Yes - it's part of the traffic calming program we wrote about recently.

Direct link:
http://www.seattle.gov/Transportation/speedwatch.htm

Does Seattle have radar guns to lend to folks in neighborhoods who want to monitor speeders? We had them where I lived back east and they really did wonders. We didn't try to make citizens arrests, but just knowing that your neighbors knew (and were watching) acted as a deterrent.

I love those reminders... I wish there were more of them across West Seattle.

A coutesy reminder in my opinion is better than a ticket.

I seem to remember reading somewhere that neighborhoods could request these to be temporarily placed where they feel they're having a speeding problem--but I'm too lazy/hot to take the trouble to look it up, just now.... So maybe one of your neighbors requested the trailer?

In my neighborhood there was an accident when a woman t-boned another car coming from the right and going through an unmarked intersection.
Her first words were "who was at fault." And then she made her second mistake of saying that she presumed the speed limit (on a non-arterial street) to be "40 or 45 mph, right?"

We had one on our block (40th and Genesee) a few weeks ago. I think they just randomly put them out in area's to remind people to go the speed limit. It was gone within a few days.

Nope - just a regular non-arterial. The main route is down 35th to 106th so it does seem a little weird. I thought maybe the guy installing the trailer got his paperwork wrong and was -supposed- to put it on 35th instead but the speed limit sign on the trailer is 25MPH, where I think the posted speed on 35th is 30MPH.

Wow, that is unusual. 34th isn't an arterial in that span is it? The speed limit already is 25 non-arterial streets.

We just had a one of those speed watch trailers ( http://www.seattle.gov/transportation/speedwatch.htm ) installed on our street (34th, between 104th and 106th). I asked the guy if someone had requested it or if it was a study being done by SDOT. He said he didn't know.
Anyway I have a call in to SDOT to find out but I was curious if other neighborhoods might have seen them, too.
I haven't noticed a big increase of speeders on our street - tho' we do get the occasional idiot - like someone in a pickup who thought it would be incredibly cool to peel out with bald tires after it sprinkled this week and made the road slick.