Rule of thumb,
If you have to ask about it, and your in Maryland, it is illegal. MD Law regarding vehicles is one of the most strict I have ever heard of.
Personally I have the windshield lights on my jeep, I haven't used them while driving though because I know what it is like to be blinded by a large vehicle who likes to play with they're lights, its infuriating. But I wouldn't let the fact that you can't drive with them on down the highway deter you.
I love mine, shine them on my friends cars and houses and act like the 5-0 all the time. They have many uses outside of driving with them, I mostly use them to light up the yard when I am working at night, or at the park with some friends.