jimti:
When I read the title of this thread I thought it was a trick question as tunnels by their very nature are low, underground even, had you asked the height of the Dartford bridge I could have answered, but there are a number of different answers
Height of road deck 65 metres above the river
Navigational clearance 57.5 metres above the river
Number of steel pylons 4
Height of steel pylons 84 metres
Total height of pylons 137 metres above the river
all good relevant answers, how long would it take you to hit the water if you jumped?