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 The transport sector needs to go through an extended process of decarbonisation to counter the threat of climate change. Unfortunately, the International Energy Agency forecasts an enormous growth in the number of cars and greenhouse gas emissions by 2050. Two issues can thus be identified: (1) the need for a new methodology that could evaluate the policy performances ex-ante and (2) the need for more effective policies. To help address these issues, we developed an Agent-Based Model called Mobility USA aimed at: (1) testing whether this could be an effective approach in analysing ex-ante policy implementation in the transport sector; and (2) evaluating the effects of alternative policy scenarios on commuting behaviours in the USA. Particularly, we tested the effects of two sets of policies, namely market-based and preference-change ones. The model results suggest that this type of agent-based approach will provide a useful tool for testing policy interventions and their effectiveness. © 2013 by the authors; licensee MDPI, Basel, Switzerland.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. © 2013 Elsevier Ltd.