A lot of interesting feedback.

1. I live in the Chicago area (and was in high school in Madison, WI, when bikes & buses were my only transportation options other than Mom.) It SNOWS here. And it ices up even more. Open bike paths on the street become impassable in that kind of weather. Biking basically shuts down in the Winter. You need something covered in the cold season. If you want to talk about falling and breaking various parts of your physical person, bike in that kind of mess routinely. (And I say that from personal experience with getting broken.)

2. You wouldn't be biking on the bare plastic. You could lay in a thin paved surface where the bikes & peds go.

3. Plastic breaks down over time mostly due to ultraviolet rays in sunlight. Its life could probably be extended if some kind of UV block was incorporated.

4. No way, that massive concrete and steel thing. Way too much material and bulk and expense.

5. Like was said by another poster above, 2,000 lbs of metal and an idiot on a cell phone will trump your right to be on the road regardless of liability laws. I'd way rather be completely separated from automotive traffic. People around here drive insane. They routinely do twenty miles over the limit, are under-slept, over-caffeinated, distracted, and have a massively over-developed sense of "me-first" entitlement.

6. A modular system that was over-engineered to have twice the required safety factor in materials strength would make extreme failure unlikely, while making the system fairly repairable. The key would be to think about the design in terms of durability and repairability from the very beginning.

7. There's gotta be some way to deal with rain, though with paved riding and walking surfaces, it would be less of an issue. Good drainage in the design would be a start.

8. EMT access is critical. You make sure that elevators and ramps are plentiful and can easily accommodate a pair of EMT's with a gurney.

Thanks for responding folks. But try to think about what it's like biking in the land of ice and snow and incredibly rude, self-centered drivers.