I love this idea. The problem is it has to be short and sweet because it's a poster board and mean to be seen by passing cars. I wouldn't have the room to put all of that without it being too small. I hope that makes sense.This is just an idea, see how God leads you. In line with what Mikhen7 was saying, a passage that speaks of an open invitation to those who already know they are imperfect and not right with God can be a great draw. You might preface the passage with something like
"I have known the love of Christ:
For God did not send His Son into the world that He might condemn the world, but that the world might be saved through Him. John 3:17"
It's more of a giving your own testimony rather than 'telling people what to do'. It's an approach that gives people room to consider for themselves if they would like to have Christ in their lives rather than putting up an automatic defense.
Of course follow as the Lord leads you, He can use any part of His Word for His purposes. The above is just an idea.