In fact, although hOMe uses a 20» wide step (or maybe it's even 24 ″), if clever guy wires were used, you'd get the same clear tread width out of an 18» step as you would a 20» step with traditional railings..
There are some very clever use of guywires to serve as handrails / fall protection that would not require an additional 3» beyond your proposed 17» clear width.