Civil and Environmental Engineering


Date of this Version



Transportation Research Record: Journal of the Transportation Research Board of the National Academies (2007) no. 2035: 88-96.


Copyright 2007, Transportation Board of the National Academies. Used by permission.


One control objective at high-speed isolated intersections is to provide safe phase transition by minimizing the occurrence of high-speed vehicles in the dilemma zone before the phase is terminated. An upper limit for extending the green (maximum green time) is used to avoid an indefinite extension of the main-street green. Currently, the maximum green times are chosen by using engineering judgment. This approach does not explicitly consider the trade-offs between safety and delay and hence often results in both unsafe and inefficient performance at the intersection under medium to heavy traffic volumes. The dilemma problem is recast as one of minimizing the number of vehicles entering the upstream decision conflict zone (DCZ). An economic evaluation approach is proposed to maximize both safety and efficiency at the intersection by considering the problem in terms of marginal costs and benefits.Traffic conflicts are used to estimate potential safety benefits, and the induced delay cost is used to estimate the cost accrued on side-street traffic that is associated with extending a competing phase. This approach allows the implementation of logic that minimizes DCZ exposure instead of the current approach of absolute protection until the maximum green time is reached, at which time no consideration is given to dilemma zone exposure when the phase is terminated. This approach handles efficiently and safely the periods of moderate demand volumes when current dilemma zone protection frequently encounters maximum green time exposure.