Arterial Traffic Analysis Actuated Signal Control Dr. Gang-Len Chang Professor and Director of Traffic Safety and Operations Lab. University of Maryland-College Park
Actuated Signal Control Fully Actuated Control Semi-actuated Control Main Detector: either loop detectors, or some image detectors Side Street A complete system shall include: a signal, controller, and a set of detectors
Operations of a Semi-actuated Control Mainline signal is always on a green phase, unless recalled by the side street controller The criteria to terminate the main line green phase: a) Max. No. of waiting vehicles b) Max. delay for waiting vehicles c) Max. total delay from last side-street green phase (account for both green time and volume) d) Pedestrian needs
Key parameters for actuated control Max. green time Unit extension time Min green time Recall switch (when no demand) Yellow and all-red phases Pedestrain walking time
Typical detector type Inductive loop; radar sensor Microloop; magnetic sensor If area detectors are available, one can use volume-density controller to offer: Variable min. green time Gap reduction: max and min unit extensions, gap reduction starting and ending times.
Key Parameters in Actuated Control (1/4) (A) Minimum Green Time detector L W The minimum green time is designed to allow all vehicles queued beyond the detectors to enter and pass through the intersection Gmin = (start-up delay) + (discharged saturation headway) (the Max. No. of queued vehicles)
Key Parameters in Actuated Control (2/4) Min Green time should also be consistent with the pedestrian requirements Gmin = 4 (sec) + [ 2 (sec) Integer (d/20) ] d distance between detectors and stop lines One can solve d, based on the required min. Green time How to determine the d?? 1. Pedestrian Requirements 2. Observe the number of queue vehicles per cycle under a pretimed control
Detection location Method-1: Based on the min green time which is constrained by the pedestrian walking speed (15 percentile) Method-2: If no pedestrian concern: set Gmin to process one or two vehicles Then, use Gmin to compute the detector location.
Gmin with an area detector, the mini green time can be a variable. Gmin = (start-up lost time) + 2. (number of queue vehicles in the detection area). Also may vary with the required pedestrian walking time.
Key Parameters in Actuated Control (3/4) (B) A unit extension time interval allow a vehicle detected by the detector to travel safely through the intersection Min G Actual Operation of an Actuated Control UE YAR :Yellow and all Red UE YAR UE YAR Max extension period Max Green time
Determine the unit extension time: Low bound: long enough for a subsequent vehicle operating in dense-traffic to retain a green signal. Upper bound: not so long that a straggling vehicle may retain the green extension. Traffic Detector Handbook recommends: 3.0 seconds for approaching speed < 30 mph 3.5 seconds for approaching speed > 30mph Longer than passage time= (Dis.)/15 th percentile speed
Key Parameters in Actuated Control (4/4) Gap-out strategy: If the detector reports No Vehicle over a specified Interval or Gap, then the green phase will be terminated and yellow all red phase shift Green time to other movements Q1: How to determine the Max Green time? Q2: Should the GMax change between the peak and off-peak periods? Q3: How to determine the appropriate Gap size for changing the Green-to-Yellow phase? Q4: Should the Gap size be varied between peak and off-peak periods?
Figure 22.4 Gap Reduction Feature on Volume-Density Controllers (Source: Used with Permission of Institute of Transportation Engineers, Traffic Detector Handbook, 2nd Edition, JHK & Associates, Tucson AZ, p. 68.)
Advantages of an Actuated Control Vs. Pretimed Control Minimize the loss of unused green durations per cycle ** The green phase will be gapped out if no traffic has been detected Fully utilize the effective green time, especially when traffic is under moderate congestion level Under peak-hour * mostly saturation traffic conditions? * which one can perform better?
Disadvantage of an Actuated Control a) Longer loss time due to frequent phase changes b) Need sensor or detectors, and higher maintenance cost c) May incur more delay if traffic signals are not properly coordinated platoon Gap size too large Causing the signal to change phases! the competing direction may have low volume or no queue exist such changes may not be beneficial to the intersection drivers
Semi-actuated control (Traffic Eng. By Roess, Prassas, and McShane)
Detector Placement Detectors are to be placed to produce a minimum green of 6 s for the side street.
Passage Time As a point detector is in use, the passage time (PT) should be set to the minimum allowable headway (MAH), which is generally set at 3.0 s. The passage time must be at least long enough to allow a vehicle to traverse the distance between the front edge of the detector and the intersection line:
Assuming that the standard default values for start up lost time and encroachment are in use, the total lost time in the cycle is equal to the sum of the yellow and all red intervals, or (3.2+3.1) + (4.7+0.7) = 6.3 + 5.4 = 11.7 s
Maximum Green Time (Minor) and Minimum Green Time (Major)
**standard practice: 1.5 times of the computed green allocation time
This is not viable, as the maximum green time for the minor street (3.5 s) is less than the minimum green time of 6.0 s. A reasonable maximum green time for the side street will have to be determined, and a related minimum green time for the major street estimated. Given that the high volume SB minor street approach has a demand flow rate of 146 veh/h, the average arrival rate of vehicles would be 146/60 = 2.4 veh/min. The minimum green of 6 s allows two vehicles to be services (2.0 s start up lost time + 2 vehicles at 2.0 s). If the maximum green is set at the minimum green of 6 s plus one passage time (6 + 3 = 9 s), a third vehicle could be serviced on one green.
With the low demand, anything more than this would be excessive. The minimum main street green, however, should remain at the same multiple as indicated by the original timing computation
Critical Cycle Length The critical cycle length includes the minimum major street green, the maximum side street green, and the yellow and all red intervals for each street, or
Fully-actuated control (Traffic Eng. By Roess, Prassas, and McShane
Recommend a suitable phase plan for the signal; Mini. Green time/ Unit extension time Yellow and all-red phase Max. green Cycle length Pedestrian buttons needed or not?
Phase Plan There are no left turn lanes provided, so it would be very difficult to provide for protected left turns with this geometry. All left turn volumes are well under 200 veh/h, none of the cross products are anywhere near 50,000, and none of the ITE criteria are met. A two phase full actuated signal timing will be used. Passage Time Once again, with point detectors, the passage time (PT) is set equal to the maximum allowable gap (MAH), which will be set at 3.0 s. This must be at least sufficient to allow a vehicle to move from the detector to the intersection line, or:
Assuming standard default values for start up lost time and encroachment (both 2.0 s), the sum of the yellow and all red intervals is equal to the lost time in the cycle:
Maximum Green Times
Neither minimum green time provides this. Even with 3.9 s of yellow and 1.5 s of all red added, the EW Phase provides only 8.0+3.9+1.5 = 13.4 s while the NS Phase provides only 6.0+3.9+1.5 = 11.4 s. Both streets require pedestrian push buttons and pedestrian signals for safety. When activated by a pedestrian call, each phase will provide:
A fully actuated control Minimum Green Times/Detector Location Sixty foot area detectors, with the front edge located 2 ft from the STOP line are being used. The minimum green will, therefore, be variable, based upon the number of vehicles detected in the detection zone. The minimum greens will vary as follows:
Passage Time
Yellow and All Red Intervals
Critical Cycle Length The critical cycle length includes the maximum green for each phase, and all of the lost times in the cycle, or:
Potential Alternatives (1/2) (A) Multiple detectors or a long-loop detector (B) Adaptive control Prior to Gap-out or change the signal phase Objective: min total intersection delay Compare Cost of extending one more interval Benefits of extending one more interval
Potential Alternatives (2/2) Q1: How many intervals should be included in the cost-benefit comparison? This is the core concept of adaptive control