Just added a rear passenger handlebar for my Radrunner. It really helps keep the rear adult passenger steady so their weight doesn't shift around and make the ride less stable. Since RAD (sadly) doesn't offer one, I decided to use the "Sidekick Bar" from Tern, which fits a range of seatposts including the 27.2 mm post for the Radrunner. For those who are wondering, it's 17" wide overall so it doesn't stick out the sides too much. There are numerous adjustable and angled stoker bar setups if you need something different, but this is one of the cheaper and high quality all-in-one solutions. There are cheap $17 rear handlebars on E-Bay, but those will easily bend / snap so I wouldn't advise using those.
https://www.ternbicycles.com/us/accessories/471/sidekick-bars (https://www.ternbicycles.com/us/accessories/471/sidekick-bars)
That's an awesome idea for someone hauling passengers. Love It!
This is very helpful! My son rides on the back to school everyday and is asking for handles. Were they easy to install? (I'm not very handy).