Sunglasses are vertically polarized. LCD displays with fixed viewing orientation are often vertically polarized for this reason. For displays that might be viewed in either portrait or landscape mode, it's common for LCDs to have the polarization oriented at 45 degrees so that polarized sun glasses won't make the screen does not go black in either. Some LCDs have a birefringent layer to convert the linear polarized light to circular polarized light so that there won't be angles where polarized sun glasses will turn the display black. With either the 45 degree orientation or the birefringent layer, polarized sunglasses will make the display darker than the make the surroundings.

The Garmins I have use vertically polarized displays.