Truck-Mounted Attenuators & Shadow Vehicles

A truck-mounted attenuator (TMA) is a crash cushion on the back of a heavy truck. It absorbs the energy when a car runs into the truck, protecting the driver of the crash vehicle and the crew working ahead. A shadow vehicle with a TMA is one of the most important protections on highway work, and it only works if it is used correctly.

• Road Crews
• TMA & Shadow Truck Drivers
• Flaggers
• Traffic Control Supervisors

What a TMA does

• The TMA is a crushable, energy-absorbing cartridge hinged to the rear of a truck. It is raised to a working position behind the truck and folded up for travel.
• It is meant to be hit by an errant vehicle at speed and to reduce the severity of the crash. Units are crash tested (against NCHRP 350 or the newer MASH criteria) and labeled with test levels.
• It is also called a crash cushion or impact attenuator. See Road Construction and Traffic Control Crews.
• The shadow vehicle is the truck with the TMA that protects the work area or a mobile operation, usually with an arrow board and flashing lights.

How it is used

• Mobile and short-duration operations: the shadow truck follows a slow-moving crew (striping, sweeping, potholes) to protect it. See Mobile & Short-Duration Work Zones.
• Stationary work zones: the TMA truck is parked upstream of the work area as a barrier, with space between it and the workers.
• Lane closures: a TMA truck supports the taper while cones are set and removed.

Main hazards

• Driver injury if hit: the TMA truck driver is exposed to rear impact. Seat belts, head restraints, brakes set and the truck in gear matter.
• Workers in the "roll-ahead" zone: when a truck is hit, it can roll forward. Workers must be placed far enough ahead (per the plan) to stay clear.
• Improperly raised or damaged TMAs that don't work as designed.
• Pinch and crush hazards when raising, lowering or servicing the TMA.
• Reduced visibility and following too closely behind other vehicles.
• Misuse: parking a TMA in a position where traffic can hit it at an angle, or using it without arrow boards and signs.

Safe use

• Follow the traffic control plan and the maker's manual. Use TMAs approved for the road speed.
• Inspect the TMA daily: look for damaged cartridges, cracked frames, loose pins, hydraulic leaks, broken lights and missing reflectors. Take damaged units out of service. A hit TMA must be inspected and often replaced before reuse.
• Raise the TMA in a safe spot out of traffic and confirm it locks in the working position before use. Lower it only when parked and stopped, and stay clear of the folding area.
• Stationary use: set the parking brake, put the truck in park or neutral as the plan directs, wheels turned away from the work area (as the plan says), and the driver seat-belted with head against the restraint.
• Position the truck so a hit is straight on, and place a distance buffer between the TMA and workers as the plan specifies. Never put workers inside the roll-ahead area.
• Keep people out of the TMA zone: nobody should stand behind or beside the TMA, or between the TMA and the work area, while traffic is passing.
• Use arrow boards, flashing lights and signs and keep lights working. Wear high-visibility clothing. See High-Visibility Clothing.
• If hit, the driver should stay belted until the vehicle stops, then check for injuries and call 911.

TMA checklist

• The TMA is approved for the road speed and inspected daily
• It is raised and locked, and the truck is braked with the driver belted in
• Workers are placed beyond the roll-ahead distance in the plan
• Arrow board and warning lights are working
• A hit TMA is taken out of service for inspection

Last updated: October 8, 2026