All of the folks that are shown in the street view don't seem to be scrambling to get out of the way, which indicates to me that it's a bicycle , or possibly a Segway...I don't see any shadows either; they're obscured by the blurry area around the camera.
I don't know about this bridge, but I know for sure that Google has a bicycle outfitted with the Streetview cam. They have completed street view for city parks and college campuses in Chicago and other major cities with this.
Look at the street view halfway across to see a line of Segways. Maybe Google has a camera Segway. Probably a bicycle, though. Look for a shadow.
So...it appears that Google has a Street View Bicycle, unless the car violated some very serious access rules! But, there are street views in the area of this bridge where cars would not fit, so either a bicycle or someone on foot with a camera was on the loose...this would be great, as we would be provided street views of some of the incredible historic trail bridges out there!
I bike across this bridge regularly and what it is not showing are the concrete K-rails on either side of the bridge that have been in place for some time now. Even the Park Patrol Officers can only cross on bike although although the 4 wheel version is visible 24 hours a day.