The department quietly has outfitted eight of its duty motorcycles, more than half, with the upright mounts for semiautomatic rifles.
Chief Sylvia Moir, the former chief of El Cerrito, California, brought the idea with her when she was hired by Tempe in September 2016, and it was embraced by officers, said Tempe police Commander Michael Pooley.
When serious trouble strikes and traffic blocks the way, sometimes a police motorcycle is the only vehicle that can get through, Pooley said.
Motorcycles can cut through traffic, drive on sidewalks, or fit through tight places where four-wheeled vehicles can't go. That means police motorcycles can arrive at the source of trouble before any other first responders.
Sometimes, pistols aren't enough. Criminals are using more high-powered rifles and long guns, Pooley said, and police don't want to be outgunned.
Pointed at the ground, the black AR-15s add an eye-catching and militaristic look to the bikes. American police don't typically walk their beats with long guns like their European counterparts. And with heavy news coverage recently of mass shootings that involved long guns, some local residents may feel nervous at the sight of the well-armed motorcycles.
Pooley said the agency was braced for a backlash when it deployed the guns: "We're always concerned about public perceptions. We don't want people to think we're carrying these around for no reason."
So far, he said, the agency has "gotten a lot of positive feedback" from people in the community.
The mounts have a tricky release system that doesn't allow easy access by the public while the motorcycles are out and about, according to Pooley.
That makes sense, since criminals or the simply curious might be overly interested in the the highly visible weapons. In 2013, an elementary school student in Chino Valley, California, pulled the trigger on a motorcycle-mounted AR-15 during a demonstration, causing an apparent ricochet that injured two students.
No motorcycle officers have yet fired their AR-15s, but they've had to draw the weapons several times in the past few months, Pooley said.
Chief's Moir's accessory idea comes as concerns still rage nationally over heavy-handed police shootings and "police militarization." Last year, Maricopa County Attorney Bill Montgomery prosecuted a Mesa police officer who killed an unarmed suspect with a his department-issued AR-15; a jury acquitted the officer of all charges in December.
Pooley said he's not aware of any other Arizona police agency that has outfitted its motorcycles with AR-15s. (But if you've seen this in other cities, feel free to drop a line in the comments section. One reader said this morning that he's seen Apache Junction police motorcycles outfitted with AR-15s.)