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51 APPENDIX A Vendors' Descriptions of Major Adaptive Traffic Control Systems References cited in this appendix can be found in Appendix C. ACS LITE ACS Lite could be operated from a traffic management center--such as a traditional central system, or FHWA initiated development of Adaptive Control Software Lite ACS Lite could be operated in on-street manner--such as (ACS Lite), prescribing a lower cost and more easily managed sys- a traditional field master. tem, to surmount the major deployment impediments and bring this rarely used state-of-the art technology to the mainstream state In either case, the adaptive control software was to be encapsu- of practice (Curtis et al. 2009). ACS Lite offers significant cost sav- lated in a single computer, to avoid the expense associated with ings relative to earlier FHWA-sponsored adaptive systems by bet- distributed adaptive systems sponsored by FHWA in the 1980s ter leveraging existing infrastructure; evaluations based on both and 1990s (Luyanda et al. 2003). These earlier non-Lite ACS field trials and simulation studies have shown significant benefits, systems had required that each intersection be equipped with a such as delay reductions of up to 35% (Shelby et al. 2008). The dedicated processor (aside from that of the traffic controller following discussion provides a more detailed description of the itself) in order to host adaptive optimization algorithms at each ACS Lite system architecture, adaptive logic, and evaluation intersection. These prior systems used the 2070 controller, results to date. specifically to utilize its unique Virtual Machine Environment (VME) expansion slot, which accommodates the installation of Project History such add-on processors. The research and development of ACS Lite has been done by Intersection Controllers Siemens, with collaboration from other partners including major signal controller manufacturers Eagle (now Siemens), Econolite, FHWA, focused on cost minimization, intended ACS Lite to be McCain, and Peek, who accepted an invitation from FHWA to par- compatible with National Electrical Manufacturers of America ticipate in the project (Ghaman 2006). The project began in 2002, (NEMA) model controllers, which have historically been less and the last of four field evaluations (one with each of the four con- expensive than 2070 controllers, although McCain opted to inte- troller vendors) was completed in 2007. A concise summary of the grate with its 170 controller (which is generally less expensive). ACS Lite project and findings can be found in Shelby et al. (2008). This compatibility enabled deployment of ACS Lite without the need (or expense) for controller upgrades in most of the four field As of 2009, ACS Lite was in the midst of a secondary research trials. However, in each case, a firmware (controller software) effort by FHWA to incorporate cycle time adjustment. Currently, upgrade was required to accommodate support for additional cycle length settings are changed according to a time-of-day ACS Lite status messages. schedule. Despite this limitation, ACS Lite has been able to pro- duce substantial benefits based on its adaptive split and offset Another "intangible" and often overlooked cost saving ben- capabilities (e.g., 35% delay reduction in Houston). That being efit comes from the capability to retain familiar controller the case, ACS Lite is currently being marketed and deployed as is firmware--the core of which is largely unchanged by the commu- by the aforementioned controller vendors who participated in the nications upgrade. The time and effort that would otherwise be original project. necessary for staff to learn to use and maintain completely new controller firmware could be significant. ACS Lite Architecture Communications An ACS Lite system is composed of the following hardware: FHWA required that ACS Lite be designed to use National Trans- An ACS Lite system computer, portation Communications for ITS Protocol (NTCIP) as its com- The traffic signal controllers, munications protocol, given the desire to encourage adoption of Communications between ACS Lite and the controllers, and this national standard. It was also specified that ACS Lite be able Vehicle detectors. to communicate over low-speed serial communications, which constitutes the existing infrastructure to most signal controllers In the four field tests--one with each participating controller at this time. To achieve this goal, custom NTCIP-compatible manufacturer--the exact nature of the system architecture varied status messages were developed for ACS Lite to substantially somewhat, depending on the manufacturer, as illustrated in Fig- reduce required bandwidth, such that ACS Lite was able to com- ure A1. More detail is available in Shelby (2008). Each of the municate during field trials with eight to ten signals using 9600 four system components is briefly discussed in the following baud communications. Higher data rates are necessary to sup- paragraphs. port more controllers. In each of the four field trials, ACS Lite was deployed leveraging existing twisted-pair communications. ACS Lite System Computer Modem upgrades were required in some cases; however, this expense compares favorably with the costs associated with In the interest of the widest possible applicability, FHWA envi- installation and maintenance of fiber optic, peer-to-peer com- sioned ACS Lite as being capable of governing traffic signals in munications as has been used in deployment of non-Lite ACS either of the following scenarios (Crenshaw 2000): systems previously developed by FHWA. At the time of this

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52 ACS Lite NTCIP protocol Local Controllers (a) ACS Lite ASC/2M Field Master (b) NTCIP protocol Econolite protocol (solid lines) ACS Lite 2070 Field Master NTCIP protocol (c) BI Tran protocol ACS Lite NTCIP Translators NTCIP protocol (d) Peek protocol Local Controllers FIGURE A1 ACS Lite as deployed with (a) Siemens, (b) Econolite, (c) McCain, and (d ) Peek. writing, ACS Lite supports 16 controllers in a single system. Vehicle Detectors ACS Lite also supports Internet Protocol (IP)-based (Ethernet) communications. The detection scheme required by ACS Lite is compatible with typical layouts used for intersections under fully actuated ACS Lite monitors traffic signals by polling each controller on control. This reduces the total cost to instrument a typical arte- a per-minute basis for time-stamped state changes. If a poll request rial as required for adaptive control. ACS Lite also incorpo- fails, the status report is still available from the local controller until rates detector processing in such a way as to be relatively the end of the minute, so the system has ample opportunity to poll flexible with respect to the size, location, and capability of again. This affords a measure of insulation from occasional com- detectors. This capability often reduces the need to resize or munication errors, which can compromise systems that require relocate existing loop detectors that are in the right location, but per-second communications. In such per-second systems, a missed are not of ideal dimension. The demand measurement scheme poll generally means a hole in the data that cannot be recovered also aims to address concerns that have been raised about the once the 1-s polling window has passed. This appears to be partic- sensitivity of adaptive control performance with respect to pre- ularly relevant in the context of wireless communications. cise detector count accuracy.

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53 To adjust splits, ACS Lite requires stop-line detectors for Adaptive Traffic Control Logic each phase or movement, preferably separated out for individual lane-by-lane monitoring, although lanes serving the same phase ACS Lite operates by monitoring traffic signals that are run- or movement may be tied together if necessary. Stop line detec- ning normal, coordinated timing plans and then making incre- tors monitor volume and occupancy on green, and the process- mental adjustments to split and offset parameters as often as ing logic accounts or adjusts for the detector length and maxi- every 5 to 10 min. Thus, ACS Lite does not take over second- mum vehicle speed. Detectors sized from 4 to 70 ft long have by-second control of the sequence or duration of each phase, but shown good results (although 70 ft is generally too long in many rather the normal actuated logic allows skipped phases, gap-out, scenarios). max-out, and/or force-off in a normal manner. Cycle length is cur- rently not adjusted by ACS Lite, although future enhancements On approaches where progression is desired (generally the are planned. The cycle length is currently dictated by the "under- arterial approaches), advance loops (typically 6 ft 6 ft) are lying" or "baseline" timing plan, which is selected according to used to monitor cyclic flow profile, to identify the arrival of pla- the time-of-day schedule (Luyanda et al. 2003). toons, and use these data for adjustment of offsets to improve progression. Split adjustments are based on measures of the "utilization" of each phase (Luyanda et al. 2003). Detector volume and occu- Detector processing has been designed to reduce sensitivity to pancy data are processed, primarily during green intervals, to the accuracy of count data. Count accuracy may deteriorate sub- gauge the amount of time that traffic is flowing across the stop- stantially if a loop begins counting axles instead of cars or if video line. ACS Lite estimates the degree of saturation of each phase. detection cannot precisely separate out two vehicles traveling The adjustment logic reallocates split time to balance (with closely together, owing to the angle of the camera. As examples optional biasing) the degree of saturation across all phases, of "sensitivity reduction," consider that green-occupancy flow subject to configured minimum green times, pedestrian interval measures are less sensitive to errors than pure volume-based flow requirements (optionally), and maximum green times (when they measures. However, the technique is somewhat more involved are not inhibited during coordination). Thus, time is reallocated than measuring only green-occupancy. from a phase with an excessively long (i.e., underutilized) split time to provide more split time for an oversaturated phase. The Accurate turning probabilities and saturation flow rates are split adjustment logic provides an optional "progression bias- generally a source of sensitivity (to error) for signal timing opti- ing" mechanism that distributes "extra" or "slack" green time in mization. These values are certainly subject to change through- the cycle (if it is available) in greater proportion to designated out the day, according to daily travel patterns, and are also influ- progression phases, which are typically arterial through phases. enced by unexpected weather. However, ACS Lite does not This option is almost always used, as it has been shown very require calibration or configuration of these parameters and is effective in exploiting the availability of "slack" time to pro- designed to gauge traffic demand well over a wide range of con- vide a wider green band for improved progression (Shelby et al. ditions (Shelby et al. 2008). Aside from basic verification that a 2008). detector indeed works, there is no need for field studies or elabo- rate detector calibration in ACS Lite. Configuration is limited to Figure A2 provides a screen-capture of the ACS Lite's web- known facts, such as the location and dimensions of a detector. based user interface, which provides a color-coded bar chart indi- FIGURE A2 Monitoring degree of saturation in ACS Lite.

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54 cating the degree of saturation for each phase. These measures are BALANCE overlaid, in the enlarged view in Figure A3, on a ring diagram, to portray the trade-offs of adjusting split time between phases. This The traffic-adaptive network control BALANCE started in two particular screenshot illustrates that phase 3 (a cross-street, left- research projects supported by the European Union (Munich turn phase using typical NEMA phase numbering) is 100% satu- Comfort and TABASCO). Beginning in 1992, BALANCE was rated, whereas all of the main-street phases (1, 2, 5, and 6) are less developed at Technische Universitt Mnchen and later also at than 60% saturated. The figure also illustrates that phase 3 cur- the companies GEVAS software and TRANSVER. Since 2002, rently has a 13-s split, which could be reduced to a minimum of GEVAS software has been responsible for maintenance and fur- 10 s, or a maximum of 20 s (which allows up to 3 s of room to ther development of BALANCE. The system has been imple- reduce split time or 7 s of room to increase time). mented in several cities including Remscheid, Hamburg, and Ingolstadt. Oversaturation poses a daily challenge to some intersections. If oversaturation afflicts one phase (or more, but not all phases), then splits will be relocated to alleviate the situation if possible. Adaptive Traffic Control Logic However, in some cases, oversaturation on one phase (or more) cannot be alleviated with any amount of split reallocation. As BALANCE belongs to the generation of the newest German this situation develops, ACS Lite will "defend" or "protect" the traffic signal control systems. Therefore, it is not relevant traffic engineer's original split timing as follows. Suppose the whether the existing traffic signals are controlled in fixed- phase 3 oversaturation in the prior scenario (see ring diagram in time, traffic-actuated, or with public transport prioritization. Figure A2) cannot be alleviated, despite a substantial realloca- BALANCE can deal with any kind of existing control, as tion of split time from other phases. This traffic flow scenario long as these can be accessed and controlled by the traffic has been observed most predictably at shopping center access computer. signals during late November and December. If ACS Lite were to attempt to balance saturation across all phases, then all phases The data packages provided by BALANCE are small and would become saturated and arterial progression would be com- are transmitted only every 5 min. BALANCE's use of traffic promised. To counteract that result, ACS Lite manages phase modeling allows for minimal use of traffic detectors. BALANCE splits such that no phase is allowed to experience more than 95% will develop optimal signal timings for the existing detection in saturation with less than the traffic engineer's original split allo- the field and does not require that every intersection be equipped cation. For example, if phase 4 was originally allocated 30 s, with detectors. then ACS Lite might be willing to reallocate some of that time to phase 3, reducing phase 4 to 20 s, so long as phase 4 remains The basic system architecture of the BALANCE system no more than 95% saturated. If traffic flow picks up again on (shown in Figure A4) divides the functionality for traffic signal phase 4, such that it exceeds 95% saturation, then ACS Lite will control hierarchically on two levels (Braun et al. 2008): return some or all of its original split time to bring maximum sat- uration just below 95%, despite phase 3 being oversaturated. On the local or operational level for single intersections, The traffic engineer's original split time allocation for desig- the local actuated control reacts on short-term changes in nated "progression bias" phases (generally arterial through the current traffic demand (every second); and phases) will be "defended" at a lower saturation threshold of 90%. On the tactical level of a traffic signal network, the Thus, despite oversaturation on a cross street phase, the main BALANCE algorithm works as a macroscopic system and street progression phases will not surrender split time to the extent covers the middle-term and long-term area (5 to 15 min) of that it approaches oversaturation. This policy has proven effec- the traffic-actuated control. tive in alleviating surges in traffic on a particular approach to a reasonable extent, while also protecting arterial progression to a Minimization of transition times, caused by traffic-actuated reasonable extent. At intersections that experience oversatura- signal coordination (offset optimization, green wave), and rough tion on a daily basis (often predictably during peak flow), this adjustment of release times of the signal groups are done cen- policy results in splits gravitating back to the traffic engineers trally on the network level. Precise adjustment of release times, original settings, such that the traffic engineer can still dictate on the other hand, takes place inside the traffic controller. behavior in this scenario, and drivers can expect reasonably pre- BALANCE influences traffic light signals in the network by two dictable similar day-to-day service during these periods, which types of traffic control commands: could provide the most reasonably consistent and predictable commute times during these periods. Framework signal plans--for every interstage, the earliest and the latest points at which the interstage could start are ACS Lite offset adjustment decisions are based on monitor- defined. The interval between the two points is available to ing advance detectors (and phases) on each approach and aver- the local, traffic-actuated control for its own local deci- aging data over successive cycles to form cyclic flow profiles sions. Apart from these local decisions, the framework sig- (and green phase profiles), which also reveal early-return-to- nal plan defines the greens at the single signal groups and green behavior. Figure A3 shows a screen shot from ACS Lite, the coordination of the signal devices. illustrating the cycle flow profiles (blue bars) and green time pro- Signal program--BALANCE selects the program with the files for all inbound and outbound arterial links of an intersection. best cycle time for the current traffic situation from a pre- ACS Lite adjusts the offset of each intersection in turn, by con- arranged set of signal programs. The signal program index sidering three offset options--keep it the same, adjust a few sec- serves as output date. The selection is done at the same onds earlier, or adjust a few seconds later--and selecting the time for all traffic signals of one control group. A control option that maximizes the percentage of arrivals on green for all group is defined as a subset of traffic signals in the network designated inbound and outbound links. (e.g., a group of signals in close proximity).

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FIGURE A3 Cyclic flows (blue) and green times for two-directional arterial link.

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56 FIGURE A4 Fundamental system architecture in BALANCE. Traffic Control Optimization could be provided in as a disaggregated form as possible so that BALANCE can reconstruct (approximate) the raw detection The optimization model of BALANCE network control deter- inputs before the data are further processed. However, transmis- mines the length of release times (split) and the offsets accord- sion of raw detection data requires higher communication capacity ing to a common cycle length. The optimization is done by than if the data were preprocessed and then sent to a central com- Genetic Algorithm, which imitates the process of natural evolu- puter. This need for high-capacity communication media may tion. Interaction between optimization process and field imple- cause problems for certain communication networks. In such a mentation is shown in Figure A5. The final signal timings are case, BALANCE can operate with aggregated data, but at mini- achieved over several generations in the constrained time win- mum traffic volumes and detector's occupancy needs to be pro- dow of 5 min. Although the solution space for the appropriate vided. It is important that the length of an aggregation interval does signal timings is very complex, the Genetic Algorithms have not exceed 2 min. proven that a solution close to the theoretical optimum can be reached (Braun et al. 2008). In Germany, traffic detectors are traditionally placed at a short distance (10 to 50 m) upstream of the intersection stop-lines. This The result of optimization is a framework signal plan created type of detector allows good vehicle-actuation operations; how- for every intersection in the network. A framework signal plan ever, it cannot be used to estimate input flows and queues in the determines fixed or variable signal timings for all traffic signals same way as (far) upstream detectors, or for estimation of satu- in a coordinated group of signals. Within given framework plans, ration flow rates, in the same way as stop-line detectors. To get a local intersection controllers can execute traffic-actuated opera- good estimation of current traffic conditions a new cycle-based tions based on the local traffic demand. Priority for public trans- queue length estimation method was developed. This method port is also executed locally. allows estimating back-up lengths of distances five or ten times greater than the distance between detector and stop-line. The heart Detection Requirements of the estimation method uses fill-up time. Time needed to fill-up, with cars, an area between the intersection stop-line and the end BALANCE can use the data from any type of detectors, regardless of the detector. For that approach, this time is measured from of their placement in regard to the intersection. Detection data the beginning of the red phase until the detector is continuously FIGURE A5 Genetic Algorithm (GA) optimization process in BALANCE.

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57 occupied. From the length of this time can be estimated the speed A forecasting module of BALANCE estimates impacts of vari- of the vehicles approaching the traffic signal at the end of the ous traffic control strategies for the following time period by green time. A self-calibrating method was developed to estimate calculating performance measures such as delays, stops, and queue length based on the fill-up time. This estimation method is queue lengths for all intersection approaches. a standard way to use detector data for both German ATCSs: BALANCE and MOTION. The performance measures are computed with the assistance of both mesoscopic and macroscopic BALANCE models. The mesoscopic model estimates queues at intersection approaches Operations and other deterministic performance measures. Stochastic fluc- tuations in traffic and origin are estimated through the macro- The BALANCE traffic model creates an internal spatiotemporal scopic model. The delays from both models are integrated into representation of the current traffic conditions based on the the calculation of the Performance Index (PI). detected traffic loads. The traffic model has two modules: macro- scopic and mesoscopic: The GEVAS software program VTnet/view is used as a visu- 1. The macroscopic traffic model estimates the origin alization interface for BALANCE. VTnet/view displays all traffic destination matrix between each pair of entries and exits parameters computed by BALANCE, such as traffic volumes, in the network. The origindestination matrix is based on link traffic densities, and link level-of-service. Road works and a predefined weight function matrix and traffic inflows messages posted on variable message signs also can be visualized. and outflows measured at the borders of the network. Diagrams and journals can be created directly from the visual- 2. The mesoscopic traffic model iteratively takes into con- ization interface and can be aggregated for any time period. sideration current traffic signal status, link travel times, Two-dimensional display of the network geometry (shown in Fig- and platoon dispersion factors to develop traffic flow pro- ure A6) allows visual control of the supplied detector loops and files for all links in the network. signal groups during operation. FIGURE A6 VTnet view of traffic load in BALANCE.

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58 Network geometry for traffic visualization in VTnet/view can signals within the minimum/maximum parameters that users be taken from all common network formats (e.g., ESRI shape have input into the initial configuration settings of the system. files). Some network elements, such as type and location of turn- ing lanes, detectors, and stop-lines, need to be entered separately. For turning lanes, signal location plans can be displayed as a back- The Global Element: Time Tunnels and Adjustable ground image. Detectors and stop-lines can be read from German Periods for Optimizing Progression standard exchange format for signal engineering (e.g., OCIT-I- VD), and then they need to be placed properly on the map. Phases, Users need to determine the main directions within the grid, but phase transitions, and signal groups are as well read from standard can also redefine and automatically toggle between arterials by exchange formats. Follow-up supply of the special parameters (Time-of-Day) TOD/DOW (Day-of-Week). Special parameters required for BALANCE can be done directly in the traffic engi- can be set in for intersecting main arterials that provide effective neer's workstation CROSSIG. coordination within the grid. INSYNC Time Tunnels Green waves/time tunnels are guaranteed by suc- cessively turning each light green at the expected arrival time of InSync is an adaptive traffic signal system developed by Rhythm vehicles from upstream intersections. This can be illustrated Engineering (Lenexa, Kansas) that uses innovative sensor tech- using speed lines as shown in Figure A7. Speed lines are con- nology, image processing, and artificial intelligence. These ele- figured starting with a chosen facilitator intersection. By default, ments are integrated into a system that automatically optimizes the speed lines for the main two directions of travel intersect at local traffic signals and coordinates signals along roadway arteri- this facilitator intersection. Time tunnels are made to occur at this als according to real-time traffic demand. The use of InSync elim- intersection by requiring the simultaneous initialization of green inates the need for static signal coordination plans. lights for both directions. The facilitator intersection decides a time at which it will serve a green band for the coordinated tun- nel phases and communicates that time with the adjacent inter- Adaptive Traffic Control Logic sections. Each adjacent intersection uses the expected vehicle travel time between it and the intersection it received the tunnel There are two aspects to InSync's signal optimization that deal message from to decide when it needs to turn the tunnel phases with the conflicting objectives of providing the progression of green for both its downstream tunnel phase from the facilitator platoons of vehicles along a main arterial and the clearance of intersection and its upstream tunnel phase to the facilitator inter- vehicles involved with secondary traffic movements within the section. Start times for downstream tunnel phases to the facilita- grid: the global and the local. InSync operates and optimizes tor (or upstream from the facilitator) at the adjacent intersection FIGURE A7 Concept of "Time Tunnels" in InSync.

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59 are adjusted by (- travel_time) so that vehicles are released Intelligently Adjustable Periods Through its global interactive from the upstream intersection in time to reach the facilitator communications with the intelligent processors at each inter- intersection when it initiates its tunnel time. Start times for down- section (also see the next section: The Local Element) the facili- stream tunnel phases from the facilitator (or upstream to the facil- tator will, if necessary and as soon as possible, expand or contract itator) at the adjacent intersection are adjusted by (+ travel_time) the time between tunnels to provide the optimal period lengths to so that vehicles are released from the downstream intersection as serve each phase along the arterial. At the end of every period, they arrive from the facilitator intersection. each local processor is "polled" by the facilitator and "reports in" if it needs more time, the same time, or less time to clear its local Using the travel times between each adjacent intersection phases. These period length adjustments serve to efficiently these tunnel times are calculated by each intersection as it progress vehicles and clear out queues, both globally and locally. receives the dynamic tunnel phase timing messages from each adjacent intersection along the artery. Visually, the speeds lines for an artery with two main directions of north bound and south The Local Element: Logic and Features of bound with four "listener" intersections and one facilitator inter- the Optimization Algorithm section in the middle would look like this: Beyond the constraints communicated by the facilitator as "tun- Expected travel times between intersections are listed on nel messages" that guarantee coordinated green lights for the the x-axis, and elapsed time is the y-axis. The slope of a speed main arterial, the signals operate in "intelligent fully-actuated" line is always 1, because the expected travel time between mode. The time between tunnels is called a period. If a period is intersections has a 1 to 1 relationship with wall time (or time 90 s in duration and a green light is guaranteed for the main direc- on a clock). tions at each intersection for 10 s, then 80 s are available for the local optimizer to schedule states (phase pairs) at each inter- Tunnel start times at each intersection for the north and south section according to its intelligent scheduling. The local opti- phases are relative to the tunnel start times at the facilitator. If we mizer embodies the dominant logic and algorithm of the adaptive say that the north and south tunnel phases start at t = 0 at the capacities of the system. facilitator intersection, then tcN (the time of the start of the north bound downstream tunnel phase at Intersection C) = 0 + Scheduling of States There are three main factors the opti- travel_time_to_facilitator = 0 + 10 = +10. This says that Inter- mizer considers in its scheduling logic: section C needs to force a green light for the north bound phase 10 s after the facilitator intersection does. tcS (the time of the 1. If it is close to the initiation of a new tunnel, it will sched- start of the south bound upstream tunnel phase at Intersection C) = ule a main street sequence of states. This sequence of main 0 - travel_time_to_facilitator = 0 - 10 = -10. This says that Inter- states is only allowed to be scheduled such that after it section C needs to force a green light for the south bound phase, completes there is sufficient time to schedule a sequence 10 s before the facilitator intersection does. To complete the of cross street states. If the main direction requires a lead- example, the other start times at Intersections A, B, and D are: ing left turn, its clearance time is also included in the cal- culation for time needed on the cross street. tdN: +35 2. If a tunnel has recently ended, it will schedule a cross tdS: -35 street sequence as its priority. The amount of time needed tbS: +30 for the cross street is based on a balance between the actual tbN: -30 amount of clearance time needed and anticipated time taS: +50 needed. If there are no cross street queues it will schedule taN: -50 a miscellaneous main state. 3. A miscellaneous main state is scheduled for phases with Additionally, the speed lines are not actually required to intersect queues that have been waiting the longest. Wait times at the facilitator or even intersect at an intersection. The tunnels being equal, the phase with the largest queue is scheduled. can be offset to allow any arrangement of speed lines as desired. Any available miscellaneous time is used to schedule any This flexibility can provide for more efficient progression. Also, phase with real-time demand including protective permis- travel times for both directions between adjacent intersections do sive left turns on the main directions. not have to be the same. The south bound direction may take 5 s longer to travel than the north bound direction between two inter- Empty Queues In the calculation of a state sequence every vehi- sections. The minimum tunnel bandwidth (or guaranteed green cle phase is assumed to have a queue of 1 vehicle, if no queue time for a tunnel phase) is configurable. exists. These phases typically find their way into the states sched- uled toward the end of the state sequence. This way, if vehicles do Vehicles traveling along the main arterial that arrive at an arrive on these previously empty phases, they can be served. In intersection at the beginning of the time tunnel ideally progress this sense, those states act as place holders for vehicles that may unstopped all the way through the coordinated arterial. InSync arrive. If a phase remains empty when it becomes time to serve automatically extends green lights beyond the set parameter if it that state, that state is either removed or modified to contain phases observes that the moving platoon has not sufficiently gapped out that do contain a queue of vehicles. at a user changeable percentage of occupancy (calculated every second by InSync) or set gap time. InSync may, if permitted by Duration of States After an initial sequence of states is sched- the user, provide a green light along the main street before the uled, the durations of each state are continually modified to con- light is guaranteed to be green. Real-time traffic data are contin- tain enough clearance time to serve vehicles that may have arrived ually passed to downstream processors by their upstream part- after the state was initially scheduled. As each previous state in the ners that can also be factored into the optimizing process. sequence reaches extension time, all states scheduled after adjust

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60 their initial durations for newly arrived vehicles. Adjusted dura- ically logged flow rate, then it is assumed that the current pattern tions and extensions are limited by the amount of time left to serve of traffic can be served using a period that was adequate enough vehicles on pending phases. For a state with two phases, ph1 and to serve the historical pattern of traffic. This process is known as ph2, if ph1 clears out, a phase concurrent with ph2 that has a period prediction and is communicated to the facilitator inter- queue can be put in place of ph1, assuming there is enough time section along with the real-time load analysis of the local inter- remaining in the scheduled state to fulfill the new phase's mini- section. The facilitator intersection uses all of this information to mum green time, amber time, and red time. determine if/how it will adjust the current period. Termination of States States being served are ended or dynam- ically modified as phases are terminated. Termination of a phase Digital Signal Control Concepts--Finite Number occurs in one of two ways. Both of these methods employ a of Signal States (No Transition) model of linear change with time, such that a phase will termi- nate at a higher percentage of capacity as time increases or the InSync does away with set cycle lengths, set splits, and offsets to phase's gap time will decrease to a point at which, once presence a fixed point in the cycle that have traditionally been considered for a detection zone is lost, the phase will terminate as time essential for signal coordination. These concepts are germane to a increases. linear/analog approach to signal coordination. InSync is an artifi- cially intelligent/digitally based finite-state changing machine. By Calculating a Sequence The optimizer gathers the data for the its method of externally influencing a controller it causes any con- calculation: current queues, pending pedestrian calls, and any troller to effectively function digitally. This digitization does not upcoming plans for tunnels. These requirements are converted refer to the nature of the component parts of the controller, but into restrictions on the beginning and ending times of green lights rather a "digital methodology" of how traffic signal phases are for phases. A queue is converted into a minimum clearance time: chosen. In relation to traffic movements, there is a maximum of the ending time minus the beginning time for the phase must be at 16 possible sequences of phase pairs (states) at any quad inter- least equal to the time to clear the queue plus the change time section. Because it knows the real-time traffic demand, InSync is required for that light to turn green; similarly for a pedestrian call, able to instantaneously select and input to the controller any user- except that the clearance time is the time required for pedestrians permitted phase pair associated with these 16 sequences that to walk across the intersection. A plan for a tunnel is converted it deems optimal. InSync needs to decide: (1) optimal sequence, into restrictions on the beginning and ending times for the phase: (2) when to initiate a state (phase pair), and (3) duration of that the beginning time must be less than or equal to the beginning state. time of the tunnel minus the change time, and the ending time must be greater than or equal to the ending time of the tunnel. The It is not limited in its choices or their duration times by a set optimizer considers each permitted sequence as a sequence of cycle length, split, or offset. Except for minimums and maxi- transition times at which phases begin and end, with the restric- mums and 1 s passage times, all typical volume density inputs to tions transformed into inequalities in these transition times. For the controller are disabled so that it runs in free mode. This per- each sequence, a minimum-total-time solution satisfying these mits the controller to quickly react to and change the traffic sig- restrictions and the total waiting time for queues are calculated. nal according to the optimized calls coming through InSync. The sequence with the least total waiting time is chosen. If Another important advantage of this "state-changing" architec- no solution satisfying the restrictions exists for any permitted ture and methodology of signal optimization is that the traffic sequence, then the lowest priority of the restrictions are relaxed flow disruption caused by the transition from one static timing until a solution is possible: first, queues on permissive left turns plan to the next, or by preemptions, is eliminated. are transferred to their adjacent through movements, then the queues with the least waiting times have their clearance times reduced, then, finally, only the plans for tunnels are considered. Hardware and Software Requirements Once a solution is obtained, the times are translated into a sched- ule of states and the first state of the schedule is initiated. Hardware Overview--Cameras, Processor, Detector Cards, Ethernet Communications Early Release Sometimes, it is useful to prevent cars from leaving an intersection too early and collecting at a downstream InSync uses high-end IP digital cameras in weatherproof enclo- intersection, either because there is a limited amount of space sures that are normally mounted on mast arms with standard available for cars to queue or to absolutely ensure that the down- brackets. The cameras are connected to an InSync processor stream light is green when they arrive. This is sometimes called installed within each local traffic cabinet through a CAT-5 Eth- metering. To handle these situations, the optimizer has the con- ernet cable and a 24-volt electrical wire that provides power. The figuration option of restricting early release of a tunnel phase at InSync processor is placed in the local traffic cabinet and inter- an intersection. faces with the local signal controller using detector cards that are plugged into existing detector card racks. Other system hard- Period Length Evaluation During local optimization, the inter- ware includes a 110/24-volt transformer, surge protectors, an un- section continually analyzes its queue lengths and percentage of managed Ethernet switch, and a pigtail cable for red/green returns occupancy for each phase. If the intersection determines that it has to be fed back into the processor from the controller's leads. Except not been given enough overall time to adequately clear out its for an I/O board within the processor and the associated detector queues (drop the percentage of occupancy to a desired threshold), cards that are each required to communicate with the various pro- the intersection reports this to the facilitator (see the previous sec- prietary controllers, the system uses off-the-shelf components. tion: The Global Element). This method of adjusting the period is For arterial coordination to take place, Ethernet communica- reactionary. To be more proactive, each intersection constantly tions need to exist between the networked intersections. Because analyzes the flow rate of vehicles in each phase. If the current flow InSync uses distributed network architecture, an unlimited num- rates over the past 15-min durations are comparable to the histor- ber of signalized intersections may be coordinated.

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61 Configuration Procedures--Standard Web Interface A camera fails to talk to a video processing detector subsystem. InSync is both Ethernet and web-centric in its functionalities. The video processor determines the view is not sufficiently Each processor and every camera has an IP address. These com- clear. ponents can be accessed directly by means of the network without The processor does not hear from a particular detector any proprietary software. All the necessary configurations of and subsystem. A text alert is seen on the video image when a any software upgrades to the system software can be accom- sensor is in emergency/fog mode. plished remotely over the Ethernet network. The onsite cameras are properly aimed, zoomed, focused, and tightened to effec- Calls on all phases are automatically input when: tively view vehicles arriving at and progressing through a traffic signal. A web page associated with the InSync system is accessed InSync determines that the detected traffic is significantly through a standard Internet browser (Internet Explorer or Firefox) lower than historical averages, which indicates a sensor that leads a user through the process of drawing all the necessary failure. detection, count, and contrast zones that quantify the traffic data The I/O board fails to hear from the processor for 2 s. generated by approaching vehicles. It also provides a dropdown A detector card fails to hear from the I/O board for 2 s. menu page for all the adaptive system parameters. If communications between networked intersections fail, indi- vidual processors will continue to perform local optimization Interface Methodology--Determination of Inputs functions. Optimized Calls to Signal Controllers InSync is a plug-in technology that interfaces with all existing LA ADAPTIVE TRAFFIC CONTROL SYSTEM traffic signal controller and cabinet architectures. It controls traf- fic signals by submitting calls to the traffic controller through The Adaptive Traffic Control System (ATCS), developed by detector cards, just as inductive loops do. However, InSync only Los Angeles Department of Transportation (LA DOT), was first allows one phase pair at a time to be input to the controller by fil- deployed as a part of the Automated Traffic Surveillance and tering, prioritizing, and suppressing the demands generated by the Control (ATSAC) Center in 1984 for the Los Angeles Olympic detection of vehicles that are approaching an intersection in real Games. Prior to the implementation of the ATCS, the heart of the time. Pedestrian calls are also filtered by InSync and are permit- system was a group of mainframe computers that communicated ted at the times deemed optimal by InSync's real-time coordi- with both the control center operators and traffic signal equipment nation. InSync's calls are passive in that InSync will yield to any in the field. The software used by the mainframe computer was the higher priority calls that are directly communicated by users' Urban Traffic Control Software (UTCS) on an OS/9 real-time choice into the controllers; that is, preemptions or central system operating system. Funding for the system was provided by the city software priorities. In these cases, InSync will continue to serve as of Los Angeles, Los Angeles County Metropolitan Transportation Authority (LACMTA), and FHWA (Hu 2000). a detection device and then revert to its optimization mode when the controller begins to respond to its calls again. It can also be configured to toggle automatically between detection mode and Adaptive Traffic Control Logic optimization mode by TOD/DOW if users desire to use pre- determined timing plans. One of the primary goals of the LA ATCS development team was to develop an open system that can be used to test various control algorithms. The basic principle of ATCS adaptive operations is to Detector Requirements adjust signal timings on a cycle-by-cycle basis by changing cycle length, splits, and offset. Optimizers for splits and offsets are The video/data collection sensors (the IP cameras) capture and called in ATCS "Critical Intersection Control" and "Critical Link communicate real-time images of vehicles approaching an inter- Control," respectively. Each optimizer can function indepen- section to the InSync processors. The processor reads and inter- dently of others. The system allows for maximum flexibility when prets these images for its optimization processes. This kind of individual intersections are assigned to various sections (groups image tracking provides a sufficient estimation of real-time of signals). ATCS is a very responsive system that can respond to queue lengths and the percentage of occupancy of each lane and spikes in traffic demand. On the other hand, the system has some approach for optimization purposes. Advanced detection is not attenuating features that help the system avoid overreacting to essential to create an effective traffic-adaptive dynamic, although short-term variations. LA ATCS does not perform any optimiza- it can be incorporated seamlessly into the system. These data are tion when adjusting basic signal timings; however, it applies updated by the processor every second. (Similar traffic data heuristic formulas based on extensive operational experience. could also be input using other kinds of sensors.) Critical link and critical intersection approaches are used to calculate intersection splits and offsets. ATCS configuration parameters can be easily adjusted to adapt to different street con- Failure Mitigation figurations. The adaptive adjustment of signal timings is based on changes in volumes and occupancies, which are collected every When a sensor is placed in emergency/fog mode, InSync will second but utilized every cycle. The system allows for limitation access 4 weeks of historic green split data for specific TOD/ in variation of cycle lengths by providing its upper and lower lim- DOW at that particular approach. These data are normalized into its. When splits are adjusted minimum phase green times are con- a split time to put in to the controller until the sensor is function- sidered. LA ATCS does not alternate phase sequence, which is ing again. A call is issued for every phase for at least a minimum fixed all the time, but phases can be omitted based on the existing split time, which happens in the following cases: traffic demand. Figure A8 shows a Dynamic Map functionality in

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72 FIGURE A15 RHODES detection requirements. The performance of RHODES is highly dependent on the SCATS adapts and coordinates the intersections within each accuracy and quality of the detection system, but is not depen- subsystem and is able to coordinate adjacent subsystems. This dent on a particular type of detection technology. Therefore, coordination aims to divide the traffic on major roads into "pla- proper maintenance and monitoring of the detection system is a toons" (groups of vehicles) and to allow just enough time for each requirement for any RHODES installation. To this end, future platoon of vehicles to progress through the system while allowing revisions of RHODES will incorporate algorithms to recognize the green time required for competing flows. This maximizes the detector failures so that an alarm can be set for notification. network capacity for the benefit of all users. In addition, the extent of the failure upon RHODES will be assessed so that the system can either continue operating or be taken off line, as appropriate. Adaptive Traffic Control Logic Strategic and Tactical Control SCATS In SCATS traffic control is affected at two levels, strategic and The Sydney Coordinated Adaptive Traffic System (SCATS) tactical. Strategic control is managed by the regional computers (current version 6.7) is an Area Traffic Control (ATC) or Urban and is known as the Masterlink mode of operation. Using flow Traffic Control (UTC) system consisting of hardware, software, and occupancy data collected at the intersection from loop detec- and a unique traffic control philosophy that operates in real time; tors in the road pavement the computers determine, on an area adjusting signal timings in response to variations in traffic basis, the optimum cycle time, phase splits, and offsets to suit the demand and system capacity as they occur. Rather than changing prevailing traffic conditions as they occur. The strategic and tac- individual intersections in isolation, SCATS manages groups of tical control methods operate together to provide a powerful but intersections that are called "subsystems," the basic unit of the flexible operation. Strategic control provides overall system con- system. Each subsystem will consist of a number of intersections, trol of cycle time, phase split, and offset. Tactical control provides usually between one and ten. One of those intersections is des- significant local flexibility within the constraints of the strategic ignated as the controlling or "critical" intersection. control parameters.

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73 Tactical control is undertaken at the intersection by the local demand for that phase. The sub-phases have no direct SCATS traffic signal controller (local controller) and meets the cyclic specification or control; however, they are normally labeled with a variation in demand. Tactical control primarily allows for green numerical suffix (e.g., B1, B2). The sub-phases effectively extend phases to be terminated early when the demand is low and for the flexibility of signal control within the bounds of the primary phases to be omitted entirely from the sequence if there is no phase (e.g., left overlap phases). demand. The local controller bases its tactical decisions on infor- mation from vehicle detector loops at the intersection, some of Cycle Time Cycle time is increased or decreased to maintain the which may also be strategic detectors. DS at a user-definable value (90% is typical) on the lane with the highest value. Cycle time can range between 20 s and 240 s and the actual lower and upper limits used are configurable on a sub- Masterlink system basis. Cycle time can vary by up to 21 s per cycle; however, this upper limit is resisted unless a strong trend is recognized. This is the real-time adaptive mode of operation. In Masterlink mode the regional computer determines the phase sequence, the Phase Split Phase splits are specified as a percentage of the maximum phase duration, and the duration of the pedestrian cycle time or as a fixed time in seconds. For critical inter- green signal displays. The local controller may terminate any sections, phase splits in percentage are varied by a small amount phase under the control of the local vehicle actuation timers or for each cycle in such a way as to maintain equal DS on com- skip phases without a demand, unless prohibited by instructions peting approaches. The minimum split that can be allocated to a from the regional computer. phase can be configured, but is limited by a value determined from the local controller's minimum phase length. The current cycle The regional computer controls the phase transition points in time and the minimum requirements of the other phases limit the the local controller, but subject to the local controller safety inter- maximum split that can be allocated to a particular phase. val times being satisfied (e.g., minimum green, intergreen, and pedestrian clearance). On completion of the transition to a new Offset A number of offsets are configured for each intersection phase, the local controller times the minimum green and mini- within each subsystem and also between the subsystems that can mum pedestrian green intervals, and then waits for a phase termi- link together. Offsets are selected on the basis of traffic flow for nation command from the regional computer. On receipt of the each subsystem. The higher traffic flow links select the offsets command to move to the next phase, the local controller then that provide good progression for that link. Optimal offsets on the independently times the necessary clearance intervals (e.g., inter- higher flow links tend to minimize the total number of stops in green) for the phase termination. the system, reducing fuel consumption and increasing the capac- ity of the network overall. Subsystems The subsystem is the basic unit of SCATS strate- gic control. One subsystem is configured for each critical inter- section, which are intersections that require accurate and vari- Other SCATS Operating Modes able phase splits owing to their operational characteristics. The intersections in a subsystem form a discrete group, which are Besides the real-time adaptive traffic control mode (Masterlink), always coordinated together. They share a common cycle time, SCATS can run a variety of other "auxiliary" traffic control with an inter-related phase split and offset. Phase splits for all modes. Table A1 identifies these modes accompanied by short other intersections in the subsystem are by definition non- descriptions. critical, and are therefore either non-variable, or are allocated phase splits that are compatible with the splits in operation at Other important SCATS features are: the critical intersection. To provide coordination over larger groups of signals, subsystems can be configured to link with Hurry Call--the local controller invokes a pre-programmed other subsystems to form larger systems, all operating on a com- mode usually associated with an emergency phase or local mon cycle time as determined by the links at the time. These pre-emption such as a railway-level crossing phase. links may be permanent or may link and unlink adaptively to suit Schedule--SCATS allows for system operation to be the prevailing traffic patterns. A SCATS regional computer has scheduled. Scheduling can operate with any mode of SCATS a maximum of 250 subsystems. operation and can be used to switch between modes. Almost any function that can be executed manually can Degree of Saturation SCATS strategic control bases its deci- also be configured to occur at specified times on speci- sions on a measure of traffic demand known within SCATS as fied days. Degree of Saturation. In the SCATS context, Degree of Saturation Special Routines--a range of special routines is available is an empirical measure that is defined as the ratio of effectively in SCATS, which allows the user to vary operations to suit used green time to the total available green time. Using loop special conditions. Special Routines generally provide detectors at the critical intersections, the local controller collects an extension to the default adaptive behavior of the Master- flow and occupancy data during the respective green phase. These link mode. It is features of this type that enable every data are sent to the regional computer, which calculates the DS. detail of signal operation to be tailored to meet the oper- The DS is used as a basis for determining whether an increase or ational needs of each individual intersection. decrease in both cycle time and phase split is required. Phase Sequencing The signal cycle is divided into phases, and Hardware and Software Requirements up to seven primary phases are available. Each primary phase may additionally have several optional sub-phases, subject to certain The SCATS regional traffic control software has a maximum criteria. The primary phases (A, B, C, etc.) can be introduced in capacity of 250 intersections per region. With a maximum of any defined sequence. Any phase can be skipped if there is no 64 regions, the total capacity is 16,000 intersections.

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74 TABLE A1 NONADAPTIVE SCATS OPERATING MODES Operating Description Mode Flexilink Intersections are synchronized by local controllers clock and are therefore coordinated without any connection. Signal timings and phasing sequence (stored at the local controller) are determined according to a time of day schedule. Local tactical control is still operational in this mode, unless prohibited by instruction from Flexilink. Police Off The lamp state at the local controller has been turned off. Police Red All lamps at the intersection have been turned to red. Police Manual The phases at the local controller are being manually introduced. Maintenance This mode provides an indication to an operator that a technician is on site Mode servicing the controller. Flashing The normal signal display is replaced by flashing yellow displays on all Yellow approaches, or flashing yellow and flashing red to competing approaches. Regional Computers System Architecture and Communications The regional traffic control function uses standard PCs operat- Architecture ing under the Microsoft Windows operating system. A range of intersection communication methods is provided and includes SCATS has been designed in a modular configuration to suit network (TCP/IP), serial, dial-out, and dial-in. the varying needs of small, medium, and large cities. In its simplest form, a single regional computer can control up to 250 intersections. Expansion of the system is achieved by Central Management Computer installing additional regional computers on a TCP/IP network. SCATS also has the ability to internally manage several instances The Central Management Computer is a PC operating under the of the regional traffic control software on one physical com- Microsoft Windows operating system. Communications with puter. This provides flexibility in hardware configuration and regional computers and workstations is through TCP/IP. for simulation use. All systems have a Central Management Computer to manage global data, access control, graphics data, and data backup. A typical SCATS system is shown in Software Figure A16. SCATS comes with the Central Management Computer soft- ware that allows other software packages including SCATS sup- Communications port software to be used as part of the traffic management package. All SCATS software modules (i.e., regional, central, SCATS 6 supports the following communication methods picture developer, alarming and monitoring, and simulation) use between a region and an intersection: a PC platform and are compatible with the Microsoft Windows operating system. SCATS provides the end user with a modern Serial--for example, dedicated cable, leased line. GUI with a full capability in monitoring and controlling SCATS Network (TCP/IP)--for example, dial IP or ADSL using and traffic signal functions. TCP/IP. FIGURE A16 Typical SCATS system architecture.

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75 Dial-out--using the SCATS DIDO unit. detectors (i.e., including tactical detectors) might be provided Dial-in--using the SCATS DIDO unit. at a length suitable for strategic detectors so that strategic detectors can be selected from any of the detectors provided at There are messages to and from each intersection controller any time. every second. The minimum requirement is 300 bits per second (baud). The low speed rate required for SCATS communica- tions allows for a high degree of tolerance in the reliability of the Special Features communications network. Route Preemption In the event of regional computer failure, loss of communi- cations between the computer and any local controller, failure of Route preemption allows a user to manage the sequential all strategic detectors, or certain other local malfunctions, the introduction of a green window or green wave through several affected intersections will revert to a user-defined fallback mode intersections and is typically used for emergency vehicles or of operation. This may be either Flexilink (the usual fallback convoys. mode) or Isolated mode. If specified by the user, fallback at one intersection will also Time/Distance Display cause other intersections in the subsystem to operate their fallback mode and, optionally, intersections in adjacent linked subsystems. In this way, if Flexilink is specified as the fallback mode, a sig- Figure A17 shows a time distance diagram for viewing signal nificant degree of coordination can be maintained between inter- coordination in real time. The relationship of coordinated phases sections affected by the failure during the period of fallback. and offsets is displayed dynamically in real time. SCOOT Detection Requirements Adaptive Traffic Control Logic Tactical Detectors In SCOOT optimization of traffic control in the network is Tactical detectors are located at the stop-line to enable differen- achieved using small, regular changes in signal timings designed tiation between the left-turn, straight-through (ahead), and right- to avoid major disturbance of traffic flow. Loop detectors are turn movements at the intersection, both by knowledge of the polled by the controller for occupancy every one-quarter second lane usage in lanes of exclusive use, and by speed differential in and typically transmitted once per second to the central com- a lane shared by two or more movements. Tactical detectors puter, although the latest version of SCOOT relaxes this require- could be provided on all lanes of an approach (or movement) ment. Detector data are processed at central in 1-s intervals. that would benefit from tactical control. At a minimum, tactical SCOOT uses a hybrid measure of volume and occupancy [also detectors could be provided for minor movements. known as Link Profile Units (LPUs)] to express traffic demand at detectors. Approximately 17 LPUs equal 1 vehicle, although this value is variable depending on traffic behavior. The LPUs are Strategic Detectors then processed through a platoon-dispersion model, similar to the one used by TRANSYT, to create Cyclic Flow Profiles (CFPs). Strategic detectors measure how effectively the green time is Using CFPs together with the red/green signal status, SCOOT is used by traffic that is controlled by SCATS. Correspondingly, able to model a traffic demand profile at the stop-line (the queue SCATS uses strategic detectors to accurately determine the on the approach). SCOOT's internal traffic model maintains a required green time for an intersection approach. Stop-line detailed, real-time image of the traffic network (similar to detectors installed for tactical control are used as strategic detec- TRANSYT-7F or CORSIM). As such, a major part of a SCOOT tors, subject to certain criteria. Strategic detectors can be system installation involves calibrating and validating the net- also located upstream from the stop-line, in which case the cal- work model to match network field conditions. Overview of culation of DS will be biased and detection of traffic queues SCOOT operations is shown in Figure A18. becomes possible. At times, stop-line detectors at the upstream intersection are used to control a downstream intersection (it SCOOT has three optimization procedures by which it adjusts helps to identify queues). It is logical that approaches most signal timings--the Split Optimizer, the Offset Optimizer, and the requiring strategic detection are those least requiring tactical Cycle Optimizer. Each optimizer estimates the effect of a small detection, and vice-versa. However, the installation of detectors incremental change in signal timings on the overall performance at all intersection stop-lines regardless of fundamental need pro- of the region's traffic signal network, which is measured through vides a degree of redundancy and increased strategic control a performance index, a composite measure based on vehicle flexibility. delays, and stops on each link. Calculated signal timings are trans- mitted to the local controller every second. The length of strategic detectors is critical for accurate cal- culation of DS. Detectors shorter than a critical length tend to The Split Optimizer works at every phase change by analyzing perceive traffic as widely spaced in the conditions of slow mov- the current split timings to determine whether the split time is to ing, closely spaced traffic. Conversely, if the detectors are too be advanced, retarded, or remain the same to achieve the degree long they would not measure any spaces when traffic moves of saturation. Split changes are typically in increments of 1 or freely. Historical research has shown that a suitable detector 4 s by default, but include the ability for operator-configured val- length is 4.5 m, but acceptable lengths go as low as 3.5 m. All ues to be used.

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76 FIGURE A17 Time/distance display in SCATS. The Offset Optimizer works once per cycle for each inter- The Cycle Optimization operates on a region basis once every section. It operates by analyzing the current situation at each 5 min, or every two and one-half minutes when cycle times are intersection using the CFP predicted for each of the links with rapidly changing. It identifies the "critical intersection" within upstream or downstream intersections. It then assesses whether the region (any of the intersections in a system or sub-area can the existing offset time is to be advanced, retarded, or remain the determine the system cycle length), and will attempt to adjust the same. Offset changes are also in 4-s intervals. cycle time to maintain this intersection with 90% link saturation FIGURE A18 Overview of SCOOT operations.

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77 on each phase. If it calculates that a change in cycle time A customized congestion management tool kit, and is required, it can increase or decrease the cycle time in 4-, 8-, or Improved access to data management. 16-s increments depending on the current cycle time value. SCOOT is not constrained by a "master" intersection in deter- mining system cycle lengths. System Architecture and Communications Figure A19 shows a typical SCOOT architecture. A SCOOT Hardware and Software Requirements system can implement a centralized strategic traffic policy, reacting to variations in demand in real time. The centralized Standard Controller Firmware system also allows system-wide strategies to be employed. Examples of these strategies are: SCOOT runs with standard Siemens SEPAC firmware, the same firmware used for standard intersection control at 50,000 intersec- Peak hour routes, tions across the United States. Older SCOOT systems used an Keeping emergency and evacuation routes clear, EPAC controller firmware upgrade for the existing controllers. Traffic metering (gating) on the outskirts of congested SCOOT can also be installed with pre-programmed logic on new areas, and EPAC and 2070 controllers. Central bus priority. Traditionally, SCOOT has been using dedicated (leased line, SCOOT Software Platforms copper cable, fiber optic, or combinations) multi-drop transmis- sion lines to outstations. SCOOT requires second-by-second communications between the central computer and outstations. The kernel software at the heart of a SCOOT system is standard Typically, six to eight intersections can be served by 1200 baud for all installations. The additional software that links the rate. Recently, the PC version of SCOOT was enhanced to SCOOT kernel to on-street equipment and that also provides the enable the use of modern communications technology used by user interface is supplier-specific. ITS solutions. This approach absorbs inconsistencies and delays in data delivery with less impact on the system. This new approach Traditionally, SCOOT kernel has been operating on Alpha reduces dependency on traditional leased-line communications DEC computers and Open VMS operating system. Recent ver- techniques and opens up the potential to use a wide range of sions of SCOOT also operate on a PC platform with a Microsoft modern communications technologies previously unavailable to Windows operating system. The PC platform provides the fol- SCOOT systems. lowing benefits: Use of standard PC components, Detection Requirements Reduced hardware and software costs, Improved network efficiency, SCOOT uses upstream detection to collect its traffic information. Ease of use and training for new users, Upstream detectors are usually installed in the vicinity of the pre- SCOOT/ASTRID/ ACTRA Server INGRID Server Modem Remote Diagnostics Hub Terminal Private Ethernet Server 10/100 Public Ethernet Laser Report Printer Comm. SCOOT Detector Servers Dot-Matrix Log Printer Once Controller 1 per sec. Modem Dial-in Local Detector Once per min. Validation SCOOT Detector Roving Terminal Up to 16 drops per channel Validation @ 9600 baud Local Detector Controller n FIGURE A19 Typical SCOOT architecture.

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78 vious upstream intersection to reduce communication costs. Ide- It can be noted that there is no guarantee of priority to buses ally, SCOOT detection needs to be located at least 7 s of travel time at an intersection. With each split decision, SCOOT will still upstream (further is acceptable and often better). Upstream detec- take into account the percent saturation of all approaches, and tion provides a view of the traffic approaching an intersection in a will still maintain the principle of small but frequent changes. TRANSYT type "flow profile." Utilization of the upstream detec- The detection of a bus merely gives a higher priority to that tors allows SCOOT to: stage. This is in contrast to a full over-riding priority system that is constrained only by maximum and minimum stage lengths. Be more sensitive to sudden changes in traffic conditions, Buses may be detected by either static means (e.g., loop detec- Be able to respond more quickly in congested conditions, tors or 3M Opticom) or by means of an automatic vehicle loca- Calculate queue lengths more accurately, and tion system. Base its changes on incoming "traffic flows," rather than latent "traffic demand." Over-Saturated Conditions Special Features SCOOT has several methods with which to handle over-saturated conditions: ASTRID and INGRID Congestion importance factors/congestion offset per link, Data used by the SCOOT model in the optimization process such Congestion links with congestion importance factors, as stops, delays, flows, and congestion levels, are available to the Gating, and user through the ASTRID (Automatic SCOOT Traffic Informa- Variable-Intersection Based Target Saturation for cycle tion Database) system, which automatically collects, stores, and time optimization. processes traffic information for display and analysis. When a detector fails and link data cannot be collected in real time, his- A congestion importance factor is specified for each link. It is torical link data from ASTRID can be used by the SCOOT opti- used to influence split calculations in favor of the link when mizers to maintain a high level of system efficiency. congestion is detected. Another factor, congestion offset, is a fixed offset, specified by the traffic engineer, to be used in con- The INGRID (INteGRated Incident Detection) system was gested conditions. Congestion weighting factor allows the developed to automatically detect traffic incidents in urban engineer to specify the importance of achieving the congestion areas. The system uses information from SCOOT and the offset. ASTRID database to compare current conditions with historic values. Information provided by INGRID includes time of inci- Gating, or action at a distance, allows the restriction of the dent, duration, location, area affected, severity, and confidence green times of the entry links to the congested area; or, exiting level. links downstream of a congested area may be granted more green time to allow traffic to clear. Bus Priority A cycle optimizer normally uses 90% as its target satura- tion level (80% when the "Trend Flag" is set, to give more Two approaches to bus priority are available with the SCOOT rapid response). Intersection-based target saturation levels system: may be set by the traffic engineer, whereby a low threshold value will produce an early increase in cycle time and a high Local-based priority, and threshold value will allow an early drop in cycle time at the SCOOT-based priority (only for intersections under end of peak period. SCOOT). Local Priority Siemens ITS controllers provide up to six pre- UTOPIA emption (high-level) and six priority (low-level) routines. When the local equipment (the most common system is 3M's Opti- MIZAR's Traffic Light Control and Priority System, UTOPIA, comTM) detects an approaching bus, it sends a request to the was developed during the 1980s in response to the need for a local controller to initiate the appropriate priority routine. This fully automated system able to increase the fluidity of traffic and routine determines the current signal phasing, evaluates the transport, and to reduce travel times across a wide-area network. direction of travel and whether the priority is for the main street UTOPIA is installed and operates in numerous cities throughout or the minor street, and executes the preprogrammed response. Europe. SCOOT-Based Priority SCOOT is able to specially accom- The basic principle of UTOPIA is to perform a real-time modate buses within its normal optimization routines. Bus pri- optimization of the signal timings to minimize the total socio- ority may be provided by green extensions, stage recalls, or economic cost of the traffic system. These costs are usually both. An extension is given when a detected bus could be served expressed as traffic congestion, vehicular emissions, and travel by an extension of the current green. A recall is implemented times both for private traffic and for public transit vehicles. Con- when a detected bus is expected to arrive at the stop-line at a red trol strategies are computed in real-time taking into account the light (i.e., the signal is currently red or a maximum length exten- measured traffic metrics at intersections as well as forecasted sion would not be sufficient to serve the bus in the current stage). private traffic demand at both the intersection and network lev- In this case, the intersection cycles as quickly as possible to els, and the predicted arrival times of the public transit vehicles return to the bus stage. at the intersections.

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79 UTOPIA offers a full suite of traffic control strategies The cost elements of the objective function optimized at the including: local level are: Fully Adaptive Travel time of the vehicles on the incoming links, Time Based Plan Selection Number of stopped vehicles on the incoming links, Traffic Actuated Plan Selection Excess queuing on the incoming links (queues that exceed Traffic Responsive (micro-regulation). thresholds proportional to the link capacity), Travel time of the vehicles on the outgoing links, and These various strategies can be applied simultaneously at dif- Travel time of public transit vehicles. ferent zones within the same network. They can also be modi- fied independently at any time of the day. Cost elements are evaluated during the entire horizon taking into account existing signal timings and phasing constraints (e.g., minimum and maximum green times). UTOPIA allows for Fully Adaptive Control utilization of various weights for various links and priority rules. Figure A20 describes interaction between UTOPIA's two funda- mental modules operational at both the local and central levels: The highest performance of UTOPIA traffic control is achieved in the State Observer and the Controller. the fully adaptive mode in which UTOPIA responds in real time to traffic conditions on the network. The system calculates the con- trol strategy using information sent every second from sensors The distributed architecture of the system derives directly from the method adopted to decompose the area optimal control located near each intersection. problem into a set of simpler and strongly interrelated sub- problems. Problem decomposition is performed following a Traffic control is determined through optimization processes topological rule: first, the area is subdivided into overlapping and by the application of the rolling horizon technique at both zones, where each zone is logically centered on an intersection the central and local levels: and includes neighboring intersections as well. Then an optimal control problem is defined for each zone, which takes into At the central level, the network control strategy is opti- account traffic data and traffic light control information related mized over the next 30 to 60 min time horizon (depending to the all the intersections within the zone. The solution of the on the size of the network controlled) and is updated every zone control problem determines the traffic light control to be 5 min. Each strategy is effective for not more than 5 min, actuated at the central intersection only but, owing to the over- and is then replaced by a new strategy. lap between neighboring zones, is strongly interrelated with the At the intersection level, signal timing optimization is per- control at all surrounding intersections. formed on the time horizon of the next 120 s and is repeated every 3 s. The resulting optimal signal settings are actually Zone-by-zone control optimization is iterated frequently based in operation only for 3 s. on a rolling horizon technique to detect demand variations promptly and to react consequently. Because of the strong Objective function, which is optimized at the intersection level, interaction, the effects of any demand variation in one zone are consists of terms related to the traffic observed on approaching rapidly propagated to all the surrounding zones. This scheme allows UTOPIA to implement a fully adaptive optimal area con- links to the intersection and also those links to which can be trol. Also, the control scheme supports the physical organization applied one of the following two fundamental interaction of the system according to a hierarchical and decentralized archi- principles: tecture, where the SPOT roadside unit performs the zone control functions at the intersection, whereas the central level deter- 1. A strong interaction principle that accounts for the delay mines dynamically the criteria and the reference strategies that at the downstream intersections experienced by vehicles need to be considered during local optimization. leaving the intersection under consideration; or 2. A look-ahead principle that accounts for the traffic fore- cast during the entire optimization horizon (120 s) for all Hardware and Software Requirements incoming links. Fully compatible with 32-bit and 64-bit architecture, the UTOPIA Implementation of the strong interaction principle requires server(s) provides high performance with minimal require- knowledge of the traffic light status at the downstream intersec- tions. On the other hand, implementation of the look-ahead principle requires knowledge of the traffic light status for the upstream intersections and the availability of traffic informa- tion for the incoming links of the upstream intersections. To achieve stability and robustness at the network level, inter- actions are defined between the local level and the central level. At the central level, the optimal network traffic control problem is developed based on the macroscopic traffic model of the network, and control strategies such as minimum, average, and maximum length of each stage, offsets, and weights for all the elements con- stituting the objective function optimized locally are defined for FIGURE A20 Interaction between modules each intersection. in UTOPIA.

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80 TABLE A2 REQUIREMENTS FOR UTOPIA SERVERS Servers Applications Operating system Microsoft Windows Server 2003 Database engine Microsoft SQL server 2000/2005 Web Server Microsoft Internet Information Services (IIS) Map Server Autodesk MapGuide Application framework Visual C++ Microsoft .NET Framework 2.0 ments. Typical requirements for UTOPA servers are provided in strained by the network control strategy from the higher level. Table A2. UTOPIA's architecture is modular, which makes UTOPIA a sys- tem easy to extend and integrate with other ITS applications (e.g., public transport management). The distributed architecture of the Requirements for Utopia Servers system derives directly from the method adopted to decompose the area optimal control problem into a set of simpler and UTOPIA uses Browser Based Clients to provide high accessi- strongly interrelated sub-problems. bility to the system without requiring any specific workstation configuration. Although essentially independent of the Operat- ing System, the UTOPIA web-based interface works best in SPOT common web browsers such as Internet Explorer 7 and Firefox 2. UTOPIA's interface offers full multi-tasking capabilities and SPOT is the software that performs local UTOPIA functions in different privileges can be provided for users of different admin- the intersection controller's cabinets. SPOT is installed as a sep- istrative levels. arate unit that communicates with intersection controllers. SPOT also exchanges information with: UTOPIA can be interfaced with the new OMNIA platform to facilitate monitoring of the traffic network and ATCS oper- Neighboring SPOT units to cooperate in the definition of the ations. Almost all of the UTOPIA's user interface functional- local control strategy and to implement dynamic coordina- ities are directly accessible from the OMNIA Common GUI. tion suitable for both private traffic and transit priority. Specialized clients of the UTOPIA system need a specialized Central system to receive commands, priority requests, UTOPIA workstation (the MS Windows XP operating system is and traffic control strategies, and to send traffic parame- recommended). ters, a locally actuated traffic control strategy, and diag- nostic information to the central system. System Architecture UTOPIA Central Functions UTOPIA has a two-level hierarchical and distributed architec- ture, which is shown in Figure A21. The higher level is responsi- The UTOPIA central functionalities can be assigned to three ble for setting the network control strategies, whereas the lower major groups, as shown in Figure A22. In the Traffic Network level (SPOT--at local intersection controllers) implements Monitoring group (far right) traffic measures (volumes, speed, signal timings according to the actual local traffic conditions con- classification data, etc.) and parameters (clearance capacities, turning proportions, actuated signal plans, etc.) are gathered and stored in the central system archive together with their statistical profiles. Automatic incident detection and congestion warning SPOT environment FIGURE A21 UTOPIA and SPOT interaction. FIGURE A22 Functional architecture of UTOPIA.

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81 functions feed further DB files. All data are made available for Communications on-line, off-line, and export system processes. In the System Diagnostics group (second from right) information on the oper- The communications network provides the communication links ational status is collected for all system components and made within UTOPIA architecture and between UTOPIA and external available to the operator through screen displays and special systems. The communications are based on a flexible WAN archi- reports for monitoring and maintenance purposes. Automatic tecture, which supports several different media such as fiber optic, alarms are generated for abnormal situations. Finally, in the dedicated telecommunication lines, VPN based on DSL connec- Traffic Control and Priority group (left) traffic control strate- tions, and private copper cables; wireless technology; and various gies are developed and delivered for implementation to the communication protocols (e.g., standard TCP/IP and proprietary intersection level. Transit signal priority is handled in two serial protocol). Particularly for fully adaptive operations, ways: priority requests can be generated internally through the UTOPIA requires a robust and reliable communications network PT Locator functionality or received from an external fleet between the local SPOT units and between the local level and the management system. In both cases arrival times of the prior- central system, with the minimal capacity of 9.6K bauds. ity vehicles are forecasted and forwarded to the intersections. Also, a backup priority management function is implemented at the local level based on the continuous exchange of infor- Special Features mation between the SPOT units. Traffic data and control strate- gies are also exchanged with other mobility management systems Plan Selection Strategies through a gateway to allow for cooperative monitoring and con- trol of the area. These traffic control strategies are suitable for networks with pre- dictable TOD and DOW traffic patterns. Once a set of typical sig- nal timing plans is defined (based on the historic traffic data from Detector Requirements detectors) and stored in the system library, it can be activated by any of these three methods: When UTOPIA runs fully adaptive control strategies, the traffic state estimation requires traffic detectors located on entry and Automatic pattern-matching plan selection--activation is exit lanes of the intersection approaches (shown in Figure A23): automatically performed whenever a certain "traffic pat- tern" is recognized in the measured traffic data; Entry detectors are needed to measure incoming traffic Time-based plan selection--activation is based on date, platoons and model expected queues at stop-lines, and time, and type of day criteria; and Exit detectors are used to dynamically estimate parameters Manual mode--operator activates the signal timing plan such as turning proportions and saturation flow rates. manually. UTOPIA requires detectors only on the intersection approaches The signal timing plan is implemented at the intersection level by with significant traffic volumes and where traffic fluctuates sig- the SPOT software. SPOT switches between signal timing plans nificantly. One can note that in an urban grid environment exit using a "smooth" transition technique (i.e., the green splits and off- detectors from upstream intersections can serve as entry detec- sets are changed gradually to avoid discontinuity or jumps in the tors for the downstream intersections. UTOPIA usually uses phase sequence). Also, if traffic control is executed through the queue detectors at the approaches where queues typically are plan selection strategies SPOT takes control of traffic signal prior- critical. Any detection technology can be used providing that it ity for public transit, emergency, and VIP vehicles. reports two fundamental detection measures: traffic counts and occupancy time. Traffic Light Priority Management UTOPIA is able to assign absolute, weighted, and selective prior- ity to buses and trams at signalized intersections. This function can also be extended to emergency and VIP vehicles. For public tran- sit vehicles, traffic signal priority is implemented through: Functional integration with an automatic vehicle loca- tion/automated vehicle maintenance system, Local detectors (active and passive), Dedicated detectors managed by the UTOPIA PT Locator central functionality, and A combination of these methods. In UTOPIA the priority requests are represented by forecasting the arrival time of the priority vehicles at the intersection. In gen- eral, the priority requests are prepared by the central level (auto- mated vehicle maintenance or PT Locator) and forwarded to the local level, where the SPOT control function handles properly. FIGURE A23 A common intersection detection When local detection is implemented, SPOT also generates the layout in UTOPIA. priority requests.

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82 In all cases, the SPOT unit implements a distributed forecast curves centred on the predicted arrival times. These curves function, which complements the central forecast functionality become steeper as the vehicle approaches the intersection and the and consists of the local propagation of the arrival forecasts from forecast variations decrease. UTOPIA begins to calculate the SPOT to the downstream intersections along the priority vehicle control strategy (stage duration, offset, etc.) when the public tran- route. Forecasts are promptly corrected by SPOT when priority sit vehicle is approaching the intersection. In this way, it ensures vehicles are delayed at the traffic light. that the traffic signal phases are managed in a way that minimizes the impact on other vehicles. To determine the level of priority, "weights" are assigned to specific vehicles (e.g., according to the line, direction, or vehicle adherence to the schedule) locally or centrally. In UTOPIA, pub- The SPOT control function is also responsible for recovering lic transit priority is achieved within the intersection optimization any disturbances to the local optimization strategy resulting from process and not as a result of post-processing actions. Opti- the priority request. For adaptive mode, the recovery action is mization is carried out every 3 s; therefore, the system can react based on a fine optimization of the waiting times on all the traffic quickly to any changes in the predicted arrival time. When the movements. In the plan selection mode, the system is "re-hooked" vehicle has safely passed the intersection, the priority request is gradually to the selected plan to compensate for the effects of the "cleared" by the SPOT unit. priority provision. The system can provide priority to emergency and VIP vehicles as long as these are equipped with on-board In the definition of the intersection state, PT vehicles are con- transmitters so that they can be detected. The priority in this case sidered in the same way as private vehicles; they are represented is managed at the local level and is based on vehicledetector by equivalent "vehicle platoons," which appear as probability communication.