How to get Video Input
The fundamental question here is how fast you need to update the sign's image.

If you don't need to update it more often than once every few seconds, DVI is WAAAAAY overkill. (And placing something that shows video next to a highway seems like a very bad idea)

You're probably best off using a frame-buffer in your hardware, and using something like a FT232 or FT245 to dump images to it.  The FT2** IC Gives you ~8 MBps of interface bandwith over USB, and a dead-easy software driver you can talk to. 

Alternatively, depending on your sign's resolution, you could use a simple serial interface. 

I would assume you would be sending raw bitmaps to the sign (it makes the software end easier). Then you just cache them in the hardware, and then serve them up to whatever interface the sign implements from your hardware.