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28 CHAPTER FIVE SYSTEM REQUIREMENTS INTRODUCTION much more detectorization than conventional traffic-actuated signal systems. There are many system requirements that define the quality of an ATCS, its deployment, and success. Even the best adaptive Responding agencies reported that their ATCSs utilize traffic control algorithms will not function properly if their anywhere from 4 to 24 detectors (8 to 12 detectors on aver- operations are not supported by adequate hardware, software, age) to cover a single four-leg intersection with one through communications, and system integration. This chapter identi- lane and two turning bays (left and right) for each leg. Although fies those system or operational requirements that are consid- these results might be viewed with some caution, because var- ered critical for ATCS operations. ATCS users were asked to ious agencies deploy detectors differently for their traffic- describe their experiences with ATCS requirements. Their actuated operations, the findings do not fully support the descriptions were captured through a set of questions regard- notion that ATCSs require much more detection coverage ing detection requirements, hardware, software, integration than operations of traffic-actuated signal systems. with legacy systems, and communications. In addition to dis- cussing the agencies' practices, this chapter reviews some of Various ATCSs use a combination of detection layouts to the implementation problems and some lessons learned in estimate the current state of traffic, which is later used to adjust practice. traffic control in a network. There are generally four major detector locations used by most ATCSs: DETECTION · Stop-line detectors (e.g., as seen in common actuated Any traffic-responsive control system depends on its ability operations in the United States and SCATS). to detect traffic either for local intersection control or for · Upstream detectors located close to the stop-line (10 network-wide adjustment of timing plans. ATCSs rely heavily 15 m), which cannot be used (owing to their proximity to on the quantity and quality of traffic data available from detec- the stop-line) to easily estimate queue length (e.g., as tors. Poor or improperly installed detectors can affect ATCS used in Germany by BALANCE and MOTION). performance, which can eventually lead to the removal of · Upstream (mid-block) detectors, which can be used to ATCS operations. estimate reasonably long queue lengths (e.g., as seen at some Californian intersections and used by ACS Lite). Historically, ATCS predominantly used inductive loops as · Upstream (far-side) detectors located at the exit point of a detection technology. Over the past several decades video the upstream intersection (as used by SCOOT, UTOPIA, detection has emerged as a cost-efficient and reliable replace- and optionally by RHODES). ment for the inductive loops. This trend was also observed in the analysis of the survey conducted for this project. Some of Detection layout used by an ATCS correlates with the the ATCS agencies almost exclusively use video detection and adaptive control logic that is used to adjust signal timings for are quite satisfied with its performance. On the other hand, the prevailing traffic conditions. Sometimes detection layout some ATCS users overseas expressed reservations about the is established to provide good measures for the adaptive con- quality and reliability of video detection and exclusively use trol logic [e.g., in SCOOT--upstream detectors selected to inductive loops. However, the survey showed that most of accommodate for Traffic Network Study Tool (TRANSYT) the agencies use a mixture of various detection technologies logic]. Other times, adaptive traffic control logic is developed for their ATCS deployments. Although approximately 93% of for the existing detection layouts (e.g., SCATS logic for stop- the agencies use inductive loops, almost half (43%) also use line detectors). video detection. Approximately 18% of the agencies use radar detection, whereas only 9% use other types of detection not When asked which of the four detection types they use, the contained in any of these three major technologies. responding agencies were not able to make a clear distinction between mid-block and upstream detectors on one side and Detection coverage is very important for the success of an stop-line and near stop-line detection on the other side. ATCS. One of the most significant barriers for widespread Therefore, aggregated results were provided for these two deployment of ATCSs is a notion that such a system requires major detection placement categories. Approximately 42% of
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29 interviewed agencies reported that their systems use upstream to short-term inputs from detectors tend to be more robust and detection. Distance between these detectors and the down- work better when minor detection failures occur. However, stream intersection varies anywhere from 10 to almost 300 m these ATCSs may sometimes be insensitive to the changes in (40 to 800 ft). On the other hand, approximately 50% of traffic flows. Most of the ATCSs provide some features that the respondent's ATCSs use stop-line detection exclusively. allow for replacement of the missing detection data using The rest of the respondents (approximately 8%) use various historic traffic records. Therefore, if a certain detector fails, combinations of the upstream and stop-line detection in their the system finds and uses data from the respective day and ATCS operations. time of day, which will approximate current operations. Such ATCS use of historic traffic data may reduce the Left-turn detection is handled by 50% of the interviewed impact of the detector malfunction on overall performance agencies at stop-lines. The other 50% of the respondents use of the ATCS. upstream detection for left-turn movements, but a wide variety of solutions is applied. Some agencies use common (for the Minor detector failures are relatively frequent events in United States) queue detectors located two to three car lengths everyday ATCS operations. Although these minor failures behind the stop bars, whereas others use combinations of may have a significant impact on ATCS performance (e.g., upstream detectors and filter detectors based on the local con- detectors for a major signal group fail at the critical inter- ditions at each left-turn movement. Placement of the upstream section), their impact is usually limited. Low impact of minor left-turn detectors varies from approximately 20 m (50 ft) to detection failures on overall operations may not trigger a the full length of the left-turn bay. Filter detectors are usually quick response from the agency and detection repair time not placed in the storage bay of the left-turn movement but in might be prolonged. For this reason, it is important to find out the through exiting lane of the intersection leg that receives how major ATCS users perceive the quality of ATCS opera- left-turn traffic. tions during the minor (by its scope) detection problems. The results from the survey are shown in Figure 7. Fewer than Depending on the sensitivity of ATCS operations on detec- 20% of interviewed ATCS users reported that their systems tion inputs, the system may have more or less significant prob- perform poorly (or very poorly) during the minor detection lems when certain detectors fail. ATCSs that are less sensitive problems. Very poorly; 1; 2% Very well; 7; 16% Poorly; 7; 16% Neutrally; 7; 16% Acceptably; 21; 50% FIGURE 7 ATCS operations with minor detection malfunction.