The basic nature of a pipeline is unique in comparison to any other mode of transport. Pipelines operate on a 24-hour basis, 7 days per week, and are limited only by commodity changeover and maintenance. Unlike other modes, there is no empty container or vehicle that must be returned. Pipelines have the highest fixed cost and lowest variable cost among transport modes. High fixed costs result from the right-of-way for pipeline, construction and requirements for control stations, and pumping capacity. Since pipelines are not labor intensive, the variable operating cost is extremely low once the pipeline has been constructed. An obvious disadvantage is that pipelines are not flexible and are limited with respect to commodities that can be transported, as only products in the forms of gas, liquid or slurry can be handled.