Browsing by Author "Isik, Gul"

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Citation - WoS: 1
A Case Study To Reduce Accident Rates and Waiting Times at A Selected Roundabout
(Scientific Research Center Ltd Belgrade, 2012) Oner, Erdinc; Çeli̇k, Burcu; Isik, Gul
In this study, the crash statistics for the roundabouts in Izmir, Turkey were analyzed. One of the roundabouts with the highest crash rates was selected and countermeasures for crash reduction at the selected roundabout were identified. The expected benefits were estimated by the use of before and after crash analysis data found in the literature. In addition to the countermeasures for crash reduction, the maximum waiting times at the roundabout were analyzed and minimized. The roundabout has 28 units of traffic signals. Phases of the traffic signals at the roundabout were evaluated for all directions and improved according to the Highway Capacity Manuel traffic signal timing formulas. The improved phases of the traffic signals were evaluated using ARENA simulation software. The maximum waiting time reduction at the roundabout was documented using the outputs of the simulation model. The fuel usage for the vehicles and the amount of emissions transmitted to the atmosphere are expected to reduce due to the reduction in the maximum waiting times at the roundabout.
Citation - WoS: 26
Citation - Scopus: 30
Dynamic Economic Lot Size Model With Perishable Inventory and Capacity Constraints
(Elsevier Science Inc, 2017) Sargut, F Zeynep; Isik, Gul
In this study, we consider a dynamic economic lot sizing problem for a single perishable item under production capacities. We aim to identify the production, inventory and backlogging decisions over the planning horizon, where (i) the parameters of the problem are deterministic but changing over time, and (ii) producer has a constant production capacity that limits the production amount at each period and is allowed to backorder the unmet demand later on. All cost functions are assumed to be concave. A similar problem without production capacities was studied in the literature and a polynomial time algorithm was suggested (Hsu, 2003 [1]). We assume age-dependent holding cost functions and the deterioration rates, which are more realistic for perishable items. Backordering cost functions are period-pair dependent. We prove the NP-hardness of the problem even with zero inventory holding and backlogging costs under our assumptions. We show the structural properties of the optimal solution and suggest a heuristic that finds a good production and distribution plan when the production periods are given. We discuss the performance of the heuristic. We also give a Dynamic Programing-based heuristic for the solution of the overall problem. (C) 2017 Elsevier Inc. All rights reserved.