Efficiency versus Fairness in Priority Control: Waterway Lock Case

Delay at a congested service facility, such as a waterway lock, depends on the control policy used. The shortest processing time first (SPF) policy, which is a promising priority control policy, can significantly reduce the average delay/barge compared to the normally used first come first served (FCFS) policy. SPF tends to favor large groups of barges, i.e., tows, at the expense of smaller ones. This paper modifies the SPF policy to consider fairness among tows in queues. One modified algorithm, called fairer SPF (FSPF), limits the number of tows allowed to pass any particular tow. The case study indicates that FSPF can yield most of the benefits of SPF without greatly sacrificing fairness. Compared with FCFS, FSPF can reduce barge delay up to 53.89%. When tow sizes (and thus service times) are similar, the benefits from resequencing tows are limited. Results confirm that larger tow size variance yields larger barge-delay savings. The barge-delay savings for FSPF compared to FCFS range from 6.68% for constant tow size to 60.74% in a high-variance case. The results indicate that significant delay reductions are achievable without significantly compromising fairness.