Good idea. Are there other services such as GPS information that should be included in some sort of base service that the user can control.
For example, a gps service that could be turned on and off by the user and would be able to be queried by another service or app that is not in the car? Perhaps other things such as the app that is being run and the screen the user is on?
I'm just thinking aloud here. Not sure if these make sense or not but I was aiming towards trying to give other services the ability to understand your context - say, if you've been sitting still for more than 3 or 4 minutes, a traffic service might look for congestion information or traffic cameras nearby.
The traffic service doesn't need to query your car for the information. It could query some type of status service like this one and it would either get access or be denied access depending on user controlled settings.