Avoiding graphical CAPTCHA by checking against OpenStreetMap

Question

I am writing a subscription app for a rallye site. As the subscription doesn't need former registration, a captcha should be useful.

The subscription is team-based. One team include 1 to 3 person. for each person an postal address must be provided.

Now I wonder, if it makes sense to check if the city exists and the provided street is in the provided city. Or would it be to fragile, i.e. the subscribers use abbreviation like st. instead af street....

Do geo-based captchas apps already exists?

EDIT:

I dont mean to build a bullet-proof captcha system. It was meant as a hint to the site owner

• Address from a country that isn't in the target group: flag as review needed.
• several supscription from the same town: flag as review needed. (here geocoding of course isn't needed)
• ... (do you have ideas?)

Answer 1

The idea of a captcha is to provide some sort of challenge that will be hard to solve by a piece of software. Looking up valid addresses is not a hard thing to do, so in the end it's really hard to prove that the poster is indeed human. If you really want to avoid graphical captcha perhaps you should look into some invisible captcha approach?