[magmastonealex]

> (maybe with some kind of publicly deployed differential gps?)

I believe this is supported/expected in the DSRC specifications. As I understand it, a well-known point, such as a post at a corner of an intersection, is located with high accuracy. A DSRC Roadside Unit can then broadcast GPS corrections to provide significantly higher accuracy to units that are able to receive and use this information.

It does require some infrastructure support, but it's something the specification group has thought of.