It was done in 2006 at Penobscot cable-stayed bridge at Maine, USA, where an observatory was incorporated into the top of a 136m high tower.
This observatory is on three levels, within the tower curtilage, and serviced by a lift taking less than a minute to reach the top.
The proposed Forth Bridge is understood to be also of the cable-stayed type with three pylons or towers some 200m to 250m high and would offer a vantage point with views of up to 160km.
At the proposed Forth Bridge, access to an observatory lift tower would require a coach bay clear of the main carriageway. A small area observation deck, if similar to Penobscot, would limit the number of visitors to a coach party perhaps every half hour from an off-bridge ticket office.
If in principle you think this a good idea, please contact us at www.forthbridges.org.uk
PROFESSOR ROLAND PAXTON, chairman, Forth Bridges Visitor Centre Trust, School of the Built Environment, Heriot-Watt University, firstname.lastname@example.org