There are a set of private fleets in Seattle that are large enough and distinct enough to benefit from having their own electrification policies. These are:
Transportation Network Companies: e.g., Uber and Lyft. These are companies where individual subcontractors offer their private vehicles on a part-time basis. See here for more information and policies on them. The City has rules about the vehicles that may be used for this. The companies have rules also.
Taxis. The City has rules about vehicles used as taxis.
Package delivery. These are the small vans that drop packages to commercial and residential locations (e.g. UPS, Amazon).
Corporate fleets. This includes companies like Puget Sound Energy, Comcast, CenturyLink, etc. that have small trucks and vans making house calls.
The State is compiling a plan for electrifying publicly owned fleets, and this plan may end up serving as a useful model for these private entities. The State could require these private entities to draw up their own plans for electrification.
Electrification of these fleets is getting more cost-effective all the time. The cost of lithium batteries has dropped from $1183/kWh in 2010 to $135/kWh in 2030. Just in the last year, 2019-2020, it has dropped 13%.