That could easily have happened.
I'm not talking about making people late for dinner. I'm talking about making people lose their jobs because they couldn't get to work. I'm talking about people unable to reach a hospital before their loved one dies. I'm talking about stranding kids at school when mom and/or dad couldn't get there to pick them up, or people suffering legal expenses because they missed a court appearance.
And in many cases, such a this one, those being made uncomfortable are allies. Not too bright to antagonize an ally.
There are ways to make people uncomfortable that don't prevent fire and medical services from reaching an emergency. And that's actually happened. (The most prominent examples involve environmental activists in the UK. Again - I agree very much with what they want accomplished, but if someone needs an ambulance, the ambulance has to get through.)
These also don't involve putting marbles on stairs, which could have caused an innocent person not involved in the event to fall and break their neck. Had that been me, you can bet everything you own the scumbag who laid down the marbles would end up paying my wife a pension and for my daughter's college education.
And like I said, people stuck behind protests don't become more aware of a social problem. They grow annoyed at the people standing in their way. Those stuck in traffic are more likely to turn away from your message than listen. Blocking traffic is as useless as it is potentially dangerous.