bridge vibrations

As I was spending one of my relaxing times on my balcony today I noticed an interesting phenomenon. The river was very still - if this river flowed any more slowly it would be a pond - and the reflection of the bridge was sharp and clear in the water. Then the light changed, cars started to move across the bridge, and under the bridge the water was vibrating, rippling in a curious cross current kind of way in time to the traffic overhead.  Who knew bridges vibrated so much? I don't know if I like the idea of the bridge vibrating beneath me when I drive over it tomorrow. Maybe I'll just go really fast.  "I'm sorry officer, I just wanted to get across before the bridge collapses."