Hi Dean - thanks for your feedback. We need to develop a native app to do that, as web apps like our can't provide those sort of features when you're device is locked, but this is something we're working on.
Could you implement voice navigation when your iphone is locked and in your pocket? Unless I am missing something voice only appears to work when phone is unlocked and screen is on. Would be handy to know i have a turn ahead without having to keep taking my phone out of my pocket. thanks