ML20235E605
| ML20235E605 | |
| Person / Time | |
|---|---|
| Site: | Shoreham File:Long Island Lighting Company icon.png |
| Issue date: | 07/23/1987 |
| From: | Fullerton I, Kell J INSTITUTE OF TRANSPORTATION ENGINEERS |
| To: | |
| References | |
| OL-3-A-036, OL-3-A-36, NUDOCS 8709280196 | |
| Download: ML20235E605 (6) | |
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.,I 7l2 3/g 7 INSTITUTE D?t$il:
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Manual of Traffic Signal Design James H. Kell JHK & Associates, President Iris J. Fullerton Transportation Consultant I
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I PRENTICE-HALL, INC., Englewood Cliffs, New Jersey 07632 8709280196 870723 PDR ADOCK 05000322 i
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75 interconnection l'f the recallis off, the controller operates in a full-actuated mode and rests in the phase i
last served providing there has not been a serviceable demand on a conflicting ph tse and the Red Rest has not been asserted. This is the normal mode of operation for secondary phases (e.g., minor street and left turns).
If the vehicle recallis on for a given phase, there are two forms of operation: Minimum Recall and Maximum Recall. In both cases, the controller automatically returns to this phase and provides vehicle right of way, once each cycle, without the need for any vehicle demand for service. The WALK and pedestrian clearance intervals are omitted unless called for by pedestrian demand, in the Minimum Recall mode, the controller responds as if a single vehicle requires service. A minimum green time will be displayed, providing de-mand exists on conflicting phases. In the Maximum Recall mode, the selected phase times to a maximum as though there were a continuing demand for service. Termination of the i
green interval will not occur when the maximum timing expires, unless there is a serviceable demand on a conflicting phase.
In the Pedestrian Recall mode, the controller automatically returns to thelelected phase once each cycle and provides both vehicle right of way and the subordinate WALK and pedestrian clearance intervals. This timing is provided without the need for either veni-cle or pedestrian demand for service. Utilization of this mode of operation adversely af-fects the efficiency of actuated control and, in general, should only be used m emergency situations where vehicular and/or pedestrian detectors have failed.
In the nonactuated mode, the controller returns to the nonactuated phase once each 4
cycle and times the WALK interval. When WALK timing expires, response of the con-troller will depend on the presence of a serviceable conflicting call and the assertion (or nonassertion) of external inputs such as HOLD, FORCE-OFF, WALK REST, and so forth.
Each phase module of the NEMA controller has a series of indicator lights which pro-vides information on the current operation. One group of indicators, when illuminated.
signifies that the phase controlled by this module is active. The lighted indicators identify whether the phase is timing a green, yellow change, red clearance, WALK, or pedestrian clearance interval. The sequence indicator, when illuminated, signifies that the phase con-trolled by this phase module is committed to be the next phase timed.
A second group of indicators are used to portray demand for that phase.The Vehicle Call Indicator flashes with each vehicle actuation during the green interval. It remains 11-luminated when the callis received during the yellow change, red clearance, or red intervals unless nonlocking detector circuits are used. The Pedestrian Call Indicator, when il-luminated, signifies the presence of pedestrian demand on this phase.This indicator is not active during the WALK interval.
INTERCONNECTION Controllers that are coordinated receive information from a master control source which establishes the base line condition. The individual controller then operates its intersection in the predetermined relationship with the base line. Direct communication can be pro-vided by hard wire cable, coaxial cable, fiber optic cable, or radio. Indirect communication can be provided by time-based coordinators.
it is also possible to hand set timing relationships between noninterconnected signals.
This technique relies on the local power frequency to keep the tirning relationships con-stant. However, frequent short-term power interruptions disrupt this relationship and re-quire manual recalibration. Some form of communication, either direct or indirect, is highly desirable to avoid these disruptions.
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Chap. 11 Signst Timing set-up procedures, there are basically three types of operational modes associated with par-
.l ticular types of actuated equipment: nonactuated modes (used with semi-actuated con-trol), actuated modes, and volume-density modes. Timing for each mode of opera-tion is discussed below.
Nonactuated Mode In semi-actuated operation,the major street normally operates in a nonactuated mode.
That is, green will remain on the major street for the predetermined minimum time and.
thereafter, until there is a vehicular or pedestrian call for service from a conflicting phase.
. When such a call is received, the signal proceeds through tu normal change (and clearance) interval to transfer the right of way to the calling phase provided that the minimum green.on the major street.has elapsed. Timing for the nonactuated mode pro- -
vides for a guaranteed minimum green on the major street before a conflicting call for serv.
x " ice will be accommodated.'In this type of operation,Inc cycle length is variable and is a function of the demand from the side street.
The length of the guaranteed green for the major street depends on the type ofintersec-tion. Where there are only occasional vehicles on the side street and the primary street is a secondary arterial, relatively short settings (25 to 40 seconds) may be used.' On the other hand, where the side street discharges large numbers of vehicles at times (e.g., a factory en-trance) with almost no demand at other times, the guaranteed green setting for the major street may be quite long'(40 to 75 seconds) to ensure that major street traffic is not inter-rupted too frequently. In addition to the setting for guaranteed minimutn green time, there are settings for the yellow change and all-red clearance intervals for the nonactuated phase.-
Because of the unpredictable variability of the major street green, pedestrians are fre-quently allowed to cross the side street concurrently with the vehicular green with no special indications or timing. When pedestrian indications are used, the DON'T WALK appears when a side street call is registered (providing the minimum green has expired).
Normal pedestrian clearance (as described in the Pretimed Control section) is timed prior to release of the green to the side street. This clearance time (which is also green for the ma-jor street) should be taken into account in establishing the minimum green setting.
In a systems operation, the cycle length is fixed and will only allow a given part of the cycle to be used by the side street. Although a prescribed portion of the cycle is available for the side street movement,' only the time actually required by side street traffic will be assigned. The remaining time, if any, will be allocated to the nonactuated phase.
' Actuated Mode Normally in an actuated phase, there are three timing parameters (in addition to the yellow change and all-red clearance interval). These include: the initia/ interval, the vehicle interval (also referred to as the extension interval or unit extension), and the maximum in-terval. Theseintervals are timed as a function of the type and configuration of theintersec-tion's detector installation.
initialInterval 1
In the early stages of detector technology, most detectors were." point detectors."-'
l' Originally, these detectors were treadles or pressure plates in the roadway. Today's 6 x 6-foot loop detector is also essentially a point detector. With this form of detection, the iri-
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itialintervalis established to permit those vehicles stopped between the detection point and the stop bar to get started and move into the intersection. Therefore, the timing of this in-terval will usually depend on the distance between the detector and the stop bar and the l
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. tion" which will increase capacity and will tend to red'uce dela"ys by not wasting green time f-to accommodate a few late arriving vehicles.
The initialinterval plus one vehicle interval comprises the minimum green period for a -
single vehicle. If other vehicles are detected during the vehicle interval, the timer is reset,-
and timing of a new vehicle intervalis begun. This process is repeated for each new detec.-
tion until a preset maximum is reached after a vehicle or pedestrian places geall for another phase, or until the gap between detections is greater than the vehicle interval. Figure 11 ~ illustrates this' operation. In this example, the actuations occurred so frequently that there was no gap greater than the vehicle interval. Accordingly, the preset maximum interval was reached and the green iadication was terminated. Some controllers provide a full vehicle in-terval (termed last car passage) when the phase is terminated by the maximum interval in the example, the controller will return the green at the first opportunity since the last vehi.
t cle may not have had sufficient time to enter the intersection.
If, in the example illustrated, the first actuation had not occurred after a call on a con-flicting phase, the vehicle interval would have timed out and the green would have ter-minated. The green phase, in this case, would have equaled the minimum green time.
' When long loops are used in approaches to the intersection and especially'when used in -
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left-turn bays, the vehicle interval generally is set to zero (or as near zero as possible). The long loops operate in the presence mode and the controller continuously extends the green -
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L as long as the loop is occupied. The critical time gap is the length of time it takes a vehicle to l
travel the length of the loop plus the length of the vehicle.
When a series of short loops is used, the series acts as a long loop, provided that the f
space between loops is less than the length of a vehicle. If the spacing is greater than the
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length of a vehicle, a short vehicle interval can be used to provide the same effect as a single'
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Maximum Green Interval The maximum green interval limits the' time a phase can hold the green. $t may be in terms of total green time or it may be in terms of vehicle extension limit. Ordinarily,
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Timing for Actuated Control 153
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maximum intervals are set between 30 and 60 seconds. When the signal is properly timed with appropriately short vehicle intervals, the maximum interval will not consistently time.
out unless the intersection is badly overloaded.
9,' y a-One' method for determimag maximum intervals for each phase is to compute op.
timum cycle length and green times in the same way these intervals are determined for pretimed controllers..The computed green intervals are then multiplied by a factor ranging between 1.25 and 1.50 to obtain the maximum green.
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Some actuated controllers are capable of providing two maximum intervals per phase (called in by a time clock or other means). This allows for a longer maximum during peak periods when very heavy traffic flows are expected on the major street.
Pedestrian Timing Modern controllers have actuated phases with or without pedestrica timing. The latter I
are used especially for turning phases where pedestrian timing is inappropriate. For major phases with concurrent pedestrian movements, phases with pedestrian timing capabilities are desirable to maximize timing efficiency. The pedestrian timing parameters are the WALK period and the DON'T WALK pedestrian clearance. These intervals should be calculated as described in the Pretimed Control section.
There are occasions where very heavy pedestrian flow way warrant an exclusive pedestrian phase. A separate pedestrian phase can be used. >r this purpose. Timing is normally based upon diagonal crossing times.
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Volume. Density Mode l
The volume-density mode available on many models of controllers has more timing parameters than the standard actuated mode. For this type of operation, detectors are nor-mally placed further back of the intersection, particularly on high-speed approaches to the intersection of major streets. This placement provides earlier detection information.
l The single value for the initialinterval (as described in the Actuated Mode section) can l
be expanded to provide an appropriate assured green time for the actual traffic present.
This variable initial interval can be provided by three different options as defined in the NEM A specifications (Ref.12). These options are added initial, computed initial, and ex-tensible initial. These options, their timing parameters, and the.ir timing ranges are shown in Table 11-4.
Table 11-4 Volume. Density initial interval Options Option Parameters Timing Range.
Added initial Minimum initial, sec.
1-30 Actuations before added initial, veh.
2-60 Added initial per actuation, sec.
0-2.25 Computed initial Minimum initial, sec.
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Actuations to reach maximum, veh.
0-89 Extensible initial Minimum initial, sec.
1-30 Added initial per actuation, sec.
0-2.25 Maximum initial, sec.
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