I installed one at a gated community. It is on a pole next to the call box. Their entrance is about 20' wide, and it is a high end community. This means there is a variation of vehicles from the highend mercedes, which have metallic windshields to the jacked up dodge ram trucks.
The mercedes or any vehicle that have the metallic/heat reflective windshields are a nightmare when you use the sticker tags that you stick on the windows because the owners dont want a bumper tag. It takes experimenting with the vehicle, and thru research, there are "spots" on the windshields that are purposely without the metallic to allow toll tags to work, but still, it was a pain. Ive had good luck with putting the tags on the inside of the moonroof.
The dodge ram seems to have the best read with a windshield sticker...I think it is because it rides high.
There is a "sweet spot" that exists across the board for all vehicles, and the best way to find it is to slap a tag on your vehicle, and drive up to it in different ways, while someone is at the reader making adjustments.
The "sunvisor tag" somehow uses the actual sunvisor to amplify its signal because if you hold the tag with your hand, the signal is greatly reduced.
The bumper tag uses the actual metal bumper to amplify its signal, although it works ok without a metal bumper.
Once you have the reader positioned where it gets a good read, take a silver/metalic sharpie or paint pen, and mark the location, so if for some reason the reader gets knocked out of place, you dont have to spend time re-aiming it...just match up the paint pen marks. (This is from experience, at this HOA, the landscapers managed to smack the reader while installing a new tree, and putting it back in place only took less than 5 minutes).
Advise drivers it is not a race track and to go slowly towards the reader &/or stop.