Correction! In my letter last week, heights were stated in feet. They should have been in metres.
This makes the tunnel a much more difficult problem.
An overall drop of over 120 feet in 1.5 miles is some gradient.
With regard to the bridge the general rule of thumb for the area needed for the construction site facilities, offices, construction plant etc is double the area of the finished construction. There isn’t that amount of room by the Kingston roundabout.