The evaluation and design refinement of an integrated traffic management system is a complex process. The first step of this process is to identify the objectives of the traffic management system. Alternative traffic management systems should be evaluated based on how successful they are in achieving the desired objectives. Potential objectives include: improving the level of service, reducing emissions, improving safety, maintaining control stability, and assuring prompt response to emergency situations.
A large number of diverse and complex interactions between the various objectives are expected. Moreover, the number of control devices, the different types of control strategies, and the various ways the devices can be used (independently or in combinations) allow for many possible management strategies. The use of simulation allows engineers to test many potential system designs and control strategies under various hypothetical conditions.
Figure 2-1 illustrates the overall framework for evaluation and design refinement. The process begins with the specification of the traffic control and routing systems. The system specification, one of the two inputs to the simulation laboratory, defines the elements (e.g. surveillance and control devices) and the logic that determines how the system operates. The other input is the scenarios representing the operational environment under which the system will be evaluated. Scenarios are determined by the following factors: (i) traffic demand; (ii) events (including incidents and surveillance and control device failures); and, (iii) driver behavior and vehicle characteristics.
In this evaluation process, a set of candidate designs are usually considered and compared against a ``base'' case. Alternatives or candidate designs are defined by specific logic and parameters and then tested over a range of scenarios. The initial scenarios represent expected conditions under which the system operates. The simulated performance measures quantify the effectiveness of the system and illuminate potential design shortfalls. This knowledge is used to formulate additional scenarios in order to further challenge the system and test its robustness. In this way the scenarios under which each design is tested are generated iteratively, and will likely involve an engineer's professional judgment. These results are then analyzed to suggest modifications to improve the original design.
Figure 2-1: Overall design-evaluation
framework
Qi Yang