# ITE Journal - January 2020 - 37

```The following example illustrates the point. The source for this
example is Example Problem 1 from Chapter 30 Urban Streets:
Supplemental in the Highway Capacity Manual, 6th Edition. Hourly
traffic volumes from Intersection 1 are used but have been inflated
by 50 percent so that some operations for some of the lane groups
are oversaturated. These are illustrated in Figure 1.

Figure 1. Example of Peak Hour Intersection Movement Demand Volumes.
The example focuses on the eastbound left turn and through
movements. The left turn movement is slightly under capacity
(with a volume-to-capacity ratio slightly less than 1.00), while the
through movement is oversaturated. Two analyses were performed:
1) a single period analysis using PHF = 1.00 (i.e. an equal distribution of traffic demand across the peak hour); and 2) a multiple

period analysis consisting of four 15-minute periods with traffic
demand equally distributed.
For selected performance measures, results of the comparison
between the two approaches is summarized in Table 1.
The eastbound left turn lane group is undersaturated. There is
no Initial Queue Delay (d3) component and the results are nearly
identical. This is not the same for the eastbound through lanes,
where the volume-to-capacity ratio is greater than 1.00. Compared
with the single period analysis, the Incremental Delay (d2) increases
with successive time periods. The most remarkable difference is in
the d3 term, the unmet demand that carries over from the end of
one 15-minute time period to the beginning of the next. This value
is zero for the single period analysis, assuming no unmet demand
exists when the analysis period begins. Even if an initial queue is
recorded at the beginning of a single period analysis, the cumulative
effects of cycle failures are not carried forward and the delay is
underestimated. In many cases, it can be grossly underestimated,
as this illustration shows. The table for this example shows a control
delay of 900 seconds per vehicle (s/veh) in TP 4 of the multiple-period analysis, compared with 141 s/veh in the single-period analysis,
for a difference of 574 percent. Similar differences occur for back of
queue with 32 vehicles per lane (veh/ln) in the single-period analysis
compared with 159 veh/ln in TP 4 of the multiple-period analysis for
a 408 percent difference. These dramatic differences occur for this
lane group with a volume-to-capacity ratio of only 1.3, which can be
much higher at many signalized intersections during peak periods.

Underestimating the Demand
Counting vehicles as they cross the stop line is not adequate for
collecting data to analyze congested conditions. When conditions
become congested, stop line counts reflect capacity and not actual

Table 1. Summary Comparison of Single Period and Multiple Period Analysis Results.

Performance Measure
Volume-to-Capacity Ratio ( X )
Uniform Delay (d1), s/veh
Incremental Delay (d2), s/veh

Initial Queue Delay (d3), s/veh
Control Delay (d), s/veh
Control Delay Difference, %*
Back of Queue (veh/ln)
Back of Queue Difference, %*
Initial Queue (veh)**

* Compared to Single Period Anaysis

** For the entire EB Through lane group

Single
Period
0.911
39.6

25.4

Eastbound Left Turn
Multiple 15-Minute Periods
Single
TP1
TP2
TP3
TP4
Period
0.911
0.911
0.911
0.911
1.239
39.6
39.6
39.6
39.6
26.4
25.8

25.8

25.8

25.8

114.6

65.0

65.4

65.4

65.4

65.4

141.0

9.4

9.4

9.4

9.4

9.4

31.3

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

No significant difference for undersaturated

0.0

0.0

Eastbound Through
Multiple 15-Minute Periods
TP1
TP2
TP3
TP4
1.243
1.305
1.305
1.305
26.5
28.7
28.7
28.7
116.6
0.0

143
1%
31.6
1%
0.0

143.6

229.8

402.1
185%
71.4
128%
73.7

143.6

504.2

676.5
380%
115.2
268%
161.0

143.6

778.6

950.9
574%
159.0
408%
248.6

Significant differences in successive time
periods for oversaturated
w w w .i t e.or g

J a n u ar y 2020

37

```
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