Can't speak to any miles above Havre de Grace other than rt40, which is good.
Bridge over Susq. is fine, however check their bike crossing hours. It is still time restricted. Super annoying that they charge bikes $8 to cross. tsk, tsk, DOT.

southbound on rt40 to Martin State Airport is fine. (northbound is NOT) you will want to come off 40 and get to Eastern ave to get into Baltimore. (I can give directions if you want).

I lead a ride from Baltimore to DC every year and am well acquainted with it. My route:
https://drive.google.com/file/d/0By9...ew?usp=sharing
http://ridewithgps.com/routes/7391490



-Bob