One recurrent problem with on-pavement bike lanes is that they lose the right of way at access points to the road. If you were cycling along the road, cars would be required to wait for you while they were pulling on or off the road. But on the pavement, you are expected to stop. Which makes them less efficient for cycling, unless you ignore the give way signs.
Clearly in Oxford, the students have been doing this, so the fact that bikes were meant to give way had to be backed up by something more substantial. In this case, a large iron bar across the bike lane appears to do the trick
Now fast moving bicycles are forced to swerve into the oncoming bicycle lane before crossing the junction at exactly the same speed as they would have done before.
This shows precisely why bike lane design is a highly skilled task left to professionals. An amateur would have looked at this and described it as useless. A professional traffic planner would have collected data on throughput and velocity, run simulations on risk of injury before and after the bar was put in, and demonstrated that having a metal bar across an unlit bicycle lane was in fact safer for cyclists.