This was meant to happen, via the LinkedIn OpenStreetMap group I learned about the OpenTraffic project.
Here's their introduction: "OpenTraffic is a open data project initiated by doroga.tv to create open traffic jam map of the world. We use OpenStreetMap vector graph and data feeds from various fleet management systems to calculate the traffic. Each contributor and every developer may use the traffic APIs for mapping and navigation projects and researches. [...]
If you'd like to start OpenTraffic project in your area, you need to do the following steps:

Re: Open Traffic Project
I can't provide GPS equipped vehicles, but there is an alternative for England and Scotland :-)
http://www.tih.org.uk/index.php/Home is the main supplier of DATEX2 data, under an the UK Open Government License. It would be fabulous to see this data used by you :-)
http://trafficscotland.org/datex/
http://www.datex2.eu/datex-node