Government sectors often provide guarantees in build-operate-transfer (BOT) roads to attract private investments. Since a fiscal burden would be incurred from the contingent liability of issuing a guarantee, charging a fee for the guarantee is considered an effective remedy for management of contingent liabilities in practice. This study incorporates the charge mechanism for minimum traffic guarantee (MTG) into consideration, and proposes a method to choose the optimal concession contract variables with a paid MTG. We find that the government sector will specify a lower toll and a higher capacity for a BOT road with a paid MTG. As a result, the society suffers a social welfare loss when providing a paid MTG. Meanwhile, both the charging fee of an MTG and the guaranteed traffic volume would increase the loss of social welfare, which means the incentive mechanism of private participation and the management of contingent liabilities (charging fee for MTG) are all at the expense of social welfare. Moreover, since the welfare loss is magnified under a high marginal social cost condition, the government should set the charging fee and guaranteed traffic volume according to the marginal social cost level. In addition, to obtain more insights, this paper has made two extensions to further investigate the effects of a paid MTG (1) when contracting with a risk averse private investor; (2) when the concession period is an endogenous variable. Based on our model results, this paper derives several policy implications regarding BOT contract design when a paid MTG is provided.

政府部门通常会在建设－运营－转让（BOT）道路上提供担保，以吸引私人投资。由于发行担保的或有负债会产生财政负担，因此在实践中收取担保费被认为是管理或有负债的有效补救措施。这项研究考虑了最低交通保证（MTG）的收费机制，并提出了一种选择有偿MTG的最优特许经营合同变量的方法。我们发现，政府部门将为有偿MTG的BOT道路指定较低的通行费和较高的通行能力。结果，当提供付费的MTG时，社会遭受社会福利损失。同时，MTG的收费和保证的交通量都会增加社会福利的损失，这意味着私人参与的激励机制和或有负债的管理（MTG的收费）全都以社会福利为代价。此外，由于在高边际社会成本的情况下福利损失会被放大，因此政府应根据边际社会成本水平来确定收费和保证交通量。此外，为了获得更多的见解，本文进行了两个扩展，以进一步研究有偿MTG的影响（1）与规避风险的私人投资者签订合同时；（2）优惠期为内生变量时。根据我们的模型结果，本文提供了提供付费MTG时有关BOT合同设计的一些政策含义。