The purpose of this paper is to investigate both environmental effects and cost effects of using less specified contracts regarding bus sizes in public bus transports. The process of choosing the best bid in the public procurement of bus transports is easier if the demands of the qualifications are well specified and detailed. On the other hand, detailed contracts can force the entrepreneurs to use less environmentally friendly and uneconomical alternatives. A mathematical model with binary variables is developed to evaluate the environmental and the economical effects of more optimized bus sizes. Computational results from a bus service provider are reported. The results of the model indicate that the emissions decrease considerably by using less detailed contracts. The results of a sub case indicate that the costs could be reduced as well, depending on how efficient the additional buses can be planned. The process of choosing the best bid in the public procurement process will be more complicated when the contracts are less detailed compared to current situations. (C) 2013 Elsevier Ltd. All rights reserved.