Managing Selected Transportation Assets: Signals, Lighting, Signs, Pavement Markings, Culverts, and Sidewalks (2007) / Chapter Skim
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Pages 13-33
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From page 13... ...
OVERVIEW Traffic signals help manage intersecting streams of automobile and truck traffic, pedestrians, cyclists, and other road and transit vehicles by assigning the right-of-way to individual streams in turn. They are placed where the volumes of traffic or crash histories justify their need, where crossings near schools require signal control, or signal installation is needed as part of a coordinated signal plan to ensure a smooth, progressive flow of vehicle platoons.
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15 Notwithstanding this consensus on the importance and the value of traffic signals, reviews of current practice by others have identified shortcomings. An earlier NCHRP synthesis study considered good-practice system engineering techniques as applied to traffic signals, including the use of a structured analysis, identification of goals and problems to be addressed, project management approach, alternatives evaluation and project evaluation, specific topics within traffic signal systems engineering (e.g., need for signals, signal timing, signal coordination, and coordination of traffic control systems)
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of America, and the FHWA. It considered signal operation in five areas (a sixth area received a small number of responses and was therefore not graded)
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17 are involved, they typically exercise management responsibility for their work. By contrast, in the majority of cases where private firms maintain signals, they are not given management responsibility.
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program for purposes of budgeting. For two other agencies, this option was chosen because signals are maintained by local governments.
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19 signals. Immediate correction of problems was the most prevalent response, as shown in Figure 5, reflecting the importance of signals to safety, good traffic movement, and the other transportation objectives discussed earlier.
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reviews of timings; a documented, centrally accessible, current inventory of approved signal phasing and timing for each intersection; analyses of appropriate information supporting timing reviews, such as turning movement counts, pedestrian volumes, accident histories, complaint histories, field observations of clearance intervals, and checks on any geometric changes to the intersection since the last review; and quick implementation of timing plans once developed (within two working days)
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21 • Overall issues -- The low scores in the report card presented earlier are the result of resource constraints that inhibit more system-based actions and proactive management, and encourage agencies to resort to a "fire-fighting" mode. As a result, many agencies strive to meet only a basic level of service that provides safety and avoids liability.
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22 More Than Once A Year Annually Biennially Less Freq Than Biennially FIGURE 7 Frequency of physical condition assessments of signals. PHYS: Structural Condition PHYS: Corrosion PHYS: Not Functioning PHYS: Use- or Time-Related PHYS: Other Asset Age Hours in Service Operational Performance System Reliability Performance or Health Index QUAL: Structural Condition QUAL: Corrosion QUAL: Not Functioning QUAL: Use- or Time-Related QUAL: Other Asset Value Customer Complaints Customer Surveys Other No Response 0 20 40 60 80 100 Percentage of Responses FIGURE 6 Measuring performance of traffic signals.
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23 ASSET SERVICE LIFE Information on service life was obtained in the study survey for three major components of signal systems: the structural supports -- poles and mast arms, the controller system, and signal display items -- the signal heads and lamps. For each of these components, agencies were given the opportunity to report service lives for different materials that are typically used or for other materials that they employ.
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Among the 40% of reporting agencies that identified at least one method, the emphasis was on collective agency knowledge, whether embodied in documented experience (e.g., a database of observed historical service lives) or in the professional judgment of their staffs.
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25 environment in which modern signals may be expected to operate -- for example, with advanced features such as closed-loop systems coordination, preemption by emergency vehicles, transit vehicle priority, and handling of bicycle traffic -- as well as the need for system component compatibility and integrated, interoperable systems (Bullock and Urbanik 2000)
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87 6 N o. o f R es po ns es 5 4 3 2 1 0 0 5 10 15 20 25 30 35 40 45 50 Estimated Service Life, Years FIGURE 12 Estimated service life of wood pole and span wire supports for signals.
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89 10 7 6 N o. o f R es po ns es 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 Estimated Service Life, Years 8 7 6 N o.
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Rank Factor 1 Assets are repaired or replaced as soon as they fail without regard to service life 2 Service life is often determined more by functional obsolescence than by wear and tear 2 Compare current age of asset with the maximum age that defines service life 4 Monitor condition of the asset on a periodic schedule 5 Monitor condition of the asset occasionally 5 The agency does not use/does not monitor service life for this type of asset 7 Assets are replaced on a preventive maintenance schedule without regard to where they are in their service life 8 Apply deterioration models to estimate where the asset is on "the curve" 8 Compare service hours to date with the maximum number of service hours that defines service life TABLE 7 RANKING OF METHODS TO DETERMINE WHERE TRAFFIC SIGNAL ASSETS ARE IN THEIR SERVICE LIVES 8 7 6 N o. o f R es po ns es 5 4 3 2 1 0 0 5 10 15 20 25 30 35 40 45 50 Estimated Service Life, Years FIGURE 17 Estimated service life of traffic signal heads.
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29 INFORMATION TECHNOLOGY SUPPORT Synthesis and AASHTO–FHWA Survey Findings As a practical matter, applying asset management to today's transportation systems typically requires substantial information technology (IT) support.
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• Workbooks or spreadsheets that address traffic signals; and • Parts of other products or procedures that the agencies were requested to describe briefly. The responses to this survey item are summarized in Figure 19, showing a relatively uniform distribution of use across the several system categories, with no strong distinctions among different levels of government.
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31 • Operating environment of the signal system -- for example, traffic volume, composition, and flow patterns; development affecting traffic growth rate; intersection geometry; pedestrian flows; and variations in these parameters and the degree to which they are predictable. • Signal system performance -- for example, operational reliability and downtime, and impact on traffic as measured, for instance, by throughput, travel time and delay, and effect on safety (e.g., number and severity of crashes)
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Adding additional loading to an existing pole can be difficult because structural information is not tracked. We need to assign a structure number for each pole, installation date for the structure, and the standard drawing with revision that was used to perform the installation.
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33 The major gap [in the existing process] is [the need to assign]
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different ways in which signal systems operate. Different jurisdictions apply different combinations and phasings of signals for certain traffic movements, particularly protected left-hand turns, which can confuse drivers.
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