Even if they trusted the sign, I have observed that people (myself included) often don't even _see_ all the signs when we're being directed by the GPS. I'd love to see a study on what changes about your perception when using a navigation system.
-- Manual on Uniform Traffic Control Devices, 2009 edition, section 2A.04, paragraph 01
Replace 'signs' with anything--no one's patience/attention will hold up in the face of a constant barrage of information.