Kenneth G Courage

COMPARISON OF QUEUE-LENGTH MODELS AT SIGNALIZED INTERSECTIONS

Several measures of effectiveness (MOEs) are associated with the queuing process at traffic signals, including delay, number of stops, fuel consumption, emissions, and queue length. The focus in this study is on queue length in general and on the storage requirements for left turns in particular. Queue length is an important MOE because queues that overflow the available storage space have an adverse effect on the overall operation of the intersection. Many traffic models now provide queue-length estimates, but the procedures used by these models are based on different queue definitions and have different computational approaches that lead to different results. A classification framework is developed for the existing models, their behavior is compared with that of the proposed Highway Capacity Manual (HCM) 2000 queue model, and queue conversion factors are provided for translating the various model outputs to their HCM 2000 equivalent. The proposed HCM 2000 model and its parent model from the Signalized and Unsignalized Intersection Design and Research Aid (SIDRA) provide a comprehensive treatment of the queuing process, accounting for control parameters such as controller type and progression quality as well as for the random and overflow effects associated with traffic flow. As such, the queue-length estimates from these models are more analytically defensible than those of the simpler theoretical models. The SIDRA and HCM 2000 queue estimates are generally higher than those of most other models and are somewhat higher than what conventional wisdom would suggest. It is suggested as a result of the comparisons presented that the queue estimates from some models are unduly optimistic when demand approaches capacity and that a goal of 90 percent confidence in the adequacy of left-turn storage lanes may be difficult to achieve under these conditions.

FRAMEWORK FOR INVESTIGATION OF LEVEL-OF-SERVICE CRITERIA AND THRESHOLDS ON RURAL FREEWAYS

The Highway Capacity Manual prescribes level-of-service (LOS) criteria as a function of traffic density to categorize the operational conditions of both rural and urban freeway sections. This density-based level of service is ideally suited to the assessment of urban freeways when the performance must be optimized to meet high traffic demand. There is, however, some question as to whether density is the appropriate indicator of the quality of service on rural freeways, since drivers may think more in terms of psychological or emotional comfort for freeways, which generally serve long, high-speed trips and rarely experience more than moderate congestion levels. Three specific measures are examined that have at least an intuitive relationship to the concept of driver comfort: (a) acceleration noise, which is a measure of the physical turbulence in the traffic stream; (b) number and duration of cruise control applications, which could serve as a general indication of driver convenience or inconvenience; and (c) percent time spent following, which is already accepted as the basis for determining the level of service on rural two-lane highways. The three candidate measures were estimated for a hypothetical section of rural freeway by simulation, using the CORSIM model. The kinematic relationships of individual vehicles within the traffic stream were estimated by postprocessing the simulation data for each second of operation. All of the measures considered in this study have conceptual appeal. All have produced interesting and potentially usable results with respect to their relationships to traffic volume. Although the simulation results are interesting, further studies focusing on driver opinions, behavior, or field measurements, or all three, would likely be necessary to support the development and recommendation of a specific set of LOS criteria that recognizes the differences between the urban and rural freeway driving environments.

Integrated Simulation-Based Method for Estimating Arrival Type for Signalized Arterial Planning Applications

One of the primary determinants of signalized intersection delay, and consequently signalized arterial level of service, is progression quality. Progression quality is represented by an arrival-type variable in the Highway Capacity Manual (HCM) procedure for calculating signal delay. Ideally, the arrival type is estimated from collected field data, in particular the percentage of vehicles that arrive at an intersection approach while that approach has the green signal indication. The Florida Department of Transportation prescribes the use of its ARTPLAN software program for planning-level analyses of signalized arterial level of service in the state of Florida. This software is a planning-level implementation of the arterial analysis methodology contained in Chapter 15 of the 2000 HCM. One of the inputs to this software program is arrival type. Given the significant impact this variable can have on delay, and subsequently level-of-service results, it is prudent to have as good an estimate of its value as possible. However, collecting accurate data on progression quality is very difficult, and the intent behind a planning-level analysis is that detailed field data should not be required. Consequently, the blanket HCM recommendations for arrival type are often applied in these analyses without much consideration of how appropriate these values really are for the specified roadway, traffic, and control conditions. A method developed to estimate arrival-type values with a simulation-based approach from the planning-level inputs of ARTPLAN is described. Also provided is a summary of simulation-derived arrival-type values from a large number of sample data sets.

Evaluation and Design of Maximum Green Time Settings for Traffic Actuated Control

A study is described that was conducted to develop an improved average green time estimation model for traffic-actuated control and to suggest a maximum green time design method that analytically minimizes intersection control delay. Improvements in the green time estimation model include revisions in the concept of additional queue service time, explicit treatment of right turns in lane groups containing both through and right-turning vehicles, and other improvements that include updates based on recent studies and modifications in the approaches taken for the modeling procedure. The proposed maximum green time design procedure consists of four components: (a) estimation of the average green time of a traffic-actuated phase, (b) performance evaluation of the system through the 2000 Highway Capacity Manual (HCM) procedure, (c) formulation of an overall average control delay minimization problem, and (d) a search process to find the most efficient set of maximum green time parameters that minimize the average control delay at an intersection. Using simulation as a surrogate for field data collection, it was demonstrated that the proposed average green time estimation models offer better results than the one in the 2000 HCM. In addition, it was suggested that on the basis of the improvements demonstrated in terms of design, the proposed maximum green parameter design procedure represents an advancement of the methodology for analysis of signalized intersections.

PREDICTION OF TRAFFIC-ACTUATED PHASE TIMES ON ARTERIAL STREETS

Traffic congestion in U.S. cities has grown rapidly in recent years, and numerous solutions are needed to address the problem. New research is described for producing basic improvements in the practice of traffic signal timing design, which is one of the many available weapons for fighting congestion and delay. Models in the literature for predicting traffic-actuated phase times are oversimplified. In some cases, they do not recognize the signal settings associated with today’s controllers, which affect phase times. In other cases, models from the literature do not recognize operational effects (for example, progression, queue blockage, permitted left turn, critical movement, early return to green, and stochastic effects) that affect phase times or that are applicable only to isolated signals instead of coordinated signals. Consequently, the recommended average phase times are not practical, and the resulting performance estimates and timing plan designs are unlikely to materialize in the field. The overall goal of this research is to describe and demonstrate an improved methodology for predicting actuated phase times. An improved methodology is needed to improve the overall timing plan design process. Experimental results indicate an improvement in the accuracy of actuated phase time calculation based on the improved methodology.

DEVELOPMENT OF AN XML-BASED SPECIFICATION FOR TRAFFIC MODEL DATA EXCHANGE

The proliferation of traffic software programs on the market has resulted in many very specialized programs, intended to analyze one or two specific items within a transportation network. Consequently, traffic engineers use multiple programs on a single project, which ironically has resulted in new inefficiency for the traffic engineer. Most of these programs deal with the same core set of data, for example, physical roadway characteristics, traffic demand levels, and traffic control variables. However, most of these programs have their own formats for saving data files. Therefore, these programs cannot share information directly or communicate with each other because of incompatible data formats. Thus, the traffic engineer is faced with manually reentering common data from one program into another. In addition to inefficiency, this also creates additional opportunities for data entry errors. XML is catching on rapidly as a means for exchanging data between two systems or users who deal with the same data but in different formats. Specific vocabularies have been developed for statistics, mathematics, chemistry, and many other disciplines. The traffic model markup language (TMML) is introduced as a resource for traffic model data representation, storage, rendering, and exchange. TMML structure and vocabulary are described, and examples of their use are presented.

Microscopic and Macroscopic Approaches to Delay Estimation with Oversaturated Conditions

Chapter 16 of the 2000 Highway Capacity Manual (HCM) contains a widely recognized and well-accepted procedure for calculating per-vehicle control delay at signalized intersections. Appendix F of that chapter discusses the relationship between various components of control delay using cumulative arrival and departure curves. The delay obtained from these curves has certain limitations that need to be recognized if proper conclusions are to be drawn. To illustrate these limitations, a comparison is made between the delay obtained from cumulative curves and the delay obtained through a detailed analysis of vehicle trajectories. A comparison of the control delay obtained from trajectory analysis and that obtained from cumulative arrival–departure curves shows that the cumulative curves omit certain valid portions of the control delay, while including other portions of time that are not delay at all. Taking a macroscopic approach to oversaturated delay estimation, the HCM procedures for dealing with oversaturated conditions are described and their shortcomings are explored using an example of a multiperiod analysis of an oversaturated approach to a signal. It is demonstrated that the current HCM equations can overestimate multiperiod delay and that the assumption of a capacity that is constant from cycle to cycle can underestimate delay at high values of the volume-to-capacity ratio.

Adaptation of Highway Capacity Manual 2000 for Planning-Level Analysis of Two-Lane and Multilane Highways in Florida

A planning-level adaptation was developed of the Highway Capacity Manual (HCM) 2000 procedure for estimating the level of service (LOS) on two-lane and multilane highways in Florida. The problems associated with planning-level adaptations in general and with uninterruptedflow highways in particular were identified. Although much of the adaptation was achieved though the use of default values for data items, some departures from the HCM procedures were required. The most significant deviation was the creation of a third class of two-lane highway to supplement the two classes currently defined in the HCM. A case was made for the existence of this class and its inclusion in a future edition of the HCM. The Florida Department of Transportation’s planning-level methodology, termed HIGHPLAN, is well suited to its intended application, which is planning-level analysis of two-lane and multilane highways in Florida. It maintains fidelity to the HCM procedures to the extent that Florida conditions will allow and Florida users will accept. As long as they are understood, the departures from the HCM should not pose significant problems for users outside of Florida. The planning-level methodology has also been implemented in a software program that produces LOS estimates and service volume tables covering sitespecific conditions.

Phase Time Prediction for Traffic-Actuated Intersections

The Highway Capacity Manual (HCM) provides a methodology in Chapter 9 to estimate the capacity and level of service at a signalized intersection as a function of traffic characteristics and signal timing. At traffic-actuated intersections, the signal timing changes from cycle to cycle in response to traffic demand. An accurate prediction of average phase times and their corresponding cycle length is required to assess the performance of intersections controlled by traffic-actuated signals. The current technique suggested in Appendix II of HCM Chapter 9 for this purpose has not been well accepted. A more comprehensive methodology and a more satisfactory analytical model are described that predict traffic-actuated signal timing for both isolated and coordinated modes with actuated phases. The proposed methodology and model have been verified by simulation augmented by limited field studies. The results are encouraging with respect to their general reliability and their compatibility with the current HCM Cha...