[en] Participatory mobile sensing applications are becoming increasingly popular. The growing population of privately-held mobile smartphones enables a plethora of new services. One of the most promising application areas is collaborative traffic sensing. Here, smartphones are used as mobile sensors to collect and share relevant location information in order to reconstruct a global picture of the traffic situation in a monitored area. There are several challenges that need to be addressed in order to provide an efficient and ubiquitous service. In this talk we present how low-cost mobile smartphones can be used for such services without compromising the everyday usage of the device. We first test several sensing policies and evaluate how they affect location accuracy and battery life. We then perform a penetration rate study to identify the proportion of participants required to provide good service by varying several parameters. The outcome shows that if configured correctly, smartphones can be used as accurate mobile traffic sensors providing important information even at low penetration rates.