The city of Alexandria has them mostly on Treks -- they went by my house (bike route goes past my front door) and there were about a dozen of them. Some of the bikes are different -- all hard tail MTBs. I'm guessing some may have been donated by local businesses.

I sat next to a Fairfax county cop at the barber's the other day. He tols me that it's a very popular assignment and that it's very competitive to get in. There are some who are volunteers and not-quite-police (I think in Arlington) as well, plus the park volunteers who mostly hel with breakdowns.

Apparently, they have special techniques including how to come off the bike to tackle someone who is running, whioch is why they don't use clipless and why they need sturdier bikes.