Home > Problem 4 > Sub-problem 4b - Page 4 of 4

Sub-problem 4b: Two-way Stop Control with a Normal Urban Intersection Treatment

Exhibit 3-30 explains why these lines 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 17-3. This equation contains two parameters:

  1. 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.

  2. The follow-up 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 3-30 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 willing to accept shorter gaps in the opposing traffic than drivers that 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 3-30. 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 3-30 with the corresponding results obtained by treating each of the conflict points separately. We will examine the separation of conflict points in the next sub-problem.

[ Back ] [ Continue ] to Sub-Problem 4c