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2. Existing Signal Timing Process

Signal timing is a task that frequently involves coordinating activities from many different departments of the jurisdiction. It is not unusual for the traffic counts and mapping data to be provided by the Planning Department, the timing optimization analysis to be conducted by the Traffic Engineering Department, with the actual parameter installation being done by the Maintenance Shop. It is important to recognize that the signal timing process is not simply executing a computer program; rather, it is a continuing series of tasks that involve persons with many different skills. Two of the most prominent are the traffic engineer and the traffic signal technician. The engineer typically uses a model, like Passer-II or Transyt-7F to derive the timing plan which is defined in terms of a cycle length, split, and offset. These data are then provided to the traffic signal technician who must convert these variables into the timing parameters used by the controller. These parameters are the phase Force-off, phase Holds, and Permissive Periods.

It is useful to examine the entire signal timing process as it is commonly practiced today in many cities and counties. The complete process is probably more complex than one might expect. Figure 2 illustrates the major activities and interfaces that are typically followed to update signal settings. Whether the process is applied to a single intersection or to an entire city, the steps are the same. It is also interesting to note that that the same steps must be followed whether the process is entirely manual or completely automated.

In the following sections, we step through the process with the purpose of identifying issues that provide an opportunity for improvement.

2.1 Trigger Event

In the real world, the signal timing process begins with a "Trigger Event". This event may be as benign as a scheduled activity to retime the controller every few years. More likely, however, the impetus for new signal timing is a citizen complaint ("The light is too short"); a major change in the road network (widening of the existing arterial); or a significant change in demand (opening of a shopping center). Whatever the cause, the initial response is usually a review of the existing timing and equipment to make sure the there is no hardware failure. One of the most common signal timing complaints is that the phase time is too short. This is frequently a result of an intermittent detector failure. This initial response, then, is to affirm that the hardware is operational and the timing parameters are operating as planned. After the Trigger Event, there are two basic paths through the process: Field Adjustments and System Retiming. Both paths are described below.

2.2 Field Adjustments

Once the hardware is determined to be operating correctly, the next task is to determine if controller timing parameters have to be adjusted to respond to changes in traffic demand. Many times, a simple adjustment of one parameter may be all that is necessary. It may be possible to accommodate longer queues on the main street, for example, by simply advancing the Offset by several seconds. Other timing problems can be resolved by simple adjustments to the Minimum Green or Vehicle Extension parameters. These types of issues are resolved by a positive output from the "Field Adjustment" decision in Figure 2. In most jurisdictions, the entire sequence, from determining the type of problem, to making the adjustments, to evaluating the results, to recording the changes is a manual process that relies on the experience of the Signal Engineer (or Signal Technician) to provide a solution. Obviously, the quality of the solution is a function of the experience and dedication of the person doing the work.

Flow chart of activities and interfaces needed to update traffic signal settings.
Figure 2. Typical Approach to Signal Timing.

Field Adjustment Issues – There are three issues that are illustrated in this path through the flow chart:

  1. The criterion used initially to diagnose the problem is arbitrary and relies on the experience of the Signal Timing Engineer (Field Adjustment or Complete Retiming) to make the correct decision. The need is to better define the diagnostic process to enable a more consistent performance in determining the extent of the problem.
  2. Once the adjustments are completed, the process relies on the experience of the Signal Timing Engineer to judge that the adjustments are an improvement ("Looks OK"). The need is to formalize the evaluation to enable a more consistent performance by non-expert personnel.
  3. The changes are typically recorded manually. The need is to mechanize this activity.

One potential improvement would be to identify specific points in the signal timing process where objective criterion can be employed to reduce the subjectivity to a minimum. Another potential improvement is to clearly define the steps that are typically performed manually (Adjust and observe), so that new practitioners have a set of guidelines to follow. The third potential improvement would focus on the documentation (recording timing plan changes) and determine ways to automate this activity.

2.3 System Retiming

Of course, signal retiming is not about making simple adjusting to a few timing parameters in a controller. Most jurisdictions follow a more complicated effort to retime a signal or group of signals using modern computer programs and procedures.

2.3.1 Turning Movement Counts

This path through the flow chart begins with a determination of whether there is adequate traffic count data. For the most part, the need is for turning movement counts that reflect the traffic demand. Most Traffic Engineers consider four plans to be the minimum required for proper signal operation: AM Peak Plan, Day Plan, PM Peak Plan, and the Night Plan. The need, therefore, is to have a turning movement count for each of these four periods.

While this seems simple enough, it is not inexpensive. Collecting these data typically costs in the range of $500 to $1,000 per intersection or more. Converting the raw count data into a format useful for analysis easily can double the cost. This is an area where significant progress has been made. For example one vendor, Jamar Technologies Inc., makes an electronic data collection board that is easy to use, accurate, and reliable. Although an observer is still required to record the movements, once the observations are completed, the data are easily uploaded to a computer for further processing.

The more elegant solution to this problem, however, is to collect the data using existing system and local detectors and to derive a complete traffic volume network with all turning movement from these detector data. Several systems, QuicNet/4, MIST, Pyramids, and Actra (probably there are others) have the capability to export traffic count data from existing count stations. The missing capability is to be able to use this information to build a complete network turning movement schedule.

Traffic count data must be considered in two dimensions, temporal and spatial. In the temporal dimension, traffic count data at any one point varies from period to period as traffic demand ebbs and flows. In the spatial dimension, we frequently require traffic count data at many different intersections for the same time period. In addition, to accommodate certain flows through a series of intersections, we need to know the upstream origin of the demand for each turning movement at the downstream intersection.

Traffic Count Issues – The need for traffic counts is not a unique demand for signal timing, most Traffic Engineering endeavors require traffic count information. Traffic signal timing, however, does require accurate turning movement counts. The following issues were identified:

  1. In many instances, total flows into and out of an intersection are known, but the turning movements are not. The need is to have a process to estimate turning movements given intersection entering and exiting flows and partial turning movement information.
  2. Turning movement observations at adjacent intersections are frequently made on different days. The need is to develop an easy-to-use process that reconciles data from adjacent intersections such that the entering and exiting flows are reasonable balanced and reflects actual traffic flows.
  3. Many existing signal systems are capable of recording traffic flows at a detector location. The need is to be able to expand these system-derived point measures into tuning movement counts that can be used for signal timing optimization.

2.3.2 Field Inventory

All signal optimization and simulation models, even manual signal timing procedures, require a physical description of the network. This description includes distance between intersections (link length), the number of lanes, lane width and grade, permitted traffic movements from each lane, and the traffic signal phase that services the flow. Building a network from scratch is a significant undertaking. But once the network is defined; in general, only traffic demand and signal timing parameters have to be updated to test a new scenario. In recent years there have been a number of programs introduced that expedite this process. One on the most pervasive is Synchro which is discussed in a following section.

However, other popular optimization programs, Transyt-7F and Passer II, have had recent upgrades to their user interface to facilitate the input of the network descriptive data. A new Transyt-7F input data editor introduced within release 9.3 is intended to provide users with a more efficient and intuitive mechanism for coding input data. Release 9.5 takes the process further with dozens of additional interface enhancements, making the software more intuitive and similar to other Windows applications. Using the new interface, Transyt-7F input data can be coded directly into software screens designed to mimic the familiar Highway Capacity Software (HCS) input data screens. A sample screen is shown in Figure 3.

Screenshot of the Transyt-7F input data editor on an input data screen, showing timing for selected signals.
Figure 3. Transyt-7F Data Input Editor.

The Texas Transportation Institute recently released a Windows upgrade to Passer II. The upgrade (Passer II-02) has a new user interface and an enhanced optimization routine. The input data requirements have not changed; however, it preserves the lane configurations supplied by the user. This was achieved by introducing a new format for the input data file. Existing users are supported because the program can read old input data files and will automatically convert them to the new format. A sample screen is shown in Figure 4.

Screenshot of a PasserII-02 input screen, showing project identification tab and data to provide systemwide data and settings.
Figure 4. PasserII02 Input Screen

The important point to recognize is that there are a number of developments that are underway that will aid the engineer in managing the optimization model data. These include upgrades to the models themselves as illustrated above; and the development of programs that are designed to manage the data, such as the Arterial Analysis Package Executive (AAPEX). The private sector has also contributed to this capability with programs like Synchro, and Signal2000.

Descriptive Data Issues – In spite of the advances that have been made by the developers of programs like Synchro, Passer and Transyt, most users still use hardcopy, manual files to keep track of the descriptive data. Issues related to these data are:

  1. Much of this information is available graphically on maps. Other data elements are recorded on paper in file folders. Still other data elements are routinely stored in the controller cabinet itself in the field. The need is to have a technique whereby all these data are indexed so the user would know what exists and where it is.
  2. As more and more agencies become more proficient in the use of computers and data management systems, there is a need to replicate the manual system that uses maps, diagrams and paper forms in the digital environment.

2.3.3 Signal Timing Optimization Models

When most Traffic Engineers consider signal timing, the first thought invariably is directed to the optimization models. Issues like, which model is best, and what are the minimum data required to use the model, are typical topics. Over the years, much research effort has been directed to developing these models.

In June 2000, Trafficware Corporation, Synchro developers, conducted a market survey[1] to find out what software packages were being used for traffic analysis. The survey asked transportation engineers about software use, quality, reports, and productivity. There were also questions about intersection analysis methods used for planning applications. The anonymous survey was mailed to randomly selected ITE members.

The survey was mailed to 400 randomly selected ITE members and 76 surveys were returned. The survey asked the respondents to discard the survey if they don't work with traffic analysis software. The response rate was considered to be quite high considering that many ITE members work in fields other than traffic analysis. The survey responder categories are shown in Table 1.

Table 1. Survey Responder Categories.
Employer Number Percent
Local Government 21 29%
Private 43 60%
State 8 11%

The average respondent had 14.8 years of traffic engineering experience and the survey results reflect a total of 976 years of experience. Of interest to the signal timing optimization issue was the question, "Which traffic modeling software packages have you used since 1995?" The responses related to signal timing optimization are shown in Table 2.

Table 2. Signal Optimization Program Useage.
Signal Timing Package Percentage Score
Synchro 54%
Transyt-7F 25%
Passer II 23%
Passer III 9%
Soap 4%
Passer IV 4%

This listing of commonly used signal optimization models provides a good starting point for describing the signal optimization. Specifically, the process must interface with these popular models as well as manual methods, and it must allow for the interface with additional models that may be developed in the future.

One way to classify these models is by the optimization technique. Several of these programs generate optimum "bandwidths" for given demand and physical constraints. TSPP/Draft, TSDWIN, Passer II, Passer IV, and Nostop are bandwidth maximization models. The other programs use various forms of deterministic models to arrive at an optimization. These models include Synchro, SOAP, Transyt-7F and Passer III. A brief description of each of these models is provided below.

TS/PP-DraftTS/PP-Draft is an arterial-based time-space and platoon progression diagram tool. With TS/PP-Draft, the user can change control parameters such as phase sequences, splits, offsets, or cycle lengths, and observe an immediate change in a graphical time-space diagram. The program allows the user to select one of two types of time-space diagrams: 1) a time-space diagram with green bands showing the approximate location of the platoon, and 2) a platoon progression diagram showing the traffic flow and queue lengths.

For a detailed analysis of platoon dispersion, the program requires the following data: speeds and distances between intersections, number of thru lanes, cycle length, phase sequences, splits, right-turns-on-red, volumes, and ideal saturation flow rates. The program calculates the "actual" saturation flow rates using the method prescribed in the Highway Capacity Manual. For a display of the time-space diagram only, the program does not require any of the traffic volumes or lanes data. TS/PP-Draft allows the user to observe platoon progression flows to enhance the fine tuning capability of the program. Based on the type of arrivals, the user may easily adjust the offsets, phase sequences, or other control parameters and view an immediate change on the monitor.

TS/PP-Draft is fully compatible with the AAP, thus allowing the user to import and export AAP files. TS/PP-Draft can also import and export delimited ASCII data and provides a context-sensitive, on-line help.

TSDWINTSDWIN is a Windows-based graphical tool, designed to assist analysts responsible for fine-tuning signal timing plans. The purpose of the program is to provide a quick and easy method to achieve graphical representation of time-space diagrams for either a single artery or a group of arteries, based on existing or proposed signal timing data (cycle length, splits, and offsets).

TSDWIN organizes intersections into arteries and arterial groups. The program has a capacity of 999 arteries and up to 12 intersections per artery. A combination of crossing arteries can be fine-tuned and analyzed in a single run. Timing for any intersection, including those that are common for crossing arteries, can be locked to prevent changes. Data and corresponding graphical displays may be selected in either metric or imperial units. Splits and offsets may be entered in seconds or percent. Coordination points can be referenced to either the beginning of the green or the yellow interval. The program also allows the user to select a double-cycle option for any intersection.

Data inputs required for TSDWIN include spacing and travel speeds between intersections, cycle length, splits, and offsets for all intersections. Traffic counts are not required. The program's outputs include graphic displays of the green band and flows. The green band is color-coded and measured in seconds. A green band for both directions of traffic movements is shown if one can be calculated based on the timing data entered by the user. If a continuous green band cannot be calculated, the link-to-link green band is presented. This allows the user to evaluate how offsets might be adjusted to achieve a continuous green band.

TSDWIN allows the user to vary the speeds between the intersections and determine the associated impact on existing or proposed progression bands. Furthermore, using a mouse, TSDWIN provides an interactive user-interface to change the offsets, splits, and lead-lag phase orders, and it recalculates the time-space diagram parameters automatically when changed. Other features of TSDWIN include its ability to import data from PASSER II, import and export delimited ASCII data, and access to context-sensitive on-line help.

Passer IIPasser II (Progression Analysis and Signal System Evaluation Routine) was originally developed in 1974 by the Texas Transportation Institute (TTI). Passer II is an arterial-based, bandwidth optimizer, which determines phase sequences, cycle length, and offsets for a maximum of 20 intersections in a single run. Splits are determined using an analytical (Webster's) method, but are fine-tuned to improve progression. Passer II assumes equivalent pre-timed control, but it does represent dual-ring phasing.

Passer II requires traffic flow and geometric data, such as design hour turning volumes, saturation flow rates, minimum phase lengths, distances between intersections, cruise speeds, and allowable phase sequencing at each intersection. The Passer II timing outputs include: design phase sequences, cycle length, splits, and offsets, and includes a time-space diagram. Performance measures include volume-to-capacity ratio, average delay, total delay, fuel consumption, number of stops, queue length, bandwidth efficiency, and level of service. In addition to the time-space diagram, Passer II has a dynamic progression simulator, which lets the user visualize the movement of vehicles along the artery using the design timing plan.

Texas Transportation Institute (TTI) has released an upgrade for Microsoft® Windows® version of this program. Passer II-02 has a new interface. Also, it uses an enhanced version of the optimization routine provided by its predecessor. The input data requirements have not changed; however, unlike the previous release, it preserves the lane configurations supplied by the user. This is achieved by introducing a new format for the input data file. Existing users need not worry because the program can read old input data files and will automatically convert them to the new format. The following is a list of features for Passer II-02:

  • Provides a revised saturation flow calculation module.
  • Provides a summary of all cycle lengths analyzed.
  • Allows the user to view output for any selected cycle length.
  • Outputs reports in rich text format and launches Microsoft® WinWord for viewing these.
  • New time-space diagrams in HTML format, and viewed by automatically launching Microsoft® Internet Explorer.
  • A Help facility.

Passer II-02 is second only to Synchro in popularity. Because of it continuous refinement over more than 25 years, it is well known in the profession and considered by many Traffic Engineers to be the best of the bandwidth optimization models.

Passer IV-96 was developed by the Texas Transportation Institute in the early 1990s and is based on the MAXBAND program. It is a DOS based program used to optimize a network of traffic signals based on maximum bandwidth. Maximum bandwidth is obtained by maximizing the time period that a car can potentially pass through a given network with minimal stopping at signalized intersections. It is able to optimize signal timings for arterials as well as closed-loop networks, such as a downtown area.

Using hourly traffic volumes, user-defined saturation flow rates and optional minimum green times, Passer IV can optimize the progression bands for main arteries as well as coordinated crossing arteries by computing the optimum cycle length, splits, phase sequences and subsequently adjusting the offsets for a maximum of 20 arteries and 35 intersections.

When it was under development, MAXBAND was shown to be able to provide slightly better bandwidth solutions than other bandwidth models. This improvement, however, came at the expense of a more complex model and it required a much more powerful computer. The Passer IV-96 incarnation of the model has had some success, but has not enjoyed the widespread popularity of its namesake, Passer II.

NOSTOP is another bandwidth optimizer that is used in some areas of the country. NOSTOP develops a set of timing plans from the point of view of linear bandwidth progression in an arterial traffic signal system. The program provides a fast and effective means of presenting the user with a graph of the variation of progression efficiency over a complete range of cycle lengths and progression speeds. After the cycle/speed analysis is tabulated, the cycle with the best efficiency is determined. Further analysis provides the optimum system control parameters, which include cycle length, speeds, and offsets. Additional refinements which are provided are the amounts of lead and lag left-turn time available at each intersection without interfering with the through bands, the amounts of through green time unused by the progression bands, and widening the band in the preferential flow direction.

The analysis provides an accurate, repeatable method of bandwidth analysis, which can be conducted in order to find the optimum cycle length and progression speed combination for the system. The program thus produces the optimum cycle/speed/offset combination for each signal, which in turn produces the best possible progression performance of an arterial street system. Once the optimum timings are determined, the program will plot a time-space diagram showing all of the offsets and the progressive bands.

While the bandwidth optimization models focus on the problem of timing a linear series of traffic signals, the next model is directed to the optimization of an isolated intersection.

Synchro is a macroscopic traffic signal timing tool that can be used to optimize signal timing parameters for isolated intersections, generate coordinated traffic signal timing plans for arteries and networks, and also develop time-space diagrams for interactive fine-tuning. Synchro can analyze fully actuated coordinated signal systems by mimicking the operation of a NEMA controller, including permissive periods and force-off points. Synchro runs under Windows 95/NT and OS/2. Using a mouse, the user can draw either individual intersections or a network of intersecting arteries, and also can import .DXF map files of individual intersections or city maps. The program has no limitations on the number of links and nodes. It can analyze multi-legged signalized intersections with up to six approaches per intersection.

Synchro is designed to optimize cycle lengths, splits, offsets, and left-turn phase sequence. The program also optimizes multiple cycle lengths and performs coordination analysis. When performing coordination analysis, Synchro determines which intersections should be coordinated and those that should run free. The decision process is based on an analysis of each pair of adjacent intersections to determine the "coordinability factor" for the links between them.

Synchro calculates intersection and approach delays either based on the Highway Capacity Manual (HCM) or a proprietary method. The major difference between the HCM method and the Synchro method is treatment of actuated controllers. The HCM procedures for calculating delays and LOS are embedded in Synchro; thus, the user does not need to acquire HCM software. Synchro is useful for agencies that want to operate groups of arteries on different cycle lengths. Using Synchro the user can optimize the entire network or groups of arteries and intersections in a single run and determine the control boundaries of the different arterial groups, based on the program's selection of the cycle lengths. Synchro requires mostly the same traffic flow and geometric data as Transyt-7F. The program can be used to evaluate existing traffic signal timing or to optimize the settings for individual intersections, arteries, or a network. The program performance measures include average approach delay, intersection delay, volume-to-capacity ratio, intersection level of service, 50- and 95-percentile queue lengths, total stops, travel time, emissions, and fuel consumption. Further, Synchro has a generous listing of user-specified reports, including capacity analysis, LOS, delay, stops, fuel consumption, blocking analysis, and signal timing settings.

Synchro has unique visual displays, including an interactive traffic flow diagram. The user can change the offsets and splits with a mouse, then observe the impacts on delay, stops, and LOS for the individual intersections, as well as the entire network. Another significant strength of Synchro is its ability to create data input streams for Transyt-7F, and CORSIM. Once the user has entered the data to run Synchro successfully, it is possible to run any one of these programs without using any of their preprocessors (these programs must be acquired separately). Following a successful Transyt-7F run, the user has an option to use the results as inputs back into Synchro, and perform further evaluations.

As indicated by the survey, Synchro is the most widely used signal timing program. As we will note later, this package also has the most highly developed database management capabilities and it is integrated with many traffic control systems. For much of the signal timing process, Synchro is the current state of the art.

SOAP 84 can be used to determine signal timing plans for pre-timed controllers and has limited capabilities for actuated controllers. Signal Operations Analysis Package (SOAP) develops effective traffic signal control timing plans for isolated intersections. It determines optimal cycle, splits, and dial assignment of isolated intersection. It handles up to 48 time periods. Multiple runs are needed to consider alternatives. It calculates average rather than optimal cycle length.

SOAP 84 provides a macroscopic analysis with the primary objective of developing signal control plans for individual intersections. It develops cycle lengths and splits that minimize a performance index. Inputs include traffic flows, truck and bus composition, left turn data, saturation flow, and signal data. Outputs include delay, percent saturation, queues, excess fuel consumption, left turn conflicts, and percent stops. Although SOAP is still used by several agencies, it has been largely overshadowed by more advanced and broader programs. SOAP 84 is important, however, because it is the only program that explicitly deals with the temporal aspects of signal timing through the use of its 48 time periods.

Transyt-7F (TRAffic Network StudY Tool, version 7, Federal) is designed to optimize traffic signal systems for arteries and networks. The program accepts user inputs on signal timing and phase sequences, geometric conditions, operational parameters, and traffic volumes. Transyt-7F is applied at the arterial or network level, where a consistent set of traffic conditions is apparent and the traffic signal system hardware can be integrated and coordinated with respect to a fixed cycle length and coordinated offsets. Although Transyt-7F can emulate actuated controllers, its application is limited in this area.

Transyt-7F optimizes signal timing by performing a macroscopic simulation of traffic flow within small time increments while signal timing parameters are varied. Design includes cycle length, offsets, and splits based on optimizing such objective functions as increasing progression opportunities; reducing delay, stops, and fuel consumption; reducing total operating cost; or a combination of these.

For simulation, the program accepts the inputs as fixed variables and reports the performance measures in terms of stops, delay, fuel consumption, and queuing. When optimization is performed the user can either fix or select the best cycle length with the least delay and stops. Detailed optimization of offsets and splits can be performed for either a user-specified cycle length or the "best" cycle length found by the program. Transyt-7F's performance measures include delay, stops, queue length, travel-time, level-of-service, volume-to-capacity ratio, speed, total travel, fuel consumption, and operating cost. When optimizing, Transyt-7F minimizes or maximizes an objective function, called the Performance Index (PI). The PI may be a combination of delay and stops; fuel consumption; and/or optionally selected excessive maximum back of queue, excess operating costs, or progression opportunities.

Transyt-7F has its own pre- and post-processors; namely, a simple data editor (T7FDIM) and the Platoon Progression Diagram (PPD). The T7FDIM provides the ability to edit all record types of an input file. T7FDIM, however, requires that the user has intimate knowledge of the Transyt-7F data record types, ordering, and contents. The Platoon Progression Diagram presents a "contour" of flow versus time and distance along an artery. Queue build-up, dispersion and arrival of platoons are clearly shown for a visual insight on the flow patterns normally occurring along the artery.

Unique features of Transyt-7F include the program's ability to analyze double cycling, multiple greens, overlaps, right-turn-on-red, non-signalized intersections, bus and carpool lanes, "bottlenecks," shared lanes, mid-block entry flows, protected and/or permitted left turns, user-specified bandwidth constraints, and desired degree of saturation for movements with actuated control. Other applications of the tool include evaluation and simulation of "grouped intersections" (such as diamond intersections and closely-spaced intersections operating from one controller) and sign-controlled intersections.

Transyt-7F is also one of the most comprehensive signal timing tools available. It is comprehensive because it has broader capabilities than other signal timing programs. To name just a few, these capabilities include:

  • Detailed simulation of existing conditions;
  • Optimization of cycle length, phasing sequence, splits and offsets;
  • Detailed analysis of traffic-actuated control;
  • Optimization based on a wide variety of objective functions;
  • Hill-climb and genetic algorithm optimization;
  • Explicit simulation of platoon dispersion, queue spillback and spillover;
  • Multi-cycle and multi-period simulation;
  • Full flexibility in modeling unusual lane configurations and timing plans; and
  • Full flexibility in modeling English and metric units, right-hand and left-hand driving.

Passer III-98 is a diamond interchange signal optimization program. The program can evaluate existing or proposed signalization strategies, determine signalization strategies that minimize the average delay per vehicle, and calculate signal timing plans for individual interchanges or up to 15 interconnected interchanges along one-way frontage roads. Three-phase, Four-phase (TTI-lead), Lead-Lead, Lead-Lag, Lag-Lead, and Lag-Lag phasing sequences can be analyzed using Passer™ III-98. In addition, the program can evaluate the effectiveness of various geometric design alternatives, e.g., lane configurations, U-turn lanes, and canalization.

Signal Timing Optimization Issues – Of all the elements of the signal timing process, optimization has received the most emphasis; and as a result, it the most developed. Unfortunately, most of the research and product development has treated signal timing optimization as a "one-time" effort – a project that begins with data collection, proceeds with running the model, and ends with a report that provides the optimum cycle, split, and offset.

With so many models to choose from, the obvious question is which one is best. The answer is, it depends. There have been many comparison studies, but none have produced a definitive result that clearly shows that one particular model is best. As a general conclusion, it would seem that any of the above models will provide good signal timing when used within their limits.

In the operating agency, once the optimization is completed, a new effort begins that takes the model results and enters the data into the format required by the controller. This exercise is called "Field Deployment" and is the topic of the next section.

2.3.4 Field Deployment

Once we have the results of the optimization, the problem is to be able to install the results in a controller. In general, the users are left to their own devices to convert the model results into the timing parameters required by their system. There have been several developments in recent years that have had a significant impact on this issue: The evolution of Synchro and the development of the Closed Loop System have greatly reduced the effort required to install the controller parameters.

The discussion of the hardware host environment in the next section provides a perspective on the interface requirements and exposes a number of the issues that are related to transferring the results from the optimization models to the hardware itself.

1. "Survey of Traffic Software Use", Trafficware Corporation, June 2000.

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