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Old 10-05-2006, 06:25 PM
Join Date: May 2006
Location: Austin, Texas
Posts: 437
Toll tag placement - does it REALLY make a difference?

Got my new toll tag (sarcastic "yay") and it came with very specific instructions on placement on my windshield: one-half inch below the rear view mirror in the center of the windshield. Also included plenty of scary warnings about how it might not work if it's close to the edge of the windshield etc.

Is this just a lotta stuff and nonsense? I would much rather have my tag down by the reg. sticker in the corner, a scant three feet away. Am I gonna start racking up toll bills cause of my sinful sticker placement?
Old 10-05-2006, 07:44 PM
Charter Member
Join Date: May 2000
Posts: 27,069
A lot of people including me sometimes don't even mount it and just hold it up going through tolls. It seems to work fine no matter how you wave it. It won't usually register if you forget to pick it up. I say the placement isn't that important within reason. Why don't you experiment like that before you lay the velcro or whatever you use there?
Old 10-05-2006, 08:09 PM
Join Date: Jun 2006
Location: Surefall Glade, Antonica
Posts: 18,786
If the tag is required to be placed in a certain spot, you might get a ticket for placing it elsewhere....
Old 10-05-2006, 10:02 PM
Charter Member
Join Date: Nov 2002
Location: Chicago's Northside
Posts: 2,540
In Illinois, you register your car's make, model, color and plate with the transponder serial number. That way, if the the signal doesn't register and your photo is taken, they simply take the toll from your account. Here, though, we have 'open road tolling' where there's no gate to stop at.
Old 10-05-2006, 10:07 PM
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Join Date: Mar 2004
Location: Sydney, NSW, Australia
Posts: 11,386
I have an electronic tag, but I don't have a car. I use the tag only on the infrequent occasions when I rent a car. So I never have it mounted. I just hold it up, as Shagnasty says, when I'm passing the tag reader. It has always worked OK.
Old 10-05-2006, 10:21 PM
Join Date: May 2001
Location: Not so North
Posts: 11,014
Our M-Tag (EZ Pass) is mounted right where they say to mount yours, and I can't even see the thing. Perhaps that's why they suggest for you to put it there.
Old 10-05-2006, 10:45 PM
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Join Date: Nov 1999
Posts: 1,552
You don't need to hold it up at all, if you can sit it on the dashboard - it just needs a clear view through the windshield to the overhead sensor (yes, it's RF, not visible light, but if a metal car roof is in the way it likely won't work).

It's possible that the instructions are specific to avoid placing it near the edge of the windshield - that could get too close to the metal car body, and could detune the antenna. The M-Tag instructions I found (PDF) mention keeping at least 1 foot away from metal - this is to avoid detuning. They also want it in the center of the vehicle, presumably so the overhead antenna is optimally aimed at it.
Old 10-05-2006, 10:55 PM
Charter Member
Join Date: Mar 1999
Location: Tysons Corner, VA, USA
Posts: 11,496
I have an EZPass and I imagine that placement is to maximiuze effective transmission and minimze obscuring the driver's vision. They say to put it directly in front of the rear view mirror where is is closest to the reader, faces through glass, and does not take up any real estate in the driver's field of view.

They also tell you it's registered to your car and not to "wave" it or use it for any other car, but I'll be darned if I know why. I mounted it (don't know why anyone wouldn't want to) but I do pop it out and "wave" if I have to rent a car.
Old 10-06-2006, 01:57 AM
Join Date: Mar 1999
Location: N/W Arkansas
Posts: 8,144
On a motorcycle, the 'wave' is the norm.

Old 10-06-2006, 02:05 AM
Join Date: Mar 2006
Posts: 14
Woo! Finally a question I can answer. CookingWithGas has it spot on.

I worked for a toll company that uses the electronic tags as the only means of payment (ie no toll booths)

Customers who did not place the tag in the correct position (behind the rearview mirror) could incur extra charges because the toll gantrys couldn't detect the toll tag.

It doesn't need to be in that precise position, but failing to put it there will decrease the chance of the tag being correctly picked up by the gantry.

Og knows why.
Old 10-06-2006, 02:39 AM
Robot Mod in Beta Testing
Join Date: Mar 2001
Location: Pennsylvania
Posts: 19,937
Originally Posted by frogger world champion 1981
Og knows why.
You don't need to be og to know why. Being a EE who has used RF tags in control systems works too.

Antennas broadcast and receive in a pattern. The exact pattern depends on the type of antenna. Some antennas focus the energy in a single direction. They have a longer range, but if you get too far from the center of the radio beam they don't work very well. Others have a wider pattern, so they work at a greater angle but a shorter range. For a toll booth, you want the angle to be wide enough that you can read a wide variety of cars and trucks, but narrow enough that you don't have to worry about picking up a tag in another lane. A monster truck is going to have its tag a lot higher than a low riding sports car, and a very thin car could be closer to one side of the road than a big old fat car.

A typical antenna pattern might look like this:

The "0" on the antenna is aimed to where the center of most car's windows will line up when the car goes through the lane. On the above antenna, if you are within about 15 degrees to either side of this, no real problem, but any further off to the side than this and the signal level is going to drop quite a bit. If you have the tag down in the corner of the window, not only do you risk having part of the metal frame of the car blocking the signal, but you've also got less signal to work with down there since you aren't near the center of the beam.

The tag is directional too, so if it's not hanging in the correct orientation (or you don't point it the right way when you wave it) you'll decrease the signal even further, giving you more of a chance that the signal won't be strong enough for the reader to understand it. Mounting the tag insures that it's oriented correctly. It also stops you from passing the tag from person to person, which they don't want you to do. They want each person to have a unique tag.

By the way, in case you were curious about what's inside that little tag, here's a pic of one:
The spiral on the outside is the antenna. The teeny bit in the middle is the microchip which does all the work. It detects the incoming signal from the tag reader and sends back a coded number which is unique to the tag.
Old 10-06-2006, 09:23 AM
Join Date: May 2006
Location: Austin, Texas
Posts: 437
Excellent answers! I guess I'll just plant it front 'n center like the thing says. Oh well. At least I can't see it while I'm behind the wheel. Thanks--
Old 10-06-2006, 10:05 AM
Join Date: Nov 2001
Location: NH, Escaped from MA
Posts: 2,868
Another reason is that the tinting common to many automobile winshields. the top several inches of the forward screen, is often metalisized. This can interfere with the signal. By placing it the "suggested" 1/2" below the rearview mirror insures that you aren't in the metallic filmed area of the windshield.
Old 10-06-2006, 10:14 AM
Join Date: Aug 2000
Location: Location: Location:
Posts: 10,482
They offer a remote version. Anyone can get one, but most who do have one own cars with the defroster built in to the front windshield (popular on some Ford products and at least one GM van I know off). Tiny, 'invisible' wires block the signal in such windshields.

I have two EZ pass tags registered to three vehicles. After a while, I even called and expressed concern that this does not match their requirement of one EZ pass tag per registered vehicle/plate. They didn't care. I told them I take one of the older two tags from and use it in my new third car. They took the plate number, and said that if I went thru a toll with the new car now and didn't have the tag they'd read the plate and bill me.

I was concerned I'd get a bill and fine, but sure enough, even when I forgot to swap, they billed me for just the toll.

Then, two months later, with a registered car I remembered the EZ PASS, but forgot to hold it up. So then I get a snapshot of my plate/car and notice of the fine, unless I disputed it.

I cannot figure out what the heck their procedure is. With the new car that had no EZ PASS, but which then had a registed tag and no EZ PASS after I called, I blew the toll at least five times w/ just being charged the 3 bucks. I go thru with my older car that has long been registered: bang, a fine.

Go figure.
Old 10-06-2006, 10:26 AM
Join Date: Oct 2001
Location: Hyattsville, MD
Posts: 941
I wouldn't recommend it, but I've driven up and down I-95 with my EZPass transponder actually in a compartment on the dash of my Subaru with nary a problem. Only time it failed was when my wife put an Altoid tin on top of it.
Old 10-06-2006, 10:53 AM
CC CC is offline
Charter Member
Join Date: Apr 2000
Location: not elsewhere
Posts: 4,291
I'm intrigued. When I got my transponder, I was told it would work everywhere (except in Indiana, and that's another story). But how can I find out where the toll roads are that would accept this method of payment? Is there a site that shows all the toll roads? I can't find one. Even my atlas doesn't show which parts of which Interstates are toll roads, outside of Illinois. Any ideas? xo, C.
Old 10-06-2006, 12:22 PM
Join Date: Mar 1999
Location: N/W Arkansas
Posts: 8,144
Generally, if it is depicted in green on the map, it is a toll road.


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