Variable neighborhood search and tabu search for auction-based waste collection synchronization

We study a less-than-truckload (LTL) transportation service procurement (TSP) in the waste collection industry, where shippers make small-volume requests and a single carrier optimizes the collection routes to realize maximum profitability. In our proposed auction-based waste collection synchronization (A-WCS) mechanism, the carrier plays a role as the auctioneer to decide on who wins the bids and the corresponding payments while the shippers act as the bidders. This is attributed to the carrier-centric structure where a carrier has to consolidate multiple LTL shipping requests so as to provide cost-effective services. Furthermore, a single shipper (e.g., a construction site) may generate multiple types of waste and highly desires for a synchronous service, requiring multiple vehicles dedicated for different waste types to arrive almost simultaneously. The synchronization is beneficial to prevent repeated handling and reduce disruption caused to the minimum. The synchronization requirements are innovatively factored by incorporating time windows into each customer bid. The A-WCS mechanism can encourage truthful bidding from shippers and obtain fair payments for both sides, therefore can benefit the market in the long run. A-WCS problems are extremely difficult to solve due to the integration of two combinatorial problems. We address the A-WCS with a two-layered algorithm, where the variable neighborhood search (VNS) is purposed to search auction allocations in the upper layer and tabu search (TS) is conducted to generate efficient vehicle routes in the lower layer. The capability of our algorithm is demonstrated by an extensive computational study, comparing with a number of state-of-the-art exact and heuristic approaches. Finally, a number of managerial implications for the practitioners are obtained, with insights into the impacts caused by changes in market size, task complexity, customer distribution pattern, and auction frequency.