The basic principle there can be succinctly expressed as "no harm, no foul". A niqab wearer doesn't harm anybody except, arguably, herself. It would therefore be impossible to justify a niqab ban as a reasonable limit, demonstrably justified in a free and democratic society. 

This is not to say that general requirements to check photo ID do not still apply. If a policeman wants to verify that the driver he is about to give a ticket to is the same person whose face is shown on the driver's licence he has been handed by that driver, then the driver will have to show her face. THAT is a reasonable limit.