Sometimes while I am in the car, I see the word "BOTH" that people spraypaint on under tunnels and on thoose tall walls maybe on highways. Does anyone know what that means? Thanks :)

It will most likely be a graffiti artists tag/signature.

I don't get it?

From Wikipedia: Some of the most common styles of graffiti have their own names. A "tag" is the most basic writing of an artist's name, it is simply a handstyle. A graffiti writer's tag is his or her personalized signature. Tagging is often the example given when opponents of graffiti refer to any acts of handstyle graffiti writing (it is by far the most common form of graffiti). Tags can contain subtle and sometimes cryptic messages, and might incorporate the artist's crew initials or other letters.

My friend took this picture for me...

https://mail.google.com/mail/?ui=2&ik=f80b81194a&view=att&th=12f994a34bd23b16&attid=0.1&disp=inline&zw

That link isn't showing anything...

What do you mean? You didn't see anything from the link? I shows a picture of the sign "BOTH"