Is it safe to run a USB cable inside a wall?
If you are going in-wall over 16 feet, you need an active extender as many have pointed out. One alternative to this is to use a Cat5 extender: this allows you to run Cat5e/6 in-wall (riser or plenum cable is rated for in-wall use), then convert to USB at both ends as needed. One I found at Monoprice is rated for 150ft.