I don't know the actual speeds, but the crescent line def. Can move when crossing the bridge... There is no noticeably slow speed. I'd say at least 50. Seems to me like I remember seeing 70 on the gps when rounding the lake, but that might not have been on the actual bridge. I also could be remembering that completely wrong.
Of course coming into town (southbound) it is probably common to get some slower signals due to freight traffic ahead.
I also agree with Edgefan, much more scenic view of the lake from the City of New Orleans, lots and lots of wildlife.