Today "cars are driven just 4% of the time". If you assume for a moment that people's transportation usage time stays the same, and "the number of vehicles on the road will be reduced by 99%", then the average car must be used 400% of the time. That would be possible if every self-driving car is carrying 4 passengers 100% of the time, 24 hours a day 7 days a week, which seems very unlikely. What am I missing?
I also don't see the justification for saying "90% reduction of vehicles in operation would reduce our overall emissions by" 90% of their greenhouse gas emissions. There might be 90% fewer vehicles, and you certainly save on environmental impact of vehicle production, but they will be used more per vehicle.
In fact, per-person transportation usage time may increase. If catching a ride is easy, fast, and cheap and allows us to work, sleep or play while riding, then many people will ride more often, or might buy a house that means a longer commute.