Most forms of clear plastic will hold up to 20 years or less of use. Recycled plastics tend to be less durable than new plastics. Most plastic construction is not particularly repairable, and it is often very prone to sudden stress fractures. So you're proposing an expensive separated facility that can't be repaired and will last perhaps 20 years... that sounds like a terrible infrastructure investment.

Further, one of the hallmarks of clear plastic surfaces is that they get slippery when wet. Often, very slippery. About the only thing I can think of that would be a worse hazard to cyclists would be making elevated bikeways out of metal. Putting a roof on but leaving the sides open means that rain will cheerfully get in. This plan sounds like a recipe for sprained knees, spiral femur fractures, pelvic fractures, and the odd tibia or fibula fracture. All of those except the sprained knee can be lethal, and the first aid procedures are not easy or manageable by a single person. Let's not talk about transport, because in general, diversion falls need to be moved on a backboard...

Oh, and better... the plan means that EMS can't reliably get to the patient!

This would have made a great April Fool's post.