Horizontal collaboration (HC) is an innovative strategy aimed at reducing distances traveled through better resource utilization and consolidated product flows. Especially concerning urban freight movements, often summarized in the concept of City Logistics, cooperation between actors on the same supply chain level can be an effective concept to ensure efficiency and sustainability. The aim of this paper is to evaluate possible business opportunities for collaborative freight operation via discrete event simulation. Certain key performance indicators (KPIs) addressing both economic and environmental aspects are used in order to present and evaluate the execution of delivery tours in the city center of Athens and its outskirts. The primary data used for this evaluation are real-life data from Greek third party logistics (3PL) operators and the results show the current impact of two collaboration scenarios involving an Urban Consolidation Center.