What you are looking for probably is AlertOmatic. As I have no car, I obviously haven't tested it personally; but it seems to match your requirements, and is well rated:
AlertOmatic (source: Google Play; click images for larger variants)
AlertOmatic features a world-wide database of speed limits which it matches against. So no need to setup a speed limit manually – though you can do so additionally if you like (see first screenshot). As the second and third screenshot show, the database also contains locations of speed traps (with cam or "manned").
Keep in mind that such an app cannot be guaranteed to always be 100% accurat – there are natural limitations:
- depending on weather conditions, your GPS might be less accurate
- depending on traffic conditions, different speed limits might apply (temporarily; think of accidents, traffic jams, and the like)
- depending on local conditions, the database might not always be up-to-date (think of construction sites etc.)
Screenshots and links are from the gratis, ad powered version. To get rid of the ads, a paid version is available for US$ 4.
There are several other Speed Limiter apps available, but most of them only work with limits you've set up manually, not considering "local conditions".