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Author Topic: Tilting of signage on overhead gantries.  (Read 2710 times)


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Tilting of signage on overhead gantries.
« on: July 22, 2009, 12:53:58 AM »

Over the years, I have noticed that some states tilt their signage on overhead gantries rather than just installing the sign flat along the gantry's truss/beam. I assume that it's probably for increased reflectivity during night hours, but what I can't figure out is what some states tilt up and why some tilt down. States like TN, PA, NM, AL, GA, etc. all tilt down. However, states like MS, LA, and SC all tilt the sign up, away from traffic.

I can understand tilting down, towards the roadway, but what does tilting up do for drivers?

Here's a sign from Birmingham, AL (tilted down):

Here's a sign from Shreveport, LA (tilted up):


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Re: Tilting of signage on overhead gantries.
« Reply #1 on: July 22, 2009, 01:17:54 AM »

I believe tilting up reduces the quantity of time during which the sun reflects directly off of it and glares out the sign.  Given that a car is moving at speed, the way the angles work out, the glare spot, which comes a tad later in the morning or earlier in the evening, is just a bit faster to vanish.
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Re: Tilting of signage on overhead gantries.
« Reply #2 on: July 22, 2009, 03:52:56 AM »

Tilting (either up or down) also helps avoid headlamp glare at night.  The problem with tilting up, and the reason I would hesitate to do it, is that the incident angle (angle between headlamp beam and line drawn normal to the retroreflective sheeting) increases faster than if the sign is tilted down.  Retroreflective sheeting will function only when the incident angle is below a certain amount (which varies according to sheeting type) and so tilting up causes the sign to go dark sooner.
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Re: Tilting of signage on overhead gantries.
« Reply #3 on: July 23, 2009, 01:58:50 AM »

Tilting down may have something to do with snow buildup.  I know Nevada DOT was doing research regarding tilting signs downward to avoid snow accumulation during storms while still maintaining sign reflection.  The study focused on freestanding signs and not overhead.  I don't recall what the result was.

(Incidentally, the sign test section is still in place along a frontage road on the east side of US 395 in Washoe Valley, just north of Carson City.  When driving by, you can see regulatory style signs with text like "South 3" "West 6", etc., which indicates the signs facing direction and degree of tilt.)
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.


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