Subproblem 4b  Page 4 of 4 
ID# C304B04 
Subproblem 4b:
Twoway Stop Control with
a Normal Urban Intersection Treatment
Exhibit 330 explains why
the capacity lines shown in
Exhibit 327 are in different places. The formula for computing the capacity
of a movement that must yield to an opposing movement is given in HCM
equation 173. This equation contains two parameters:

The critical gap,
which indicates the length of a gap in the opposing flow required to
accommodate the first queued vehicle trying to cross the opposing flow.

The followup time,
which indicates the additional gap length required to accommodate each
subsequent vehicle entering the same gap in the opposing traffic.
Larger values for each of
these parameters will lower the capacity for the entering movement. The
values shown in Exhibit 330 indicate lower values for the left turn crossing
the opposing traffic than for the minor street entry movements. This
indicates that drivers making left turns from the major street are able
to accept shorter gaps in the opposing traffic than drivers who are
entering the major street from a minor street approach. The result is a
higher capacity for the WBL movement compared to the NBL movement.
Exhibit 330. TWSC Analysis
with a Normal Urban Intersection Treatment 
Assumed
Parameters 
Movement 
Input Data 
EBT 
WBT 
NBL 
WBL 
NBR 
Volume (vph) 
2,010 
358 
257 
120 
433 
Number of lanes 
2 
2 
1 
1 
1 
Median storage
(vehicles) 
N/A 
N/A 
4 
N/A 
N/A 
Percent trucks 


20 
41 
10 
Analysis Results 
Critical gap
(sec) 
N/A 
N/A 
7.2 
4.9 
7.1 
Follow up time
(sec) 
N/A 
N/A 
3.7 
2.6 
3.4 
Adjusted flow
rate (vph) 
2010 
358 
257 
120 
433 
Adjusted capacity
(vph) 
N/A 
N/A 
69 
168 
226 
v/c ratio 
N/A 
N/A 
3.72 
0.71 
1.92 
95% queue length
(veh) 
N/A 
N/A 
27.1 
4.4 
31.1 
Delay (sev/veh) 
N/A 
N/A 
??? 
67 
464 
LOS 
N/A 
N/A 
F 
F 
F 
Simplifying Assumptions
Analysis
period=15 min
No pedestrians
No upstream
signals
PHF = 0.93 for
all movements
Level Terrain 
One of the objectives
of this exercise was to judge whether it is appropriate to consider the
intersection in the context of a normal urban intersection with TWSC
control. This task can be accomplished best by comparing the results in
Exhibit 330 with the corresponding results obtained by treating each of the
conflict points separately. We will examine the separation of conflict
points in the next subproblem. [
Back ] [
Continue ] to SubProblem
4c 