Private transport represents more than the third of all journeys carried out in large cities. The high amount of traffic in metropolitan zones implies the appearance of congestion which is one of the most important complications transport engineers must fight against. Bottle necks are one of the most common reasons of congestion 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 the evaluation of the costs that appear when on-street illegal parking is detected and specifically its effects on driving vehicles travel time. To achieve this goal, different approaches to travel time costs due to on-street illegal parking have been carried out to evaluate the magnitude of distinct variables referred to illegal parked vehicles as well as traffic conditions. The results obtained allow concluding that illegal parking reduction is a strategic issue to decrease travel time (or increase average vehicle speed).