How many antilock brake systems have been approved by the National Highway Traffic Safety Administration (NHTSA)?

"...no approvals have been granted to manufacturers of antilock brake systems by the agency. NHTSA issues standards and regulations, with which manufacturers of motor vehicle equipment must comply. However, the agency does not 'approve' products; rather, manufacturers must self-certify that their products meet Federal requirements."

Letter from NHTSA to Air Brake Systems, Inc., 3/10/2003