This happened to me last night(2:30am) on my commute home, The ambulance had their lights on, but no sirens. All I heard was a car coming up to pass. I did look over my shoulder briefly but I didn't get a good enough look to realize it was an emergency vehicle. I didn't slow or stop, but had I known it was an ambulance I would have stopped. They were in the far left lane on a 4 lane divided road. I was in the bike lane.