I have a real problem with this, and I've been on both sides of it. It really doesn't do anything to help your cause by having people stuck in traffic. There are many reasons that someone could be in their car at that moment, not just commuting to or from a job. They could be on their way to a court appointment, they could have dinner in the car, they could be going to pick their child up from school, they could be on their way to a doctor's appointment, or any of a million other reasons that make this not just an inconvenience, but a complete shithead thing for someone to put someone else through. Protestors aren't making their case against the war by pissing off every person stuck in traffic, they are just being dickheads. If you want to protest a war, go hang out outside of a government building, or in front of an elected official's house. Make them feel uncomfortable, not some poor schmuck who has somewhere else to be at the moment.
Edit: Ok, I guess I need to give a peaceful example. You want to block traffic? Block the exits to the parking garage where the elected officials park downtown. Do that for a week and see how much of an impact you make. Blocking commuters is a waste of energy.