From the SF Bridge Plaza to downtown Sausalito is probably about half an hour.
But as far as we know she wasn't in a car. From the Crissy Field area to Sausalito, walking, is 2 hours at a normal pace, obviously she could have jogged or run.
I see no scenarios in which SW could have been seen walking on the bridge and also been in a car. I'm speaking only of scenarios in which she exited the bridge away (to the north) from SF.
It's that section to the north of the bridge where you have to walk in a bike lane (with whizzing bicyclists) on what is basically a freeway exit type of road toward Sausalito (it gets better though, after a little less than a mile).
By car from Crissy Field to Sausalito is only 13 minutes in good traffic!
Again, I'm trying to address how long it would haven taken Syd to walk to Sausalito (if that was the reason for being on the bridge). I have heard nothing about Sausalito businesses being asked for video footage, which is a shame if in fact she did walk across the bridge.
If she did walk it, I doubt she knew Madeleine's advice from TA (about the catwalk) and while it is probably fairly safe on that route, I personally have felt unsafe at Baker Beach, both in the morning and in the late afternoon. That's my main worry about Syd - is that she ran into a bad actor, which still seems possible to me. Distraught/distracted young women are at risk, IMO.