We tried to use Mr. Pressure Wash in 2016, but were shut down midway by a City of Bellevue "Right of Way" Inspector. MPW supplied cones and signs but did not have a "Street Use" permit. The COB seems to be OK with MPW use of water recycling methods, to meet EPA requirements.
In applying for the "Street Use" permit ($43), we encountered other issues:
1) A "Traffic Control Plan" (TCP) needs to be drawn up by an City approved draftsman and stamped, which can cost $200
2) This TCP needed two human flaggers which can cost $40/hour per person
3) This permit can take many weeks to get approval
If the sidewalk is a hazard due to moss, this can be done by free by opening a ticket with the City of Bellevue "Street Maintenance" division.
The retaining wall is the responsibility of the COB "Parks Maintenance" division. Cleaning off the moss for aesthetics is not a valid criteria for getting this work done.