I've been driving a lot more than usual lately and find myself stuck in traffic on the 101 Freeway in San Francisco, often at a dead stop. Amazingly, when you're stuck at a standstill on that eleveated roadway, you actually keep moving quite actively because the entire freeway shakes and bounces like crazy. It feels as if someone's standing outside the car and having it back and forth, especially when a large truck goes by...

My guess is this is a deliberate design feature to absorb vibration, but it's rather frightening. I can only imagine what happens in an earthquake.