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38 gained by the implementation of an ATCS. All other times ATCS evaluations through microsimulation overcome these (33%), these studies are done in-house by the ATCS agen- shortcomings. Traffic conditions can be controlled tightly; cies. Fourteen percent of the interviewed agencies never they can be replicated and varied stochastically. Incident-and performed any evaluations to investigate the benefits of event-based traffic conditions can be constructed and tested their ATCS deployment. carefully. Installation can be simulated to be optimal with high-quality detector placement and rigorous management of ATCS evaluation studies are done either in the field or by global and local control parameters, such as timing constraints. using high-fidelity microsimulation models. Commonly, field However, field evaluations exceed microsimulation evalua- evaluations are done as a set of before-and-after studies where tions in other aspects. For instance, there is always a margin the "before" study reflects field conditions with pre-ATCS between microsimulation and reality. Data communication traffic signal control and the "after" study reflects the perfor- between the ATCS kernel, local traffic controllers, and traffic mance of an ATCS. Before-and-after studies are done to com- detectors has to be emulated, which may be a difficult task. pare all sorts of alternatives that can possibly bring benefits to traffic operations. When a new ATCS is deployed, this com- Survey results show that field evaluations are still the parison of old versus new traffic control strategies may have major way of evaluating ATCSs. Table 13 shows how popu- another dimension. If an ATCS is installed with the ability to lar various evaluation studies are with ATCS users. "On and implement various signal timing plans, evaluators of the new off" studies refer to evaluations where ATCSs are tested with system can re-apply the old traffic control timings in the full adaptive logics turned on and then turned off (with TOD new conditions for the sake of a fair comparison of the two plans running in the background). (new and old) traffic control strategies. Most ATCSs have the ability to turn off their adaptive control algorithms and imple- When performances of an ATCS and a non-ATCS traffic ment the TOD signal timings that were in effect before the signal system are comparatively evaluated a variety of perfor- ATCS was installed. In this way, both old and new traffic con- mance measures can be compared. Figure 14 shows the most trol strategies are exposed to (approximately) the same traffic common performance measures in such evaluations. The lead- conditions, as opposed to the before-and-after study where ing performance measure is travel time (or travel delay along traffic conditions could significantly change between comple- the travel time segment) followed by the number of stops, inter- tions of these studies. This approach to evaluating new and old section delays, average speeds, and queue lengths, in that order. traffic control is often called an Off versus On study. Off here refers to new traffic control being switched off (and instead BENEFITS FROM ADAPTIVE TRAFFIC CONTROL running old TOD plans) and On refers to new traffic control SYSTEMS DEPLOYMENTS being implemented. ATCSs are known to have several advantages over traditional Field evaluations of ATCSs have their limitations. The traffic signal timing operations with TOD plans. Ideally, experimental designs of such evaluations often lack rigor ATCSs work best in conditions with high levels of nonrecur- because of idiosyncratic traffic patterns that are difficult to ring congestion, such as incidents and special events, and in control. However, no less difficult are the requisites for vali- areas with fluctuating traffic demand. As mentioned earlier, an dating simulation models that also depend on field conditions. ATCS is not necessarily "the answer" for any situation. It is Field data collection is expensive and usually cannot be as important to understand that it should not be expected that an comprehensive as simulation outputs even with extensive ATCS deployment can totally resolve all traffic congestion survey instrumentation. Field evaluations typically address issues. Instead, ATCSs could be considered as tools that can limited sets of traffic conditions. Furthermore, unexpected help to reduce traffic congestion by promoting the operational traffic conditions are by their very definition tough to capture; control and management of the transportation network. The however, ATCSs are known for providing good performance primary area of benefits that can be achieved by an ATCS in such circumstances. When an installation deviates from deployment is operational efficiency, measured through the the requirements owing to a compromise by the client agency reduction of delays, stops, and other negative measures of traf- or inadequate maintenance, the ATCS is then subject to an fic performance. ATCS deployment improves the safety of unfair comparison. traffic operations only indirectly--through reduction of some TABLE 13 ATCS EVALUATION STUDIES Type of Evaluation Study Percent of Agencies Field evaluation 89 Evaluation in microsimulation 11 Before-and-after study 65 On and off study 35 Note: The first and the second evaluation types are mutually exclusive, as well as the third and the fourth; hence, total percentage equals 200.
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39 Other, please specify; 4; 3% Not applicable; 5; 4% Arterial travel times/delays; Average speeds; 18; 15% 33; 26% Queue lengths; 17; 14% Number of stops; 26; 21% Intersection delays; 21; 17% FIGURE 14 Most common performance measures to evaluate an ATCS. efficiency-related performance measures, which highly corre- When these generalized findings are disaggregated into late with some safety metrics (e.g., a reduction in the number various performance measures it is found that 60% of ATCS of stops reduces the chance of rear-end collisions). users observe a reduction in travel times/delays when such a system is deployed. Similarly, deployments of ATCSs reduce The benefits of ATCS deployments are reported in numer- the number of stops, intersection delays, and queue lengths ous studies published during the last 30 years, since the first in 37%, 37%, and 23% of the cases, respectively. Increases practical applications of the systems. There have been studies in average speeds have been observed by 35% of the ATCS that reported 40% (and higher) improvement in certain per- users. formance measures after the ATCS has been deployed and the others that did not find any improvements or found that The benefit of an ATCS in oversaturated traffic conditions the operations worsened after ATCS installations. Although is one of the most controversial aspects of the system's perfor- ATCSs have been shown to provide benefits in most cases, it mances. Many ATCS users state that the systems do not help is difficult to provide a detailed overview of the benefits for significantly in oversaturated traffic conditions, although any of the systems, as each technology works differently, and others have stated the opposite. The survey indicates that only each implementation is unique and customized to that particu- a very small percentage of the interviewed agencies (3%) rec- lar deployment site. Figure 15 shows a summary of the results ognize that their ATCSs prevent or eliminate oversatura- from evaluation studies conducted at deployment sites covered tion. The majority of the interviewed agencies reported that by the survey. Most of the ATCS users reported (based on their their ATCSs reduce or eliminate the extent of the periods of evaluation studies) that their ATCSs perform much better than oversaturation. Approximately 33% of ATCS users have found their previous conventional traffic signal systems [e.g., TOD the systems to be counterproductive in oversaturated traffic plans executed through fixed-time or actuated (coordinated or conditions. Figure 16 shows two separate categories (Ques- isolated) traffic control]. Thirty-two percent of the respondents tion 11 under points c. and e.) that were combined to obtain found that ATCSs are better, whereas 14% reported that no this percentage (comments under "Other, please describe" benefits of the ATCS were observed. Finally, only 5% of the mostly report that ATCSs are not useful for oversaturation). ATCSs have performed worse than the previous type of traf- fic control system, whereas 11% of respondents did not report Considering the responses on ATCS benefits in oversatu- any findings. ration, and that oversaturation mostly occurs in peak periods,
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40 Not applicable; 5; 11% Previous traffic control was better; 2; 5% ATCS is much better; 17; 38% Neutral; 6; 14% ATCS is better; 14; 32% FIGURE 15 Comparison of performances: ATCS vs. other traffic control. ATCS prevents or eliminates oversaturation; 1; 3% Other responses; 9; 23% Oversaturation is very rare on the corridors operated by our ATCS; 2; 5% ATCS eliminates or ATCS adversely affects the reduces the extent of the traffic conditions during periods of oversaturation; periods of oversaturation; 23; 59% 4; 10% FIGURE 16 ATCS performance in oversaturated traffic conditions.