Recently, a comment appeared on National Public Radio's Car Talk website suggesting that GPS (global positioning system) units, as well as a variety of electronic automobile accessories, may be illegal.

Why and when? They are illegal when they obstruct the front windshield or vehicle windows.The 2010 Pennsylvania Code, Title 75 - VEHICLES, section 4524 - Windshield obstructions and wipers, paragraph (a) Obstruction on front windshield, reads, "No person shall drive any motor vehicle with any sign, poster or other nontransparent material upon the front windshield which materially obstructs, obscures or impairs the driver's clear view of the highway or any intersecting highway except an inspection certificate ..."The law gives a wide latitude to the police, who are tasked with determining what "materially obstructs, obscures or impairs" means.Here's some guidelines from Lt. Jeffrey Hopkins, a spokesperson for the Pennsylvania State Police.To minimize obstruction of the driver's vision, the worst place to mount the unit is on the windshield. The best place to mount the unit is below the dashboard. Adapters are available to mount GPS units and other electronic units to the ventilation outlets.For easy readability with minimum obstruction of the driver's view, a dashboard mount offers a compromise. When mounting an electronic device on a car's dashboard, the unit should be as far to the left or right as is practical. A unit mounted directly in front of the driver would be considered an obstruction of the front windshield.Many people are using EZPass transponders. Drivers are directed to mount them on the front window for the transponders to work properly. In order that these transponders do not block the driver's view, they should be mounted on the windshield behind the mirror.Also, the mirror may not obstruct the driver's vision. Use of an oversized mirror, or locating a mirror where it may block the view of the road, could make it an obstruction.There are exceptions. In general, the top approximately three inches of the windshield is exempt from this regulation. In this location, a driver can install a tinted sun visor or a decal describing the vehicle.The police are concerned with the proliferation of electronic devices that are leading to driver distraction. Besides the GPS units, there are cell phones which should not be hand-held while driving, although even speaker units are a cause of driver distraction, music players of various types, and probably the newest and most both visually and mentally distracting, the tablet, such as the iPad. Adapters are available for windshield mounting of these units obviously not a good idea. Texting, while driving, is illegal in Pennsylvania.Violation of this law can also occur outside the vehicle. For instance, back in the days of souped-up hot rods, if you raised a portion of the hood of your car to accommodate an oversized carburetor and air cleaner, you could fail inspection if the raised portion exceeded three inches as it would obstruct the driver's vision.The law also requires that windshields have operational windshield wipers, and be clear of rain, dirt and snow. Nontransparent sun screening is not permitted nor any window tinting, other than that installed by the manufacturer or by special exemption, that does not allow a view into the vehicle.Although it is rare that a vehicle is stopped by the police for placing a GPS in the viewfield of a driver, subject to the interpretation of the statue's "materially obstructs, obscures or impairs" guideline, a driver or vehicle owner may be cited. Such a citing increases in situations such as when a "safety check" is performed by police stopping all vehicles on a road, or when the vehicle is suspect and an officer needs a justification to stop and determine whether a further inspection is required.Maintaining good driving visibility is important all the time, but even more so when outside factors such as darkness, fog, smoke, dust, mist or rain decrease the range of vision, and challenges such as traffic or deer can require awareness in all directions. Remember, when you are driving, what you can't see, can cause a collision.This article is a general discussion of the Pennsylvania regulations on vehicle window obstructions. Please refer to the referenced law or check with a police officer or attorney for additional details and interpretations.