142. Perhaps the most important issue in transportation PPP projects is the allocation of traffic risk. OECD consultations suggest that developers and investors will be reluctant to accept full responsibility for traffic risk. Traffic risk is very difficult to quantify and predict and research suggests that over estimates are common and often lead to renegotiated contracts (Better Regulation of Public - Private Partnerships for Transport Infrastructure, OECD/International Transport Forum 2013). Potential political interference in tariff levels also creates risks. These factors mean that allocating traffic/demand risk to the private sector will discourage bidders and/or will lead to more expensive bids jeopardising value for money.