Congestion of the urban traffic is growing in Tunisia, and the situation is likely to become critical in the near future. The aim of this paper is to determine the major characteristics of a solution to reduce congestion in the case of a developing country. We have developed a survey on the willingness to pay in Tunis City in order to evaluate their reaction using the Contingent Evaluation Method. The results show that Tunisians are reluctant to abandon the use of their private cars despite their awareness of the phenomenon of congestion, of its high costs and of its socio-economic and environmental constraints. This paper constitutes a first attempt in Tunisia aiming to quantify, explain and determine the factors of the Willingness to Pay of the private car users in order to reduce the congestion problems and consequently to ensure a smooth flow. Keyword: externalities of transport; urban congestion; willingness to payThe congestion generates individual and collective costs such as the loss of time for the workers, tiredness, pressure, pollution, noise, the greenhouse effect ... etc. These externalities of the transport system escape to the mechanisms of the market.