There are now an estimated 250 zip lines in the U.S.—five times more than in 2008. Since there's no official safety certification, do your homework before taking the plunge, says Steve Gustafson, owner of EBL Canopy Tours, a zip-line installation and consulting firm.
Make sure the course was designed by a structural engineer (ask the owner) and check that the guide-to-customer ratio is one to seven or fewer. Skip courses that don't require helmets and any that ask you to handle the braking yourself. Have a safe zip!