I've just switched from predominantly using the dedicated biking loop around Prospect Park (5k around) to more time on the road to get a sense of how much that slows things down... I've been using a chrono to time the whole time and then gauging how much time was ridden from the computer on the bike... just wasn't sure when people were indicating times whether they were using one or the other primarily.
Having to deal with pedestrians/cars/traffic/traffic lights definitely slows things down, especially in New York where I can't go more than a short way on the road without stopping fully. I figure it's a more conservative way to think about how long an event is going to take...