The joys of jams

I feel like I have spent most of today sitting in traffic jams and running late for all of my visits. Why is it that the traffic jam always seems to start just at the point where you are left with no choice but to sit and wait it out rather than just before the junction where you could have turned off and found an alternative route? And the traffic update that would have alerted you to the problem only comes on the radio once you already quite aware of the traffic jam as you’re now in it. Very frustrating.