Pink flowers found in Washington state

Found in Battle Point Park, Bainbridge Island, Washington.

 

I think I've been to a park there that had a lawn ringed with pink Robinia trees, maybe this is the same planting. One introduction derived from R. ambigua 'Decaisneana' x R. hispida 'Monument' and sold using the Purple Robe trademark has been prevalent in more recent plantings, this might be what you saw.