the underestimation of delay.
Why is this important? Underestimating delay can result in
signal timing with shorter cycle lengths and phase times that do
not process the actual demand. Another outcome of these incorrect
analyses would include inadequate turn bay lengths due to the
underestimation of queues. Where developer impact fees are
charged, underestimating the delay can mean the traffic impacts are
not fully mitigated and the collected fees insufficient to provide the
proper improvements.

```

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