As a general rule these things start with sincere intentions but often devolve into cynical exploitation. So if you weigh the benefits of speed cameras against the suffering they cause i think you would find they aren't a good thing.
Getting to your original question i think many people don't trust that their face and fingerprint scans will be used in a way that would be in the publics interest. Its more likely that the authorities would find a way to use that data against you.