¶ Overview
On the Adaptive ribbon tab as shown below, Adaptive configuration pages and graphs are available.
¶ More Pages
Kadence will tune intersection timing parameters in real-time. The Kadence system optimizes traffic signal timing to balance performance benefits for safety and efficiency. The system is not intended to replace or obviate the need for sound traffic engineering but rather supplement the traffic engineer’s toolbox with another tool that can handle fluctuations in demand and short and long-term changes in land use and traffic patterns.
Kadence is comprised of five principle algorithms for tuning signal splits, offsets, cycle time, phase sequence, and TOD schedule. A safety performance function is also included that allows the real-time system to predict the changes in traffic conflict rates when adjusting signal timing parameters to optimize both safety and efficiency. In the Kadence approach, new signal timing parameters are downloaded to field controllers every 3-4 cycles. The field controller then begins operating in an actuated-coordinated or actuated-free mode with these new settings. Based on past experience with adaptive systems that override the controller’s timings every second (RHODES, OPAC, SCOOT, SCATS, and InSync), this methodology of downloading new timings is more reliable, safer, and less error prone. In addition, the approach of ACSLITE is proven to require minimal capital investment, infrastructure, detectors, configuration, and calibration.
¶ Configurable Parameters
Configurable parameters include:
- Exclude any phase from split tuning by pattern
- Exclude or allow any lead-lag sequence by pattern
- Exclude or allow cycle tuning by pattern
- Exclude or allow offset tuning by pattern
- Configure maximum deviation of splits from pattern values
- Configure maximum deviation of offsets from pattern values
- Configure minimum and maximum cycle time
- Exclude or allow cycle selection
- Select any phases for coordination biasing
- Configure operation by TOD and Pattern
All Kadence algorithms and services operate at a central TMC. There are no field hardware components to install or maintain. If a Kadence algorithm fails, controllers will return to their normal TOD operation. In addition, Kadence can be turned ON or OFF from central and scheduled to run on a time-of-day, day-of-week schedule. When Kadence is OFF, the controller will return to their regular non-adaptive operation as programmed in the controller (TBC coordination or free, as configured). Kadence does not send hold or force-off commands to controllers, or suppress phase calls, so there is no risk of a controller getting stuck in a certain phase. All controller features operate normally including pedestrians, transit priority, and preemption. Kadence can run alongside an existing central system on an IP network using NTCIP or AB3418 protocols, depending on what is supported by the field device.
¶ Traffic Optimization Objectives
Kadence can meet a variety of agency objectives, including:
- Maximizing throughput on a coordinated route
- Providing smooth flow on a coordinated route
- Providing access equity for all phases at an intersection
- Manage the length of queues
- Optimize operation to minimize phase failures
- Combinations of these objectives
¶ Maximize the throughput on a coordinated route.
Kadence maximizes throughput on a coordinated route by using a combination of offset tuning, split tuning, cycle tuning, and phase sequence selection. Kadence tunes offsets to provide smooth flow, which increases the throughput on the route by reducing stops. By tuning (increasing) the coordinated phase splits, more time can be provided to that route when the level of traffic on the side streets and other competing phases is reduced. In the split tuning algorithm, coordinated phase utilization measures can be biased so that more time for that movement is protected which provides more opportunities for progression along the route. If the coordinated phases are over a specified threshold for phase utilization, the cycle time can also be increased to provide additional throughput on the critical route. In addition, Kadence can modify the phase sequence to a lead-lag combination for left turns to increase the amount of time that the critical through route receives if there is very low opposing left-turn volume.
For example, a change from lead-lead to lag-lead would be predicated if:
• Phase 5 has a heavier utilization than phase 1
• Phases 2 and 5 have heavier total utilization than phases 1 and 6
Offsets are then adjusted after the sequence change is evaluated and a similar comparison is done for the other barrier group (phases 3, 4, 7, and 8).
¶ Cycle Selection
Throughput is also improved on coordinated routes by determining a decision to either implement the cycle time that is next in the TOD schedule earlier or later than was originally planned. This can be used in conjunction with the cycle tuning method (discussed below) or alone. This allows the system to adapt to changes to the beginning or end of peak periods. The same evaluation approach for the incremental cycle tuning algorithm described previously is used but rather than changing the cycle just a few seconds, the system enables the new cycle time immediately and recalculates splits and offsets appropriately.
The algorithm begins considering implementing the next pattern in the TOD schedule early if it less than a configurable number of minutes before the next pattern change time. For example, this threshold time might be set to 30 minutes prior to the schedule change so if the pattern is scheduled to be adjusted at 10:00am, the algorithm will begin considering implementing the next pattern at 9:30. If the thresholds for phase utilization are not exceeded to implement a lower or higher cycle, the current cycle is retained. If the phase utilization thresholds are not exceeded after the scheduled time to change to the next cycle, the system can keep the current pattern in operation. After a configurable amount of time, however, the system will transition to the next pattern in the TOD schedule.
¶ Provide smooth flow along coordinated routes.
Tuning offsets improves progression performance along primary routes for phases that are coordinated. Offset tuning algorithms are particularly straightforward. The proven and robust methodology used in ACSLITE is used in Kadence with several key enhancements as described below.
The concept of the data-driven offset adjustment algorithm is to maximize the number of vehicles arriving during the green phase. Periodically, small, incremental adjustments are made to the offset to maximize the total proportion of cyclic flow arriving to a green light. This concept is then expanded to consider and mitigate the effects of such modifications to the offset value for multiple approaches (including the consideration of cross-coordination on all four approaches) and the effects of changes at a given intersection on adjacent intersections.
A user-configurable maximum deviation from the original setting (either an increase or decrease to the offset value) is defined for each offset to restrict the algorithm (if desired) from drifting too far away from the original solution. The user can also specify that this value is unbounded, which allows the system to search for any offset. For example, if the initial offset is 20s and the maximum deviation is set to 10s, the algorithm will be restricted to implement offsets with the range of 10s to 30s. Each controller considers a range of offset settings: no change, adjust up to seconds earlier, or adjust up to seconds later. The adjustment maximum step size, , is a user-configurable value. If the value is set at 10, for example, Kadence will search offsets in each step in the range of (+10, +9, +8, +7, …., 0, -1, -2, -3,….., -10). If the difference in the % arrivals on green between the evaluated offsets is not greater than small amount of improvement, say, 5%, the controller will remain at the current offset. This reduces transition events that do not result in significant improvement to performance.
¶ Distribute phase times in an equitable fashion.
Splits are tuned by collecting volume and occupancy data from detectors at the stop bar of the intersection in the same way that was implemented in ACSLITE project which is similar to the methods used by SCATS and SCOOT. The algorithm attempts to equalize the degree of saturation on all the phases at the intersection. This algorithm also allows coordinated phases (or any phase, but this biasing is typically applied to coordinated phases) to have biased splits, so that progression is protected when the saturation level of the coordinated phase is lower than that of side-streets. Without such biasing, split adjustment methods that equalize the degree of saturation on all phases tend to focus more on providing adequate LOS on side streets while degrading progression along a critical route.
Any phase can also be determined to be left out from the split tuning process. This is commonly applied to phases on minor side-streets that very occasionally experience bursty traffic flows that do not last more than a few minutes. In the absence of regular arrivals, the splits will be adjusted to the minimum possible value. Since the absence of traffic on the side-street will naturally result in additional time to the main street when the phase is skipped, the side-street phase may be set by the traffic engineer to a reasonable value that provides adequate LOS during the burst and kept fixed. Kadence will adjust the other parameters as appropriate. A good example for this would be the exit from a church. The split adjustment algorithm takes minimum and maximum constraints into account, and allows the user to either adhere to pedestrian crossing times or not. If the pedestrian crossing times are allowed to be larger than the split, then when a pedestrian pushes the button requesting service, the intersection will likely go into transition. In areas with low pedestrian volume, this is typically acceptable operation. If pedestrian volumes are quite high, it is more typical that the crossing constraints would be considered as minimum phase durations.
¶ Manage the length of queues.
Current algorithms in Kadence do not explicitly measure queue lengths or use estimates of length of queue in making decisions. Queue lengths are balanced or managed indirectly by modifying splits, offsets, cycle time, and phase sequence and changing the time that TOD plans are started and stopped in the TOD schedule. In 2014, we plan to add algorithms for managing queues based on our research as part of NCHRP 03-90 recently completed in 2011. A methodology was developed that tunes splits on a route based on measurement of the degree of oversaturation.
This process uses the TOSI (temporal oversaturation level) and SOSI (spatial oversaturation level) measures to determine the amount of green time to add and subtract, respectively, from a phase. This heuristic is denoted as the forward-backward procedure (FBP). Simply speaking, there are two ways to deal with oversaturation: one is to increase the downstream output rate and the other is to constrain the upstream input rate. These two basic actions result in three mitigation strategies for an oversaturated phase
¶ At an isolated intersection, optimize operation with a minimum of phase failure
Cycle time is adjusted on a section- or arterial-wide basis to provide adequate capacity to operate all of the signals under capacity and reduce the occurrence of phase failure. Kadence uses a heuristic rule to adjust the cycle time up or down a given step size. In a straight-forward fashion, if the cycle time is increased by 4 seconds, then every phase on the controller gets a proportion of the additional time. For example if there are four phases per ring, one additional second is provided for each phase split. The split adjustment algorithm will refine the splits at a later step if this allocation results in uneven phase utilization. The step size is user-defined. Minimum and Maximum cycle limitations are imposed including limitations by minimum green, pedestrian clearance times, and user-defined minimum and maximum cycles. As a reliability measure, there must be at least 3 cycles of vehicle-occupancy data for critical phase utilization monitoring detectors in the system to execute the cycle tuning algorithm.
This methodology will tend towards longer cycles during peak periods as traffic demand builds, which is generally accepted as an appropriate strategy. Recent research (NCHRP 03-90) we conducted is indicating that when the conditions are extremely oversaturated, shorter cycles will provide more efficient throughput. These improvements or algorithms have not yet been integrated into the system but are planned for future work. This will improve the capability of Kadence to provide sound decisions during incident response conditions, such as heavy diversion of flows from a freeway to a parallel arterial or frontage road system.
The cycle tuning algorithm used in Kadence extends from a “critical intersection” algorithm. The phases that are designated to be checked in the cycle tuning algorithm are determined by the user. When the average phase utilization on these critical phases is above the user-defined threshold (say, 80% phase utilization) to increase the cycle, a given (the user-defined step size) number of seconds are added to the cycle time. Similarly, the system cycle time is decreased by a fixed number of seconds when the average of the phase utilization on the critical phases is less than a lower threshold (say, 50% phase utilization)
¶ A combination of two or more of these strategies.
Kadence balances and optimizes combinations of operational objectives by running multiple algorithms together. Each algorithm can be enabled or disabled by pattern, so cycle tuning or other algorithms can be deliberately disabled by the traffic engineer by time of day. Five principle algorithms are included in Kadence for tuning splits, offsets, cycle time, and phase sequence. Based on the configurability of the system by TOD pattern, each of these objectives can be addressed at different times. The original ACSLITE system was designed for coordinated arterial corridors to provide smooth flow and address access equity. The additional features and algorithms added to Kadence in the last 3 years make the system applicable to a much wider range of situations including grids and interchanges. Additional features planned in the roadmap will extend Kadence’s applicability to oversaturated conditions and diversionary routes and groups.
¶ Configuration Setup
After the configuration and deployment of detection, the system can be implemented in just a few days. The larger the number of intersections and interchanges that are included, the longer the process of verification will take. Development, testing, and field verification of new features or new controller types can take time. The basic process is as follows:
Automatic import of phase parameters from KITS database
- Configure detectors
- Configure links / adjacency
- Configure algorithm parameters
- Enable monitoring mode, evaluate data
- Enable adaptive mode
- Configure and "tune" parameters of algorithms
Before configuring an arterial to run in adaptive mode, a section must be defined and all of the desired intersections must belong to that section (Configuring).
- Intersection Parameters
- Section Parameters
- Links
- Detectors
- Schedule
- Trace Log
- Action Log
- Phase Utilization
- Arrival On Green
- Coordination