Anyone in a big city will tell you that traffic patterns run your life. You always try to avoid driving anywhere during rush hour. Any unusual weather -- rain, snow, wind, excessive heat -- adds time to your trip. Since I've lived in the Chicago area my entire life, my whole view of traffic is based upon what happens here. These are the rules I live by when driving.
This is how we ended up at the wedding last Friday 45 minutes before the ceremony. The GPS told us it would take 35 minutes to get from where we were staying to the ceremony/reception. Since we were driving during rush hour on a Friday before a holiday weekend, I added 10 minutes to the trip. It was also raining, so I thought it would add another 5 - 10 minutes to the trip.
Funny thing about traffic...it is entirely local. The Chicago rules don't apply in Florida. It turns out that in the Tampa Bay area where we were travelling, these things didn't really matter. The rain didn't slow down traffic much. I didn't see anything resembling a bumper-to-bumper rush hour.
All told, the trip took 5 minutes more than the GPS predicted. We had plenty of time to visit with the family, take the girls to the bathroom before the ceremony, watch the set-up, etc. It was also one more reason to remember that while on vacation, it is important to leave home behind and remember where you are.