APPENDIX D DIAMOND INTERCHANGE PHASING, TIMING, AND DESIGN
Phases 1 and 5, s 2 Maximum Green for Phases 4 and 8, s
400 15 15 34 450 16 16 38 500 17 17 42 550 18 18 46 600 19 19 50 650 20 20 54 700 21 21 58 750 22 22 62 800 24 24 66 850 25 25 70 900 26 26 74 950 27 27 78 1000 28 28 82 Notes:
1 - Travel time is computed using the procedure described by Venglar et al. (2). 2 - Gmax,1 = Gmax,5 = T ; where, T = travel time (all values in seconds).
3 - Gmax,4 = Gmax,8 = (S/Lq) × h + 2.0; where, S = interchange spacing (in feet), Lq = length of a traffic lane occupied by a stopped by vehicle (use 25 ft/veh), and h = saturation headway (use 2 s/veh).
To illustrate the use of Table D-5, consider an interchange with 500 ft spacing and a three- phase sequence. Column 3 of the table indicates that the maximum green setting for phase 1 should be 17 s. Column 4 indicates that the maximum green setting for phase 4 should be 42 s.
The maximum green setting for phases 10 and 14 should equal the corresponding minimum green setting. This action ensures that phases 10 and 14 provide a green interval with a duration equal to the
minimum green setting for each cycle in which they are activated. Four-Phase Sequence. The maximum
green settings for phases 12 and 16 in a four- phase sequence are based on consideration of the time required to travel the length of the interchange. The maximum green setting for
these two phases should equal the corresponding minimum green setting, as specified in Table D-4. This action ensures that phases 12 and 16 provide a green interval with a duration equal to the minimum green setting for each cycle.
Maximum green setting:
Phase 10 max. green = Phase 10 min. green Phase 14 max. green = Phase 14 min. green
Maximum green setting:
Phase 12 max. green = Phase 12 min. green Phase 16 max. green = Phase 16 min. green
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Phase Recall Mode
The three-phase sequence is characterized by having good progression for the arterial through movements. To ensure this progression is provided each cycle, the minimum recall mode should be set for the
phases serving the arterial through movements (i.e., phases 2 and 6).
At an interchange served by a four-phase sequence, the external movements dictate the duration of the phases serving the internal traffic movements. However, the two arterial through movement phases do not operate together during the four-phase sequence. Hence, if both the arterial phases have the minimum recall mode set, the signal controller continuously cycles through the entire phase sequence, regardless of whether there is any traffic volume. Therefore, if recall is used, it should only be set for the phase serving the heavier volume arterial through movement (i.e., phase 2 or 6).
Guidelines for Loop Detection Layout for Low-Speed Movements
This section provides guidelines for locating detectors for low-speed interchange traffic movements. A low-speed traffic movement is defined as a movement with an 85th percentile
approach speed of 40 mph or less. The objectives of detection design for low-speed movements are to: (1) ensure that the presence of waiting traffic is made known to the controller, and (2) ensure the traffic queue is served each phase. Additional guidance on detection layout at intersections is provided in Appendix C.
Both stop line and advance detectors are used for low-speed interchange traffic movements. Regardless of the phase sequence used, the stop line detection is located near the stop line and is designed to provide a 40-ft detection zone. Advance detectors are 6 ft in length, and their location is based on consideration of the 85th percentile approach speed and the phase sequence. The location
of the advance detectors is described in the following subsections.
Three-Phase Sequence
The location of each detector for low-speed movements at an interchange with a three-phase sequence are shown in Figure D-10. This figure also indicates the detector channel to which each detector is assigned. Table D-6 provides guidelines for the advance detector location and passage time setting for the various phases that may serve a low-speed movement.
Use of minimum recall mode:
! Three-phase: Phases 2 and 6
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Figure D-10. Detection Layout for a Signalized Diamond Interchange.
Table D-6. Layout and Settings for Low-Speed Detection with Three-Phase Sequence.
85th Percentile
Speed, mph
Phases 1, 2, 5, and 6 1, 2 Frontage Road Phases 4 and 8 1, 2
Advance Detector Distance (S1), ft Passage Time (PT), s Advance Detector Distance (S1), ft Passage Time (PT), s 30 100 2.0 to 3.0 100 2.0 to 3.0 35 135 2.0 to 3.0 135 2.0 to 3.0 40 170 2.0 to 3.0 170 2.0 to 3.0 Notes:
1 - Advance detector distance is measured from the stop line to the upstream edge of the advance detector.
2 - Advance detection lead-ins are wired to a channel separate from that used for stop line detection (see Figure D-10).
Four-Phase Sequence
The advance detection design for phases 1, 2, 5, and 6 at an interchange with a four-phase sequence is the same as that for a three-phase sequence. In contrast, a different criterion is used to establish the advance detector locations for the frontage road phases. These detectors are located such that a transition interval occurs between the time the frontage road phase gaps out and its yellow indication is presented. By placing the advance detector at the proper upstream location, the last frontage road vehicle will have just arrived to the stop line at the time the yellow indication is presented to it. The advance detector locations that satisfy this criterion are listed in Table D-7. The
2 11 12 1 15 16 5 6 8 14 10 4 9 13 7 18 3 17 Left Right S1 2 2 8 8 6 6 4 4 S1 S2 S3 S1 S2 S3 S1 S1 S2 S3 S1 S2 S3 x - Detector Channel S1 S1
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calculations are keyed to the 85th percentile speed but recognize that the travel time for a frontage
road vehicle traveling at the 15th percentile speed will control the advance detector location.
Table D-7. Layout and Settings for Low-Speed Detection with Four-Phase Sequence.
85th
Percentile Speed, mph
Phases 1, 2, 5, and 6 1, 2 Frontage Road Phases 4 and 8 1, 2
Advance Detector Distance (S1), ft
Passage
Time (PT), s 100 Interchange Spacing, ft200 300 400 Time (PassagePT), s Advance Detector Distance (S1), ft 3, 4, 5
30 100 2.0 to 3.0 260 355 435 510 2.0 to 3.0
35 135 2.0 to 3.0 305 415 505 595 2.0 to 3.0
40 170 2.0 to 3.0 350 475 575 680 2.0 to 3.0
Notes:
1 - Advance detector distance is measured from the stop line to the upstream edge of the advance detector.
2 - Advance detection lead-ins are wired to a channel separate from that used for stop line detection (see Figure D-10). 3 - S14 = [PT4 + (Y5 + Rc5) + T!2.0 !(Y4 + Rc4)]×[1.13 v85] # 700 ft; where, S14 = detector distance for phase 4, PT4 = passage time for phase 4, Y5 = yellow interval for phase 5, Rc5 = red clearance interval for phase 5, T = travel time,
Y4 = yellow interval for phase 4, Rc4 = red clearance interval for phase 4, and v85 = 85th percentile speed (speed in mph, all other values in seconds). Y + Rc is assumed to equal 5 s.
4 - S18 = [PT8 + (Y1 + Rc1) + T!2.0 !(Y8 + Rc8)]×[1.13 v85] # 700 ft; where, S18 = detector distance for phase 8, PT8 = passage time for phase 8, Y1 = yellow interval for phase 1, Rc1 = red clearance interval for phase 1, T = travel time,
Y8 = yellow interval for phase 8, Rc8 = red clearance interval for phase 8, and v85 = 85th percentile speed (speed in mph, all other values in seconds). Y + Rc is assumed to equal 5 s.
5 - Travel time is computed using the procedure described by Venglar et al. (2).
The advance detector distances listed in Table D-7 are based on an assumed change period of 5 s and a passage time of 2.5 s. If other values are more appropriate, then the equations in the table footnote should be used to compute the advance detector distance. The values computed from these equations can yield distances that are relatively large for higher speeds. Such distances are often impractical from the standpoint of detector installation cost. In recognition of this practical limitation, the computed distances are limited to 700 ft. This restriction on the maximum advance detector distance will reduce the efficiency of the transition interval, but only by a small amount.
Separate Intersection Sequence
The guidelines in Table D-6 describing advance detector placement for an interchange with a three-phase sequence are also applicable to low-speed movements at interchanges with a separate intersection sequence.
Guidelines for Loop Detection Layout for High-Speed Movements
This section describes the loop detection layout for high-speed movements at an interchange. In this regard, a high-speed movement is one that has an 85th percentile approach speed of 45 mph
or more. The objectives of detection design for high-speed movements are to: (1) ensure that the presence of waiting traffic is made known to the controller, (2) ensure the traffic queue is served
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each phase, and (3) provide a safe phase termination by minimizing the chance of a driver being in his or her indecision zone at the onset of the yellow indication. As discussed in Appendix C, detection systems other than video image vehicle detection system may be better suited to advance detection for high-speed movements.
The stop line detection layout for high-speed movements is the same as that used for low- speed traffic movements. Specifically, the stop line detection is located near the stop line and is designed to provide a 40-ft detection zone. The location of the advance detectors is described in the following subsections.
Three-Phase Sequence
The location of each detector for high-speed movements at an interchange with a three-phase sequence are shown in Figure D-10. This figure also indicates the detector channel to which each detector is assigned. Table D-8 provides guidelines for the advance detector location and passage time setting for the various high-speed approaches to the interchange.
Table D-8. Layout and Settings for High-Speed Detection with Three-Phase Sequence.
85th
Percentile Speed,
mph
Phases 1, 2, 5, and 6 1, 2 Frontage Road Phases 4 and 8 1, 2
Advance Detector Distance, ft Passage Time (PT), s
Advance Detector Distance (S1), ft Passage Time (PT), s
S1 S2 S3 S1 S2 S3
45 210 330 not used 2.0 210 330 not used 2.0
50 220 350 not used 2.0 220 350 not used 2.0
55 225 320 415 1.4 to 2.0 225 320 415 1.4 to 2.0
60 275 375 475 1.6 to 2.0 275 375 475 1.6 to 2.0
65 320 430 540 1.6 to 2.0 320 430 540 1.6 to 2.0
70 350 475 600 1.4 to 2.0 350 475 600 1.4 to 2.0
Notes:
1 - Advance detector distance is measured from the stop line to the upstream edge of the advance detector.
2 - Advance detection lead-ins are wired to a channel separate from that used for stop line detection (see Figure D-10).
Four-Phase Sequence
The advance detection design for phases 1, 2, 5, and 6 at an interchange with a four-phase sequence is the same as for an interchange with a three-phase sequence. As described previously, the detectors for phases 4 and 8 are located using a different criterion. Specifically, these detectors are located such that the last frontage road vehicle will have just arrived to the stop line at the time the yellow indication is presented. The advance detector locations that satisfy this criterion are listed in Table D-9.
The advance detector distances listed in Table D-9 are based on an assumed change period of 5 s and a passage time of 2.5 s. If other values are considered to be more appropriate, then the
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equations in the table footnote should be used to compute the advance detector distance. As a practical limitation, the computed distances are limited to 700 ft. This restriction on the maximum advance detector distance will reduce the efficiency of the transition interval, but only by a small amount.
Table D-9. Layout and Settings for High-Speed Detection with Four-Phase Sequence.
85th
Percentile Speed,
mph
Phases 1, 2, 5, and 6 1, 2 Frontage Road Phases 4 and 8 1, 2
Advance Detector Distance, ft Passage Time (PT), s
Interchange Spacing, ft Passage Time (PT), s
100 200 300 400
S1 S2 S3 Advance Detector Distance (S1), ft 3, 4, 5
45 210 330 not used 2.0 390 535 650 700 2.0 to 3.0 50 220 350 not used 2.0 435 590 700 700 2.0 to 3.0 55 225 320 415 1.4 to 2.0 480 650 700 700 2.0 to 3.0 60 275 375 475 1.6 to 2.0 520 700 700 700 2.0 to 3.0 65 320 430 540 1.6 to 2.0 565 700 700 700 2.0 to 3.0 70 350 475 600 1.4 to 2.0 610 700 700 700 2.0 to 3.0 Notes:
1 - Advance detector distance is measured from the stop line to the upstream edge of the advance detector.
2 - Advance detection lead-ins are wired to a channel separate from that used for stop line detection (see Figure D-10). 3 - S14 = [PT4 + (Y5 + Rc5) + T!2.0 !(Y4 + Rc4)]×[1.13 v85] # 700 ft; where, S14 = detector distance for phase 4, PT4 = passage time for phase 4, Y5 = yellow interval for phase 5, Rc5 = red clearance interval for phase 5, T = travel time,
Y4 = yellow interval for phase 4, Rc4 = red clearance interval for phase 4, and v85 = 85th percentile speed (speed in mph, all other values in seconds). Y + Rc is assumed to equal 5 s.
4 - S18 = [PT8 + (Y1 + Rc1) + T!2.0 !(Y8 + Rc8)]×[1.13 v85] # 700 ft; where, S18 = detector distance for phase 8, PT8 = passage time for phase 8, Y1 = yellow interval for phase 1, Rc1 = red clearance interval for phase 1, T = travel time,
Y8 = yellow interval for phase 8, Rc8 = red clearance interval for phase 8, and v85 = 85th percentile speed (speed in mph, all other values in seconds). Y + Rc is assumed to equal 5 s.
5 - Travel time is computed using the procedure described by Venglar et al. (2).
Separate Intersection Sequence
The guidelines in Table D-8 describing advance detector placement for an interchange with a three-phase sequence are also applicable to high-speed movements at interchanges with a separate intersection sequence.
Guidelines for Configuration of Video Detection Outputs
When a VIVDS is used at an interchange, a total of six cameras are typically used to provide the necessary detection. The location of these cameras is illustrated in Figure D-11. When the VIVDS is housed in the detector rack, its detector outputs do not necessarily align with the predefined detector inputs needed for the Texas Diamond Controller. As a result, the detector outputs must be switched in the controller to obtain the appropriate detector channel assignment (shown previously in Table D-1).
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Rack-mounted VIVDS typically used at diamond interchanges in Texas have two video camera inputs and four detector outputs. However, additional detector outputs are needed when a six-camera configuration is used at a diamond interchange. These additional outputs are provided through the use of two extension modules (that are also mounted in the detector rack).
Figure D-11. VIVDS Camera Layout for a Typical Diamond Interchange.
A typical VIVDS detector-rack layout for Texas Diamond Controller application is illustrated in Figure D-12. The rack has 24 detector outputs. Three VIVDS cards and two extension modules are installed in the rack. The first VIVDS card on the left side accepts input from cameras 1 and 2. Camera 1 provides detection information to detector output numbers 1 and 2. These outputs are used to provide vehicle actuations to phase 1. The assignment of the other cameras to detector outputs and controller phases is described in Table D-10.
Figure D-12. Typical Video Detector Hardware Layout for a Texas Diamond Controller.
C1 C3 C2 C6 C5 C4 3 1 2 4 7 5 6 8 x 9 10 x x 13 14 x x x x x 23 21 22 24 2 & 1 6 & 5 8 & 4
2-Channel Video
Detector Card Extension Modules
C1 C6 C5 C4 C3
Camera Input Detector Output Number
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Table D-10. Typical Video Detector Switching for a Texas Diamond Controller.
Camera Number Detector Output
Number Phase Number Assigned DetectorChannel
C1 1 Φ1 1 2 not used C2 3 Φ2 11 4 2 C5 5 Φ5 5 6 not used C6 7 Φ6 15 8 6
C1 extension module 9 Overlap A
(Φ1 + Φ2)
10
10 9
C5 extension module 13 Overlap B
(Φ5 + Φ6) 14 14 13 C3 21 Φ4 12 22 4 C4 23 Φ8 16 24 8
Guidelines for Detector Settings
Delay can be used with the stop line detectors at an interchange to allow traffic turning right or left an opportunity to turn without activating the detector and placing a call on the phase. If an arterial phase is on recall, 8 to 14 s of delay may be used for the detector that serves the right-turn movement on the frontage road. Larger values in this range are used when a higher speed or conflicting volume exists on the intersecting road.
If the internal left-turn movement operates in the protected-permissive mode and the opposing arterial through phase is on recall, a delay of 5 to 12 s may be used for the detector in the left-turn lane. Larger values in this range are used when a higher speed or volume exists on the opposing approach.
If the delay setting is used, it should be programmed in the controller and not in the detector. Delay programed in the controller is only applied when the associated phase is displaying a yellow or red indication. Additional guidance regarding the use of the delay setting is provided in Chapter 2. In the Texas Diamond Controller, a 2-s delay is preset in the controller for advance detectors 2, 4, 6, and 8. This delay setting minimizes the potential for a phase to be called by vehicles passing over an advance detector. This result is desired because the stop line detection is provided for the purpose of calling a phase.
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Guidelines for Use of Conditional Service
Conditional service can be implemented at a diamond interchange when the following criteria are met:
! three-phase sequence is used, and
! the absolute difference between the average green interval duration of the two frontage road phases exceeds 10 to 12 s (3).
The conditionally served left-turn phase should also have a short minimum green value of 5 to 8 s. This short value will increase the probability that conditional service will be enabled. A short minimum green will also reduce the negative impact of the conditional service on other phases. The decision to allow conditional service for the left-turn phases should be dictated by consideration of frontage road traffic volume, rather than the volume of the conditionally served phase. This approach is more likely to minimize overall interchange delay. The conditionally served phases should operate in a non-actuated manner (i.e., the minimum green and the maximum green values should be the same or very similar).
Conditional service takes advantage of unbalanced frontage road traffic volumes and keeps the interior of the diamond interchange clear of stopped vehicles. However, if the frontage road volumes are only slightly unbalanced, conditional service may increase the delay to the arterial through movements if it continues to extend the frontage road phase after the frontage road queue is served. The operation of conditional service should always be observed in the field after its implementation to ensure that it is providing the intended operation.
REFERENCES
1. Chaudhary, N., and C-L. Chu. Guidelines for Timing and Coordinating Diamond Interchanges with Adjacent Traffic Signals. Report No. TX-00/4913-2. Texas Department of Transportation, Austin, Texas, November 2000.
2. Venglar, S., P. Koonce, and T. Urbanik, II. PASSER III-98: Application and User’s Guide.
Texas Transportation Institute, The Texas A&M University System, College Station, Texas, 1998.
3. Engelbrecht, R., S. Venglar, and Z. Tian. Research Report on Improving Diamond Interchange Operations Using Advanced Controller Features. Report No. FHWA/TX-02/0-4158-1. Texas Department of Transportation, Austin, Texas, 2001.