I'm fairly sure that if you really were doing 50mph and didn't notice until you were on the markers that you wouldn't be able to stop that quickly, unless the road had a high enough speed limit such that the markers were spread out... in which case 50mph wouldn't trip the camera anyway.

In other words, I think you're ok. I think you spotted the camera earlier than you thought and slowed down in time.

[lecture]
But Andy makes a good point. How do you know the road was empty? Slow down!
[/lecture]