Well, not the whole city, but they did help out one victim of a mugging attempt:
This cadre of PDX riders like to take the train up to the top of the hill near the zoo, then bomb down the twisty roads back into town on minibikes-hence the "Zoobombers" moniker. They've been the target of some police harassment in the past, mainly bogus tickets on the train and having their bikes confiscated once. It's a pity that local TV media aren't covering this positive story about a fun-loving group of Portlanders. Check out the link!