Interpretation ID: aiam1506
Engineer/Transit Technology
Municipality of Metropolitan Seattle
410 West Harrison Street
Seattle
WA 98119;
Dear Mr. Olson: This responds to your April 2, 1974, request for a ruling on whethe trolley and motor buses equipped with air brake systems and dynamic electric or hydraulic devices are required to be equipped with anti-lock equipment.; Standard No. 121, *Air brake systems*, requires stopping distanc performance which must be met by any bus equipped with air brakes, whether or not it is equipped with supplementary dynamic braking means, and the stops must be made with only controlled wheel lockup over 10 mph. Although the standard does not require antilock devices, many manufacturers have indicated they will use antilock devices to meet this requirement.; In evaluating a vehicle's compliance with the stopping distanc performance requirements of S5.3 and S5.7.2.3, auxiliary braking devices may be utilized in making the stops provided such devices are engaged by means of the same service brake pedal or parking brake control that operates the air brakes. It should be noted, however, that these stops must be made with the transmission selector control in neutral or the clutch disengaged (S6.1.3).; Please write again if this or other difficulties arise in th certification of your buses.; Sincerely, Richard B. Dyson, Acting Chief Counsel