Secondly, air transport growth may be constrained because of supply factors within the air transport industry (eg. Lack of runway slots, influenced terminal capacity, airspace congestion ). For leisure travel, supply constraints within the tourism industry (eg. Lack of hotel beds) may also dampen to overall demand. This is because the demand for air transport is a derived demand, depending ultimately on the demand for the overall tourism product. In addition to these physical limits there may be perceptual constraints as well. This is when travelers may decide that an airport is too congestion to be used or that a tourism resort is too crowded. Thus both physical and perceptual constraints can have the effect of curtailing growth, with the former making it not possible to travel, and with the latter diminishing the desire to travel. In many case the impact of the demand factors, such as determinants, can be clearly distinguished from effects due to lack of physical capacity. However, there are other more ‘grey’ areas when a perceptual constraints may influenced the motivations of travelers.