We did one with WRXatl a couple years ago and had a lot of success. I think we had around 15-20 teams. We organized another one two months ago and had very little interest, we also had the aim of getting our scavenger hunt together with other local clubs such as the Vette club and S2K clubs.

One rule you should implement is have the driver seal his license in an envelope signed by the planners. If the envelope is torn when the team returns they're disqualified. Granted people will speed and do stupid stuff but those who get caught are out and it does work to discourage people from driving irresponsibly.