You get the Evan way cul de sac key from a raiders camp by Diamond city where do u use the key?

At the Evan's Way Cul'de'Sac.

Actually, I think I got the key from the Raider Leader IN the Cul'de'Sac. I just picked the lock on the main gate and the key opened the two doors.

So pointless.

Yeah it was weird I didn't have expert pick locking so I kind of figure out a way to jump over the whole fence and I killed the one Raider and he gave me the Evans Way cul-de-sac key I had no idea what it was for cause I couldnt find anywhere around to use it

There are actually 2 small doorways at the corners of the cul'de'sac that lead out to the street. I didn't really need the key since I had the expert/advanced lock skills, but, it was nice to have none the less.