I find shoulders on bridges are typically unrideable spaces. On a highway debris gets kicked on into the grass by vehicle tires. On bridges the debris bounces off the wall and collects in the shoulder. If you ride there you will get a flat.
I suspect many if not most cyclist rear end collisions are the result of drivers waiting until the last second to pass. For a following driver anything in front of them is obscured until the lead driver completely changes lanes. Since the lead driver is still closing the gap as they are changing lanes they may only be 20 feet behind a cyclist before they clear the lane and the driver behind can see anything. With a string of multiple vehicles it creates a chain reaction where either the whole line slows down or the gap gets tighter and tighter with each subsequent vehicle. You can end up with a situation where a driver is doing highway speed and suddenly has a bike 15 feet in front of them. It's not limited to bikes, there are plenty of videos online where a driver plows into the back of a broken down truck.
Found a video that kind of illustrates it, occurs multiple times. First truck waits to pass, starts passing closing gap as he is changing lanes. Next group of cars does an accordion slowdown. Finally the bus waiting until the last second to pass blocks view of driver that hit cyclists. IMO failing to the bus driver failing to drive defensively contributed to it. Nice of the city to deliberately edit out the part that illustrates their employee's culpability.
This type of collision is my biggest worry, because the driver behavior that leads to it is so common. There's not much you can do to avoid it other than finding a better route, which I try to do but sometimes there are no alternatives to a road with no shoulder.