Oversaturated traffic networks with spill-back conditions are becoming more common, and their signal timing is difficult to optimize. Up through release 7, TRANSYT-7F, a signal timing tool for traffic network simulation and optimization, was designed for modeling only undersaturated networks. Release 8, which was recently completed, has the added capability to model oversaturated networks. Because signal timing optimization for oversaturated networks requires emphasis on traffic throughput and queue management rather than on traditional delay and progression, four new optimization objective functions were introduced in release 8. This paper introduces these new objective functions and compares their ability to produce optimal timing plans with the abilities of the four traditional objective functions. The types of spill-back effects explicitly modeled by release 8 are also introduced. For performance evaluation, a data set involving two closely spaced ramp terminals at an urban diamond interchange was modeled. The CORSIM microscopic simulation program was used as a tool to evaluate optimal timing designs produced by different objective functions. The results indicate that the new objective functions were able to produce superior timing plans that yield lower average vehicle delay and reduce spill-back conditions.
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