When you are putting your statement together, try and stay away from second and third hand stories. If your friend or family member has had a close run in or an accident on Nickerson, by all means mention it, but don't pull out your friend's roommate. Keep it personal and verifiable.
Numbers and facts are fine to bring up, but remember that most everyone is familiar with the points in the Stone Way study so don't linger on them. Also, don't include too many numbers as that can make an argument feel confusing. You only have two minutes or so anyway. Personal experiences, explaining why getting to the South Canal trail is impossible as is or not workable for commuters, talking about how a safe route on Nickerson for peds and bikes can give people a good option for not taking the Ballard Bridge till that gets fixed are all great things to discuss.
If your experience is with another recent road diet (Stone, Fauntleroy are good examples) by all means talk about that and how people were solidly against it and now don't complain anymore. If you've got experience with a road that has an older road diet (N. 45th) and what it was like before, that would be great to hear. Some like to complain about the car traffic there (which is pretty rough) but I have no idea what the vibrancy of that street was like beforehand. Is it better now?