Private transport represents more than a third of all the journeys carried out in large cities urban areas. The high amount of traffic in metropolitan zones implies the appearance of congestion that is one of the most important complications transport engineers must fight against. Bottle necks are one of the most common reasons of congestion effect and on-street illegal parking (double parked or driving lane parked) implies the creation of a (or successive) bottle neck. This paper is focused on to evaluate the costs that appear when on-street illegal parking is detected. To achieve the paper’s goal, first of all, the different costs of the journey have been defined and formulated; next, the effect of illegal parking has been considered and the cost related to that has been formulated for each of the different types of cost. In order to use the formulation obtained two real scenarios have been evaluated. The results obtained in real scenarios have demonstrated that the economic cost due to car parking indiscipline to the whole city is significant; also it is worth to remark that unit cost (cost provoked for a single vehicle) differs depending on the location of the violation and the affected vehicle. On-street illegal parking cost is significantly outstanding. According to data, on street illegal parking reduction has to be one basic pillar of mobility policies. This reduction would allow a higher road capacity and, consequently, greater traffic fluidity.