Many companies sell gizmos that let you mount your hand-held GPS device or smartphone to your windshield, but if your state prohibits the placement of devices, you could wind up with a ticket. A user at the POI Factory website recently collected each state's statutes on windshield visibility, and we've turned it into a color-coded map.
Source: POI Factory
The map above illustrates whether or not your state allows windshield mounts. Green indicates yes; yellow indicates yes with some exceptions; red indicates no.
See the full post at POI Factory here to get the full details, including links to each state's vehicle code regarding windshields.