We will be driving from Siena to Florence and need to drop friends at a taxi stand outside the ZTL zone so they can continue to the train station. We will be continuing on to Pisa. Any ideas where to drop them?
Having them take the train is an excellent solution - not only is it economical but also convenient (no changeovers)
However, if they want to take a taxi and you are driving up the Siena - Firenze Raccordo (the main road that connects these two cities) you will find a taxi stand after the round about heading towards Galuzzo. There is a large parking area on the right hand side, bus stop and taxi stand. BUT - you should probably have a number ready to call (055 4242 or 055 4390, because taxis are not always there waiting. When you call you can give them the location (Via Cassia, 20-22, 50023 Firenze FI)and they will give you a number in confirmation and pick you up.
You can proceed up along this road for almost as far as you want, the ZTL zone actually starts once you see a large stone gate and wall (pretty hard to miss )
This position near Galuzzo leaves you far enough out of the city center to worry about traffic and close enough to catch the main highways and start touring other great places in Tuscany.
Having them take the train into Florence would be the easiest solution even for them, they'd arrive directly at the SMN train station. If they don't want to go from Siena, you could drop them off along the way, in any of the towns you have to drive past anyways on your way to Pisa.
Lastra a Signa or Empoli would be good possibilities as well, as well as San Miniato if you both want to make a quick stop to visit the town
I'd avoid driving into Florence just to drop them off and then heading back out -- it will make you lose a lot of time in traffic in the city.