True, it's not going to happen overnight and it won't be linear, but the first horseless carriages had to be preceded by a bloke walking in front with a red flag to alert other road users of the danger that was approaching, laughable these days and would've impeded the adoption rate.
IMO the first adopters will be the professional transport industries (buses, taxis). Couriers will still need drivers - someone has to deliver the correct parcel, but trucking to one destination eg pallets from distribution centres to supermarkets, ore from mines to ports - that can all be driverless. The real initial adoption will come from industry - trying to save a buck, that will lead to scale and reduced costs and probably the eventual adoption of transport as a service (as was mentioned previously).
(20min delay)