Are "open-looped" antilock brake systems for air-braked vehicles permitted by the National Highway Traffic Safety Administration (NHTSA)? Why or why not?

“In previous notices, the agency [NHTSA] discussed in extensive detail the reasons for requiring a 'closed-loop' antilock system and for combining an equipment requirement with a dynamic test requirement for truck tractors. (60 Fed. Reg. 13224–13228) NHTSA’s definition permits any ABS, provided that it is a closed-loop system that ensures feedback between what is actually happening at the tire-road surface interface and what the device is doing to respond to changes in wheel slip. As many brake and vehicle manufacturers commented on the September 1993 NPRM, a device that satisfies these criteria is necessary to prevent wheel lockup under a wide variety of real world conditions, thereby significantly improving safety. In contrast, a definition that permitted open-loop systems would allow systems that would not necessarily prevent wheel lockup.”

60 Fed. Reg. 63966