You say its a main road, how busy is it on a sunday or after 6pm or before 7am?
Also look at the width of the pavement, some can be 15' to 20' wide.
If you are going to use a cherry picker, you will still need to create a safe working area around it.
Check with the local highways department,about closing just one lane instead of the whole road.
You will need to practice at working at 50' plus before you attempt 70'.
You also have to clean like an escaulator, do not try to move the pole sideways at that height.
Also do not work to close to the building, step back and use gravity on the brush head to created srubbing friction on the glass.