While Richardson may not apply, Washington certainly does. The cops exceeded their authority and should be slapped down. Hard.Indiana vs Richardson is specific to seatbelt stops. Seatbelt stops are their own area of law because of the following IC code:
IC 9-19-10-3.1
...However, a vehicle, the contents of a vehicle, the driver of a vehicle, or a passenger in a vehicle may not be inspected, searched, or detained solely because of a violation of this chapter...
OP was stopped for speeding.