2. State of the Practice
This section describes the state of the practice regarding traffic signal operations and maintenance. Section 2.1 provides the results of the literature surveyed. Section 2.2 documents the results of the field survey conducted under this project as well as relevant results developed under a related survey performed by the Puget Sound Regional Council (7). Section 2.3 summarizes the operations and staffing resources required. Section 2.4 relates current agency practices to the requirements for achieving OOO capability
2.1 Literature Survey
The following descriptions summarize key information relating to traffic operations and maintenance.
TRAFFIC CONTROL SYSTEMS HANDBOOK
This handbook provides an update to the 1996 Traffic Control Systems Handbook and covers traffic signal maintenance as part of Chapter 13, System Management. In this chapter, the authors describe the increased complexity of traffic signal systems since the previous Handbook update and how agencies have often erroneously estimated the maintenance requirements of these systems. The example given is that agencies assume that technology upgrades associated with traffic signal systems would result in fewer maintenance requirements and therefore under estimate budget and staffing needs as well as staff skill levels for them. The authors suggest that agencies perform a cost trade off analysis during the design of traffic signal system upgrades that considers the potential impacts on maintenance that follow.
The Handbook defines three types of traffic signal system maintenance activities: Functional, Hardware, and Software. Functional maintenance activities include updating traffic signal system databases and optimizing signal timing plans. The Handbook describes hardware maintenance activities as remedial, preventive, or modification. Remedial hardware maintenance includes the immediate replacement of malfunctioning or failed equipment. Preventative hardware maintenance includes scheduled intervals at which equipment is checked to minimize failure probability. Hardware modification relates to design flaws or changes identified after design that are needed to improve equipment characteristics.
For software maintenance, the Handbook describes debugging problems identified following system acceptance or the modification of the software to provide additional features.
The authors identify the possible results associated with inadequate traffic signal maintenance. These results include a potential for increases in accidents, degraded system performance, and increases in equipment malfunction or failures.
The Handbook also provides a summary of staffing surveys for various Traffic Management Centers (TMC) and their respective City/Area population, TMC, and traffic signal system sizes. The survey results are summarized here in Table 2.1.
2005 NATIONAL TRAFFIC SIGNAL REPORT CARD
The National Traffic Signal Report Card was developed to answer the question: How well does the nation support its traffic signal systems? The report consolidated responses of 378 agencies across the United States on this subject. The overall score and grade related to maintenance practices as well as key findings from this report are summarized below.
Maintenance practices had an overall national score of 67, or D+. The minimum level of operation and critical maintenance for traffic signal systems scored high across the surveyed agencies which were mainly attributed to ensuring public safety and limiting potential for increased liability. However, despite meeting minimum requirements, other components of maintenance programs such as planning, management, and execution contributed to the overall low score.
The report defined key components of an excellent maintenance program. These components are:
In order to achieve an excellent maintenance program and in turn improve scores and grades in future report cards, several needs were identified. These needs include more adequate maintenance resources, increased staffing levels, improvement training, more frequent signal hardware upgrades and timing updates, and more training for traffic signal technicians.
2007 NATIONAL TRAFFIC SIGNAL REPORT CARD
In the update to the 2005 National Traffic Signal Report Card, the report showed an increase in both score and grade related to maintenance practices. The report consolidated responses of 417 agencies across the United States and found the overall national score improved from 67 in 2005 to 70 in 2007 with a corresponding improvement in grade from D+ to C-. The key findings from this report are summarized below.
The 2007 report provided more specifics on the key components of an excellent maintenance program than provided in the 2005 report by adding the following:
The report findings showed that two-thirds of surveyed agencies had policies or processes to provide a traffic signal technician at intersections with a reported critical malfunction in the times specified above. In addition, 69% of the agencies reported regular preventative maintenance and operational reviews.
INTELLIGENT TRANSPORTATION SYSTEMS FOR TRAFFIC SIGNAL CONTROL (WWW.ITS.DOT.GOV)
The US DOT website provides a summary of deployment benefits, costs, and lessons learned for a number of different transportation subject matters. One of these subject matters involves traffic signal control. The website was reviewed for information on traffic signal operations and maintenance. The findings of this review are summarized below.
The website described several important benefits related to proper traffic signal operations and maintenance. Examples were given of several case studies that showed optimizing signal timing as a low-cost approach to reducing congestion that ranges from $2,500 to $3,100 per signal per update. Updating traffic signal control equipment, in conjunction with signal timing optimization, was also shown in the case studies to have a positive benefit on reducing delays.
The website also provided several general rules of thumb for operations and maintenance of traffic signals. These include providing one traffic engineer for every 75 to 100 signals and one signal technician for every 40 to 50 signals and performing signal re-timings every 2-to-3 years at a minimum.
The website also described the practices needed to properly deploy traffic signal systems for maximum benefit. These practices include devoting sustained resources to the system and the professionals who design, operate, and maintain them, making wise investments in current signal hardware, timing updates, and maintenance resources, and providing training for signal technicians and engineers to ensure proper operation and maintenance of traffic signals and to preserve the investment in the hardware and timing updates.
TRAFFIC CONTROL SYSTEM OPERATIONS GUIDE (INSTALLATION, MANAGEMENT, AND MAINTENANCE)
This guide was prepared in 2000 by ITE and includes summaries of covers topics on traffic signal systems that include operations, staffing, and maintenance.
For traffic signal operations, the guide describes the monitoring devices used to determine how well traffic signal systems are operating. Among these devices are TV surveillance, detectors, dynamic message signs, and traffic signal controllers. The guide therefore stresses the need for regular care of these devices. The day to day operation is described as a number of basic tasks and procedures that include maintaining checklists, ledgers or system parameter inputs and changes, running daily summary reports, system back-up of data, and incremental improvements.
The guide also describes the use of modern computerized equipment to facilitate information processing. Over the years, the transition from paper filing systems to electronic filing systems has improved the speed and ability of operators to process information on traffic signal systems. In addition to improved information processing, the guide describes the benefits of automated data collection. A key benefit to this includes less costly data collection efforts as compared to manual labor. The guide also describes how ITS has improved information gathering for traffic signal systems such as by providing data streams of different equipment (e.g. on-board systems of buses, and commercial truck fleets) into traffic control systems and using this information to monitor the performance of a street.
Data Synthesis, or Data fusion, is another key component of traffic signal system operations described by the guide. The data coming into the traffic control system must be in a form that is useful to the operator. It is also important for this data to be easily manageable and not require large amounts of time to reduce. Finally, the guide describes the importance data information dissemination. The information gathered through traffic signal operations is useful to a wide audience and is therefore shared with groups such as service representatives, road users, and transit patrons.
Outside of information gathering, data synthesis, and dissemination, the guide stresses the need for quality documentation of operator functions, maintenance procedures, and operating software.
The guide also provides summaries of staffing levels from three sources: Hampton, VA, Menlo Park, CA, and NCHRP Synthesis 245. The City of Hampton surveyed 23 similar sized cities (approximate pop. 141,000) which found an average of one traffic engineer per 76 traffic signals and one traffic signal technician per 47.1 traffic signals. The NCHRP survey concluded that a traffic signal technician could maintain between 38 and 43 traffic signals. When evaluating staffing needs, the City of Menlo Park recommends one traffic signal engineer for every 100 traffic signals and one traffic signal technician for every 50 traffic signals.
From this information, the guide concludes that one single maintenance person can maintain 40-50 traffic signals or other field devices.
Common provisions for maintenance were also covered in this guide. The first provision covered in the guide was on the personnel required for traffic signal systems. This included a definition of five general personnel classifications including traffic signal mechanic, supervising traffic signal mechanic, traffic signal technician, traffic signal engineer, and traffic engineer. The guide also included basic guidelines for qualifications, training, and experience for each personnel classification. This guidance is as follows:
The guide also covers provision of various maintenance equipment including maintenance vehicles, test equipment and tools, replacement parts and supplies. For each of these categories, the guide offers an itemized list of equipment, estimated quantities and prices for average conditions. The guide also provides guidance on the preferred location of repair shops for traffic signal equipment as well as considerations for layout, size, organization, outfitting, staffing, and budget for these types of shops.
The importance of good maintenance records was also stressed within the guide. The records needed for effective maintenance are described as falling into maintenance service records, signal timing charts, and maintenance manuals and as-built plans. The maintenance service records include the two main types of maintenance performed by traffic signal departments which are preventive and responsive. Another type of maintenance record required of traffic signal departments described by the guide is design modifications. This includes the documentation of changes to the approved design and operation of existing system installation to corridor recurring problems, accommodate changes in prevailing conditions, or updates since installation.
The guide also provided information on estimated maintenance requirements associated with five common traffic signal configurations for example purposes. From these examples, the guide deduced that the "average" intersection requires a total of 60 hours of annual maintenance of which 70% is devoted to preventive maintenance, 25% to responsive maintenance, and 5% to design modification maintenance. The guide also concludes that signal mechanics have 1,627 estimated annual work hours to perform maintenance which translates to the capability of maintaining 17-27 intersections per year.
2.2 Field Survey
This section provides a summary of the survey responses received from various agencies on the FHWA sponsored Traffic Signal Operations and Maintenance Staffing and Resource Requirements questionnaire. The purpose of this questionnaire was to seek input on current traffic signal operations and maintenance practices and to use this information in the development of guidelines developed in subsequent project tasks.
The 2005 and 2007 National Traffic Signal Report Cards (NTSRC) highlight a number of better practices that are believed to contribute to effective traffic signal management, operations and maintenance processes. Table 2.2 summarizes recommendations from the report card gathered from a number of sources, for staffing and resource needs for traffic signal operations and maintenance. A survey was also conducted in conjunction with the development of this report and those survey results are presented in the following table as well.
The section describes the survey distribution process and the feedback received from respondents. The report then compiles survey responses by the following subject areas:
2.2.2 Definition of Traffic Signal System Management, Operation and Maintenance
Transportation system management and operations is described by the Technical Corrections Act (6) as follows:
Concurrently with the survey conducted under this task, a related survey was conducted by the Puget Sound Regional Council (7). Results of the Puget Sound survey that are relevant to survey topics under this project are also included.
2.2.3 Survey Distribution and Respondents
The questionnaire that was distributed is included in the Appendix A of this memo. While the Traffic Signal Report Card survey provided a broad overview of operations practices and established a basis for comparatively evaluating agency performance, the survey was intended to provide an in-depth view of the practices of a limited number of agencies. It was desired to obtain responses from 10 agencies that could serve to assist in establishing the basis for superior practices, however it was only possible to obtain seven in-depth responses. The initial distribution list included a total of 34 agencies of different sizes, type, and locations. The survey was first distributed in July 2008 via email to each of the agencies. This first distribution was then followed up by subsequent reminder emails and telephone calls. Of the 34 potential respondents, a total of seven agencies provided feedback. The responding agencies and their signal system network characteristics are shown below in Table 2.3. Note that the agency names are not presented to protect the anonymity of the respondents the type of agency, state and population of the jurisdiction are germane to this discussion and are provided to characterize each survey response. Appendix B provides a tabulation of the survey responses.
Table 2.4 shows a partial set of responses regarding the operational characteristics of these traffic signal system networks. Those agencies providing responses on this indicated that traffic volumes were growing in the range of 1% to 5% and the level of saturation experienced.
Traffic delays increase modestly until a volume to capacity ratio of approximately 0.9 is reached, at which point delay increases exponentially. Thus the capability of an agency's staff to identify saturation or near-saturation conditions (by on street observation, CCTV observations and data mining) and to remedy the situation strongly influences the level of delay that motorists will experience.
Additionally, it was found that these agencies operate traffic management centers. The hours of operation for these centers varied from nine (9) hours per day to 17 hours per day. Three of the agencies reported co-location in their traffic management centers with other operators such as Department of Transportations, State Police Departments, Transit, 911, and Emergency Operations.
2.2.4 Classification of Signal System Characteristics
The respondents were asked to provide an inventory of the field devices and operational strategies used with their traffic signal system. This included traffic signal controllers, traffic detectors, ancillary devices, and communication systems.
Traffic Signal Controllers
The type and age of the equipment has a bearing on some of the challenges that a municipal traffic engineer faces, the premise being that the greater the number of different types of equipment present in a signal system, the more difficult it is to operate and maintain. This additional difficulty results from the additional spares inventories required as well as the additional training and configuration management required. Similarly, it is likely that an agency that uses older technology will have differences in hardware and software that may result in different operations, making troubleshooting more difficult for the technician in the field. The traffic signal controller classifications used by the respondents are summarized in Table 2.5.
The agencies operate all of their traffic signal controllers in time of day mode with the exception of Agency A which operates approximately 12% of their network using traffic responsive mode. The number of timing plans used varied from three to seven plans per weekday and from two to four plans per weekend day or special event. The agencies reported using other non-periodic timing plans as well such as during emergency evacuation plans, detour plans, heavy volume plans. Agency A also reported the operation of peak and off-peak seasonal plans that take into account fluctuations in traffic volumes at different times of the year. The Puget Sound Survey reported that traffic responsive operation was used by 15% of the agencies.
Four of the respondents reported retiming at three year intervals or less and one respondent reported retiming at five to seven year intervals. Two respondents retime on an as needed basis. One of these agencies uses aircraft and cameras to assist in determining the need for retiming.
The respondents also provided a breakdown on the traffic detection strategies employed at their intersections. This breakdown is shown in Table 2.6.
Five of the agencies surveyed also provided estimates on the percentage of operable local actuation detectors. These agencies reported very high percentages (90% or greater). The agencies periodically check the operability of their traffic detectors through preventative maintenance programs, routine monitoring of central system software, and when complaints are made.
The survey results also showed that only one of the agencies is analyzing detector data on a regular basis. This data is being used to determine spot traffic flow conditions, real-time volumes, peak hours, and yearly daily traffic volume trends.
The Puget Sound survey reported that 28% of the agencies did not send real-time traffic information to a central computer or master controller.
The ancillary devices also maintained by the responding agencies are provided in Table 2.7. These devices include emergency preemption/transit signal priority equipment, Closed Circuit TV (CCTV) cameras, Changeable Message Signs (CMS), and Uninterrupted Power Supplies (UPS).
Of the six agencies that reported using CCTV cameras operated by that agency, four reported on the use of these cameras to support on-line adjustment of timing plans, planning for signal timing/phasing, and incident management. One agency reported using CCTV monitoring to support on-line adjustment of timing plans and incident management and another agency uses CCTV only for incident management.
The Puget Sound survey determined that 79% of the agencies used CCTV to monitor traffic and that 53% of these agencies used the cameras for incident management as well. 23% of the agencies used CMS.
The survey sought information on the various types of communication systems being used by agencies with their traffic signal systems. These communication systems provide a data transfer link between traffic signals and in some cases provide a means for communicating remotely to a traffic management center. Table 2.8 provides a summary of the survey results showing the number of signals within each communication system type.
For the isolated traffic signals, the survey respondents indicated that only one agency uses automatic failure reporting at some locations. The respondents also indicated that portions of their traffic signal system operate with remote monitoring capabilities either as a closed loop system or through central control. The information provided on this capability is presented in Table 2.9.
The number of traffic signals reported in the table above includes coordinated systems as well as isolated locations. The redundancy characteristics of the traffic signal systems were also reported on by the survey respondents. This included redundancy of central controls and both backbone and distribution communication systems. The redundant characteristics of the central controls included software/server based systems that can be transferred to backup systems as needed, spare data storage and communication servers, and regular off-site backup of system data.
The agencies reported accomplishing redundancy with their backbone and distribution communication systems by providing alternate paths in the case of communication breaks.
2.2.5 Staff Levels and Qualifications
The number of staff responsible for traffic signal operations and maintenance along with their required qualifications were also provided by the agencies participating in this survey. This information is presented in Table 2.10 and includes the number of personnel in each of the identified positions. Where data was available, the qualifications, training, and experience level is also provided.
Analysis of the Puget Sound survey data for agencies operating in excess of 150 signals found that the number of maintenance technicians ranged from 18.4 to 78.9 per thousand signals. The large differences may be attributable to varying definitions of labor categories and maintenance tasks.
The data in Table 2.10 was analyzed according to the following Knowledge, skills and abilities categories described in Section 126.96.36.199
The survey also provided information on the average tenures of both engineering and maintenance staff. The average tenures of engineering staff was found to be between five (5) and twelve (12) years while the average tenure of maintenance staff was found to be between five (5) and twenty (20) years.
Table 2.11 shows a comparison of these results for the medium and larger signal systems. The analysis is approximate in that the categories in Table 2.9 do not directly correspond to the questionnaire categories.
The widespread variations in the engineering areas are at least partly attributable to the definitions of the positions by different agencies. Analysis of the Puget Sound data indicated that for agencies operating in excess of 150 signals, the number of maintenance technicians ranged from 18.4 to 78.9 per thousand signals. These large differences are most likely attributable to differences in labor categorizations, combined task assignments and variations in the definitions of maintenance tasks.
2.2.6 Maintenance Practices
The survey included a number of questions regarding maintenance of traffic signal systems. The feedback provided insights on the allowable and actual practices used by each agency when responding to maintenance requests. Feedback was also provided on the distance between traffic signals and maintenance facilities. The information provided is summarized in Table 2.12 and Table 2.13.
In addition, several of the responding agencies reported on the annual number of traffic signal controller failures. The information provided showed a range of failures from less than 1% per year to more than 10% per year.
2.2.7 General Observations
The following additional observations were made based on a review of the survey responses.
2.3 Operations and Maintenance Staffing and Resources
A traffic signal system conforming to Objective Oriented Operation (OOO) provides performance, reliability and functional requirements necessary to achieve a high quality of operation at a reasonable cost. This section identifies the resources that are most likely to be needed to achieve OOO. The resources include sufficiently qualified personnel and the staffing levels required to implement the functions. An estimate is provided for signal retiming costs.
While an agency's ability to achieve operational objectives is related to budget and number of staff management approach, staff commitment, dedication and training are intangible elements that dictate how effectively an agency can operate and maintain a traffic signal system. The level of maintenance and operations is not only affected by the agency's perception of its importance but also by the public's perception. Under budgetary constraints, cutting programs for training and professional development provides short-term relief but undermines the long-term viability of the system's operation and maintenance.
2.3.2 Resource Requirements for Traffic Engineering
The person responsible for traffic system engineering (planning, system operations, equipment selection, signal timing) should have professional engineering (PE) registration and should preferably have professional traffic operations engineer (PTOE) certification. Other engineering personnel should either have these qualifications or be in the process of actively pursuing their acquisition.
Staffing Level for Traffic Engineering Personnel
Table 2.14 provides a summary of the engineering staffing levels developed by various references:
Survey responses show wide variations in comparative staffing levels. To some extent, this reflects the phraseology used for the survey questions and varying interpretations by the respondents. Based on the table above, it appears that a staffing level of 75-100 signals per engineer for agencies that operate a minimum of 150 signals will be appropriate to support OOO. Smaller agencies will likely require fewer signals per engineer because economies of scale are difficult to realize. The engineering staff to support these functions should, as a minimum:
2.3.3 Cost of Signal Retiming
The cost of signal retiming generally lies within the $2500-$3500 per intersection range (8,9). Table 2.15 provides a typical consultant's estimate of the time required per intersection to accomplish key activities associated with signal timing.
2.3.4. Resource Requirements for Maintenance Personnel
Personnel in agencies that perform their own maintenance should conform to the following as a minimum (agencies that contract maintenance services should require contractors with similar qualifications):
The technician in charge of signal maintenance should have the following qualifications:
The number of subordinate positions depends on the number of signals for which the agency is responsible. Qualifications for two subordinate levels are described below:
Technician 2 – Minimum of 2 years as Technician 1 plus: certification to IMSA Traffic Signal Level II.
In addition, all technician levels must be able to work for long periods in inclement weather and may be required to lift heavy objects and work from bucket trucks.
Staffing Levels for Maintenance Personnel
Table 2.16 provides a summary of the maintenance technician staffing levels developed by various references:
Variations in the data are significant. In particular, data for the smaller agencies tends to vary to a greater extent and to skew the averages. Based on the table above, it appears that a staffing level of 30-40 signals per technician for agencies that operate a minimum of 150 signals will be appropriate to support the Constrained Ideal Traffic System. Smaller agencies will likely require fewer signals per technician because economies of scale are difficult to realize.
2.4 Evaluation of Agencies Relative to CITS Criteria
This section relates agency practices to OOO criteria. Section 2.4.1 describes this relationship for the agencies interviewed under this project while Section 2.4.2 provides a general over view of current practices relative to these criteria.
2.4.1 Evaluation of Agencies Surveyed
The interview responses were evaluated using the OOO criteria described in the preceding sections. A scale of 1-5 (5 being the best) was used to rate the interview responses against certain of these criteria. Table 2.17 describes the specific criteria used in the evaluation. A score of 5 represents achievement of OOO criteria. The results of this analysis are shown in Table 2.18. That table, and the survey responses themselves, were used to develop the evaluations described below.
2.4.2 Overview of Current Practices
Management, Operations And Signal Timing
Within the limitations of their resources, agencies generally strive to provide operations and maintenance services to the extent possible. Most of the agencies interviewed, however do not manage in a top down fashion, i.e. they do not generally systematically collect and analyze data and make systematic reviews of traffic system performance using established measures have procedures.
Among the agencies interviewed, the number of timing plans employed and the frequency of timing plan updates, while somewhat deficient, probably do not constitute the most serious deficiency. The agencies do not generally analyze detector data collected by the traffic systems for the purpose of determining the number of timing plans needed and the time periods for which they are required. These values appear to be established by anecdotal or judgmental means (that appear to be influenced by the increasing use of CCTV and, in one case, by surveillance aircraft.) This statement is supported by a the following similar observation in Tarnoff and Ordonez (1) "...more than 98% of the jurisdictions rely on engineering judgment to determine the best times of day for new plans. It must be emphasized that in many cases this approach is highly appropriate. However it is possible that within the 109 agencies that responded to this question, there are situations where a more quantitative approach should be used."
While the problem may, in part, be caused by the lack of qualifications on the part of the engineering staff, the deficiency principally appears to stem from:
While the response to field failures when the agency is notified of a failure is generally satisfactory, the following deficiencies are apparent among the responding agencies:
2.4.3 Relationship of Agency Practices to Objective Oriented Operation (OOO)
Table 2.19 compares characteristics of currently operating traffic systems with OOO criteria. The first column of Table 5.1 summarizes the general features that may be used to characterize a traffic control system. These are described in more detail in Section 3. The remaining columns classify how systems of different quality generally address these characteristics. Column A summarizes the CITS approach. It is generally characterized by strong management planning and control that in turn leads to the use of systematic processes for signal timing operation, and management of the entire signal system operation. It generally corresponds to a NTOC Report Card rating of A. Column B describes systems that range in performance from below CITS requirements to just above minimally acceptable operation. These systems generally correspond to Report Card Ratings of B or C. Column C describes systems that do not generally provide acceptable responsiveness to the transportation needs of the community and correspond to Report Card ratings of D or F.
3 Categories correspond to the those defined in Row, S., Tarnoff, P.J. Traffic Signal Training Assessment Summary Report 7, Federal Highway Administration, July 2005.previous | next
United States Department of Transportation - Federal Highway Administration