self-propelled equipment shall, when under its own power and in motion, have an operator stationed at the vehicular controls. Wow. I didn't realize there was a ban and that it's from 1977. Btw, we had an almost autonomous Case tractor ("AutoSteer") at Trimble in Sunnyvale (department since moved to Colorado) in 2000 without an operator at the controls (so it was possibly illegal), but it's surprising that it's a blanket ban rather than a selective one like human-overseen. Perhaps it was written to protect jobs or there was subconscious fear over killer machines later epitomized by the film Maximum Overdrive (1986).
Its not a ban on autonomy, which didn't exist at the time to ban. It was directed at people trying to maximize use of their time by getting off slow moving, non-autonomous, equipment to do other things and then climbing back on when needed to make course changes.
It also functions to ban unattended autonomous vehicles now that they do exist, and it may need to be adjusted to allow for an appropriate regulatory framework for limited and safe use of unattended autonomous vehicles, but that was not a subject of concern when it was written.
