Parking brake failures in commercial trucks usually appear as rollaway events. A truck, tractor-trailer, straight truck, refuse truck, yard tractor, bus, or detached trailer is parked, left at a dock, uncoupled, stopped on a grade, or being serviced. Then it begins moving without a driver in position to control it. The result can be catastrophic because a driverless commercial vehicle may roll into traffic, crush a worker, strike a pedestrian, pin a mechanic, damage a loading dock, hit parked vehicles, or crash into a building before anyone can intervene.
Federal law treats parking brakes as a safety-critical system. Under 49 C.F.R. § 393.41, commercial vehicles must have a parking brake system capable of holding the vehicle or combination stationary under the loading conditions in which it is found on a public road, free of ice and snow. For air-braked trucks, buses, truck tractors, and most trailers manufactured after the applicable dates, the rule ties parking-brake requirements to FMVSS No. 121. The same regulation also requires certain trailers, including agricultural commodity trailers, heavy haulers, and pulpwood trailers, to carry enough chocking blocks to prevent movement when parked.[1]
That means a rollaway investigation should not stop at the question, "Did the driver pull the parking brake knob?" The deeper question is whether the parking brake system actually applied, held, remained applied, and was supplemented by wheel chocks or dock restraints when required or prudent. A parked vehicle can move because the driver failed to set the brake, because the parking brake did not fully apply, because spring-brake force was reduced, because an air valve leaked, because a trailer was uncoupled too quickly, because wheels were not chocked, or because a recall-related defect allowed the brakes to release after application.
How Spring Parking Brakes Work
Most heavy air-braked trucks use spring brakes for parking and emergency braking. The FMCSA-approved Commercial Driver's License Manual explains the basic principle. Powerful springs are held back by air pressure while the vehicle is being driven. When air pressure is removed, the springs apply the brakes. The yellow parking brake control in the cab lets the driver release air from the spring brake chambers so the springs can apply the brakes.[2] The same manual warns that spring-brake performance depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular service brakes nor the emergency or parking brakes will work correctly. That is important because a parking brake failure may involve more than the parking valve. Worn linings, excessive pushrod stroke, broken springs, damaged chambers, air leaks, and poor adjustment can all reduce the ability of the spring brakes to hold the vehicle.[3]
FMVSS No. 121 also reflects the design significance of parking brake controls. The standard requires the parking brake control on trucks and buses to be separate from the service brake control, operable by a person seated in the normal driving position, and capable of controlling the vehicle's parking brakes and the parking brakes of any air-braked vehicle the truck is designed to tow.[4]
Trailer behavior can be especially important. NHTSA has explained that trailer spring brakes are kept released by air pressure and that supply-line pressure, tank pressure, and valve operation determine when parking or emergency brakes apply. A slow pressure drop can create a different application sequence than a rapid loss of air. That matters in trailer-yard and decoupling incidents, where a trailer may begin to move before the spring brakes fully engage.[5]
Driver Procedures, Wheel Chocks, and Grades
Driver procedure remains central to rollaway prevention. The CDL manual warns drivers never to leave a vehicle unattended without applying the parking brakes or chocking the wheels because the vehicle might roll away and cause injury or damage. It also instructs drivers to use parking brakes whenever they park.[6] The manual also addresses older trailers that do not have spring brakes. It warns that trailers without spring brakes may hold only as long as air remains in the trailer air tank; eventually, the air can leak away, leaving no brakes. For that reason, the manual says it is very important to use wheel chocks when parking trailers without spring brakes.[7]
Loading docks create another risk environment. OSHA's powered industrial truck regulation requires the brakes of highway trucks to be set and wheel chocks placed under the rear wheels to prevent movement while trucks are boarded with powered industrial trucks. The same regulation states that brakes must be set and wheel blocks must be in place to prevent movement of trucks, trailers, or railroad cars during loading or unloading, and that fixed jacks may be necessary to support semitrailers when a trailer is not coupled to a tractor.[8]
OSHA has also addressed the overlap between its chocking rules and FMCSA parking-brake regulations. In a 2011 interpretation, OSHA explained that FMCSA regulations can preempt OSHA chocking requirements in some circumstances involving commercial motor vehicles, but OSHA may still enforce protections in areas not covered by FMCSA rules or where the employer does not own, operate, or lease the commercial motor vehicle.[9]
That legal overlap matters because a dock rollaway may involve multiple actors: driver, carrier, shipper, receiver, warehouse, forklift operator, yard jockey, maintenance provider, and trailer owner. Investigators should ask whether the parking brake was set, whether chocks or dock locks were used, whether the trailer restraint worked, whether the dock surface was sloped, whether a forklift was inside the trailer, and whether the driver was pressured to move quickly during loading, unloading, or drop-and-hook operations.
Mechanical Failures Inside Spring Brake Systems
Parking brake failures can also occur even when the driver appears to follow the correct procedure. The National Transportation Safety Board has documented how broken spring brakes can reduce or eliminate emergency-parking brake force. In Safety Recommendation H-02-20, NTSB described broken spring brakes that reduced braking ability or rendered brakes nonfunctional; in one brake, the broken spring blocked pushrod travel and prevented the automatic adjuster from readjusting, leaving both emergency-parking and service braking nonfunctional at that wheel end.[10]
NTSB also identified corrosion and contamination as important maintenance issues. The recommendation letter explained that contaminants such as salt and water can enter spring brake chambers through caging ports when dust covers are missing, weakening spring components and increasing the risk of spring breakage. NTSB recommended adding spring-brake caging-port dust covers as an inspection item.[11]
Air-system condition is equally important. FMCSA guidance on air brake systems—developed for motorcoach operators but describing air brake fundamentals common to air-braked commercial vehicles generally—cautions that parking brakes are for parking only, not for emergency stopping, and that drivers and technicians must understand air pressure, warning systems, and system checks.[12] Beyond that general guidance, the mechanical failure modes NTSB and NHTSA have documented—broken springs, corroded caging ports, worn valves, and internal air leaks—show that a parking brake investigation must look past the cab and into the condition of the entire air system.
A proper inspection should include the spring brake chambers, caging bolts, caging-port dust covers, parking brake valve, relay valves, inversion valves, air reservoirs, supply and service lines, glad hands, trailer protection valve, pressure gauges, audible leaks, and timing of parking-brake application. The question is not only whether air pressure eventually dropped. It is whether the system applied the parking brakes quickly enough, held them with enough force, and prevented release after the driver left the cab.
Recalls Show Why Recall Status Matters
NHTSA recall records show that parking-brake-related rollaways are not limited to driver error. In 2016, Bendix filed a recall involving SR-5 trailer spring brake valves. The recall stated that under certain conditions, primarily off-highway or in trailer yards, there could be a delay in applying the spring brakes after the operator pulled the dash valve. The report described the safety risk as a "slow to park" condition that may lead to trailer rollaway after decoupling from the tractor.[13]
Vehicle recalls tied to the same type of delayed spring-brake application show why decoupling procedure matters. A 2017 recall report stated that if internal leakage occurs and the trailer is uncoupled before the spring brakes fully apply, the trailer may roll away in the direction of the ground-grade slope.[14] Other recalls involve parking brakes releasing after they were applied. In a 2021 Mack TerraPro recall, NHTSA reported that a worn PP1 park brake control valve with an internal leak could allow air to pass through the exhaust port after the park brake was applied. Under certain conditions, the leak and exhaust-hose installation could create back pressure sufficient to allow the park brakes to release. Mack identified the safety risk as vehicle rollaway with the park brake control valve in the applied position.[15]
A 2023 Autocar recall described a similar PP1 valve and vent-line issue. The report stated that a worn park brake control valve could develop an internal leak after the park brake was applied, and that the leak combined with vent-line plumbing could create enough back pressure to allow the park brakes to release. The safety risk was vehicle rollaway with the parking brake control in the applied position.[16]
Modern electronically controlled parking-brake systems create their own evidence issues. NHTSA recall materials for the Bendix Intellipark Park Valve Module described a condition in which internal air leakage could prevent the tractor channel from transitioning from delivered, or unparked, to exhausted, or parked, after the driver pulled the dash electronic control switch.[17]
A 2024 ZF/Haldex inversion relay valve recall also addressed delayed parking-brake engagement. The NHTSA report stated that the valve could intermittently delay park-brake engagement, potentially preventing the parking brake from engaging within three seconds of application and increasing rollaway risk.[18] These recalls make recall status relevant evidence in any rollaway investigation. Investigators should identify the vehicle model, VIN, valve part numbers, brake-valve manufacturer, production date, recall completion records, dealer repair records, and whether the carrier received recall notices before the incident.
Rollaway Incidents Often Crush Workers
Parking-brake-related rollaways are especially dangerous because they often occur outside ordinary traffic. The people hurt are frequently workers near the vehicle, including drivers, dock employees, mechanics, spotters, forklift operators, or bystanders.
Washington's FACE program reported two container chassis mechanic fatalities involving rollaway trucks; neither truck had parking brakes set or wheels chocked. The hazard alert also emphasized the need to set brakes, chock wheels, and prevent trailer or truck movement during service work.[19] Another Washington FACE alert described a dump truck driver who was dragged and pinned by a rollaway truck. The recommendations included turning wheels toward the curb or side of the road, chocking at least one tractor drive wheel on each side when parked on any grade, and training drivers not to try to stop a rollaway truck from outside the cab.[20]
Kentucky FACE investigated a fatal rollaway in which a commercial truck driver exited the vehicle to remove debris from a parking spot, after which the tractor-trailer began to roll, struck him, and came to rest on top of him.[21] Oregon FACE investigated a fatal incident in which a truck driver was crushed between a semi-trailer and a loading dock. The report recommended fully engaging tractor and trailer parking brakes before leaving the cab and using wheel chocks to secure trailers and tractors against inadvertent movement, especially when parked on a slope.[22]
These cases show why rollaway incidents require scene reconstruction. Investigators should document grade, vehicle path, chock placement, tire marks, brake-control position, dock-lock status, trailer restraint use, surveillance video, vehicle weight, engine status, transmission position, and whether the driver was inside or outside the cab when movement began.
What Investigators Should Request After a Rollaway
A complete parking-brake investigation should combine vehicle inspection, scene evidence, driver procedure, and recall research. The vehicle inspection should test whether the parking brakes apply, how quickly they apply, whether they hold on grade, whether air leaks are audible, and whether any valve or chamber allows release after application.
The maintenance file should include spring brake chamber replacements, parking brake valve repairs, PP1 valve service, SR-5 or SR-7 valve work, inversion relay valve records, caging bolt use, caging-port dust cover inspections, air leak complaints, DVIRs, roadside inspections, recall completion records, and fleet policies for parking on grades, chocking, docking, and decoupling.
The driver and workplace evidence matters too. Investigators should request driver training records, yard rules, loading-dock procedures, dispatch communications, route or delivery timing, surveillance video, forklift activity logs, dock restraint inspection records, and any prior rollaway or near-rollaway reports. OSHA's directives recognize that mechanical means such as dock locks or restraints may be used as protection when they are properly installed, maintained, and used, which makes dock-equipment inspection part of the investigation.[23]
Parking brake failures are dangerous because they remove the human response from the crash sequence. Once the driver exits, a loaded commercial vehicle can move faster than a person can stop it. The prevention system is therefore layered: a working spring-brake system, proper brake adjustment, leak-free air valves, completed recalls, parking-brake testing, wheel chocks when required or prudent, and dock restraints where loading operations create movement risk. A rollaway usually means one or more of those layers failed before the truck ever started moving.
Sources
- [1] 49 C.F.R. § 393.41, Parking brake system.↩
- [2] AAMVA/FMCSA, Commercial Driver's License Manual (2005 CDL Testing System, July 2014 printing).↩
- [3] Id.↩
- [4] 49 C.F.R. § 571.121, S5.6.4, Parking brake control (FMVSS No. 121).↩
- [5] NHTSA, Interpretation Letter to Berg Manufacturing (Aug. 27, 1979).↩
- [6] Id. (AAMVA/FMCSA, Commercial Driver's License Manual).↩
- [7] Id.↩
- [8] 29 C.F.R. § 1910.178(k)(1), (m)(7), Powered industrial trucks.↩
- [9] OSHA, Standard Interpretation Letter to Ron Cole (Mar. 7, 2011).↩
- [10] NTSB, Safety Recommendation H-02-20 (Mountainburg, AR school bus collision investigation).↩
- [11] Id.↩
- [12] FMCSA, Air Brake Systems (motorcoach operator guidance).↩
- [13] NHTSA Recall No. 16E-045, Bendix SR-5 Trailer Spring Brake Valve.↩
- [14] NHTSA Recall No. 17V-055, Advance Engineered Products.↩
- [15] NHTSA Recall No. 21V-953, Mack TerraPro.↩
- [16] NHTSA Recall No. 23V-023, Autocar.↩
- [17] NHTSA Recall No. 23E015, Bendix Intellipark Park Valve Module.↩
- [18] NHTSA Recall No. 24E-011, ZF/Haldex Inversion Relay Valve.↩
- [19] Washington State Dept. of Labor & Industries, FACE Program, Mechanics/Trailers Hazard Alert (2024).↩
- [20] Washington State Dept. of Labor & Industries, FACE Program, Dump Truck Driver Rollaway (2024).↩
- [21] Kentucky FACE Program, Report No. 20KY040.↩
- [22] Oregon FACE Program, Report No. 2010-06-1.↩
- [23] OSHA Directive STD 01-11-007, Mechanical Means to Secure Trucks or Trailers to a Loading Dock.↩