Avoid at all costs anything containing ferrous (iron) sulphate - it's an effective moss-killer, but leaves a brown stain (iron) on tarmac, concrete, etc.
crosspatch - google Moss Terminator it advises against anything containing chlorides and other like substances because of bleaching, but I can't disagree with divebuddy because all I use is a power hose.
Choose a time when you can avoid using the drive for 2/3 days. Sprinkle soda crystals over the area and water with a watering can. (Or if rain is due, let it rain on the crystals) Do not jet wash as you withh simply spray the crystals everywhere else!
After a couple of days, brush the dead moss away. This method is easy and effective leaving a clean non slip surface. It also prevents the moss from returning for a few years.
Don't use a power washer on tarmac - in will only break it up in the long run - just ask the guys who repair it ( the honest ones ).
I use sodium hypochlorite.