Zoltan A. Nemeth

An application of variance reduction techniques in freeway simulation

Computer simulation models are used in a variety of applications in transportation engineering and have become a prime aid in decision making. The applications range from evaluating traffic control strategies for single intersections to such complex decision processes as evaluating the impact of removing toll facilities at the George Washington Bridge in New York City. While it is widely accepted that simulation offers an unmatchable capability of evaluating alternate control policies, the high variance of the output variable presents a critical problem in such comparative analyses. The simulation models with high output variance must be run longer or replicated many times to achieve a desired precision level, and that corresponds to increased cost of computer resources. This paper describes and illustrates the application of variance reduction concepts that can improve the reliability and efficiency of the simulation experimental process by taking advantage of the simulation model structure. The two variance reduction concepts (common random numbers and antithetic variates) reduce the variance of the output variable by replacing the original sampling procedure with a new procedure that yields the same expected value but with a smaller variance. The application of the variance reduction concept was illustrated using results from experiments with a freeway simulation model. The results indicate that both common random numbers and antithetic variates sampling procedures appreciably reduce the variance of the simulation output measure.

EXPERT SYSTEM FOR ACTUATED CONTROL OF TRAFFIC AT ISOLATED INTERSECTIONS (EXACT), PART I: KNOWLEDGE ACQUISITION AND REPRESENTATION

Abstract This paper describes the application of expert systems in traffic-actuated control at isolated signalized intersections. The expert system EXACT (EXpert system of ACtuated control for Traffic at isolated intersections) was developed to assist traffic engineers in selecting the appropriate type of traffic-actuated controller and its associated features, the type of detector and its configuration, and time settings at a particular intersection. EXACT was structured to handle both design and operational problems. The first steps of EXACTs development, including variable identification, shell selection, knowledge acquisition, and knowledge representation, are presented in detail in this paper.

EXPERT SYSTEM FOR ACTUATED CONTROL OF TRAFFIC AT ISOLATED INTERSECTIONS (EXACT), PART II: THE KNOWLEDGE BASE DEVELOPMENT

Abstract This paper describes the development of the knowledge base of the expert system EXACT (EXpert system of Actuated control for Traffic at isolated intersections). EXACT was developed to assist traffic engineers in selecting the appropriate type of traffic-actuated controller and its associated features, the type of detector and its configuration, and time settings at a particular intersection. The knowledge-base of EXACT consists of three primary elements: frames, parameters, and rules. These elements are activated by a back-word chaining mechanism during a consultation. The ability to explain its reasoning process to the user is one important feature of EXACT. Moreover, EXACTs knowledge-base has the capability of interfacing with graphics program in order to enhance the understandability of the system. A full description of the knowledge-base development of EXACT as well as its graphics interface are presented below.

A simulation model of freeway lane closures

A model to simulate traffic operations at freeway lane closures is described. The model logic is based on a rational description of the behavior of the drivers in a freeway lane closure situa tion, and the program is written in SIMSCRIPT 11.5. An applica tion of the model is illustrated with evaluation of potential safety impacts of reduced speed zones in freeway lane closures at differ ent levels of assumed driver compliance.

EXPERT SYSTEM OF ACTUATED CONTROL FOR TRAFFIC AT ISOLATED INTERSECTIONS (EXACT): PART III: TESTING AND EVALUATION

Abstract This paper describes the testing stage of the knowledge base of the expert system EXACT (EXpert system of Actuated control for Traffic at isolated intersections). In Tests by two parties (our expert and external experts), more than eighty percent agreement was found between EXACTs solutions and the two parties’ solutions. EXACT was developed to assist traffic engineers in selecting the appropriate type of traffic-actuated controller and its associated features, the type of detector and its configuration, and time settings at a particular intersection. The development of EXACT, including the identification of variables, shell selection, knowledge acquisition, knowledge representation, and knowledgebase development, was presented in earlier papers.