The aim of this paper is to evaluate the effect of the on-street illegal parking on the commercial travel time of the vehicles in the area. The effect of the illegal parking in the travel time of the vehicles in the zone is analyzed in an urban scenario in order to quantify the negative impact that illegal parking implies to this, by itself, congested areas. To achieve the objective of the paper, a 3x3 street model has been designed and evaluated for different situations. In this sense, based on a traffic microsimulation model a bunch of scenarios have been considered in function of parameters referred to intensity of vehicles, illegal on street parking level and location of the illegals. Based on the scenarios mentioned, it has been analyzed the effect that the different parameters have on the commercial speed of the vehicles in order to have a first set of information that permits how to act to reduce the effect of illegal on street parking. The results obtained in this article will be able to be used in next steps in order to define direct and indirect reduction strategies referred to illegal on street parking effect.