Speeding on bridges

I recently received a fine for speeding (82 km/h) on a very long, wide bridge near where I live. The speed limit there is 60km/h but everyone does 80 - 90 across that bridge (no excuse).

Folk are now telling me that I should contest the fine because speed determination by tripod-mounted camera equipment (probably on the approach to the bridge) is not valid because a bridge is a “moving structure”. I must say that I have noticed that there is discernable vibration on a bridge when a heavy vehicle goes past.

I am not terribly fussed about this, regarding the occasional speeding fine (R400) as part of the cost of running a motor car, like toll roads and punctures.
But as a matter of interest could I ask, Duncan and Eugene (Engineering and Law), and any others; do you have an opinion on this? Would the vibration on a bridge be sufficient to invalidate the speed determination, or is this more in the nature of an urban legend?