This bridge is from the mainland over to Seil Island just south of Oban on the west coast of Scotland. Yes, that is the sea underneath the bridge. The island is that close to the land. And if you head west from the island next stop is America so technically the sea is actually the Atlantic Ocean. So this is known as the Bridge Over The Atlantic.
Now imagine the car PARKED on top of the bridge. The upward reaction force R would equal the weight mg. When you are driving the car at a steady speed over the bridge, it will be constantly changing direction due to the curvature. It will be accelerating because acceleration is rate of change of the velocity vector. Vector includes direction change. Such an acceleration requires a centripetal resultant force. The resultant is directed downwards towards the centre of curvature so resultant force = mg - R. The faster you go, the bigger the centripetal resultant needs to be. mg can't change so the only way to get (mg-R) bigger is by R getting smaller. In the extreme case, R = 0 which means no normal contact force on the ground. You have taken off!
