Current speed-camera alerting systems heavily rely on humans to check the trustworthiness of information send by their users. Hence, these systems are often either expensive or suffer from drawbacks, such as incomplete information concerning mobile speed cameras. We propose an application called FoxyTag using a computational trust engine instead of human checks. FoxyTag lets any driver equipped with a Java/GPS-enabled mobile phone to post a virtual tag about a speed camera and notify other equipped drivers who can confirm or deny the (short-lived) presence of the (mobile) camera. The novel aspect of our trust engine is that it must be location and time aware to automatically compute the trustworthiness of the given tag. We have validated FoxyTag both in real-life settings and with a simulator for large-scale scenarios. The validation showed that our novel time-based trust metrics are appropriate.