Truck-mounted attenuators: Preferred wheel direction for optimum safety

By ATSSA Chief Instructor Juan Morales

At the most recent ATSSA Instructors’ Meeting in Providence, R.I., a question was asked about the proper orientation of the front wheels of a truck-mounted attenuator TMA, a key issue for roadway safety.

TMAs are trucks equipped with energy-absorbing attenuators, to provide physical protection for roadway workers from traffic approaching from the rear.

A common myth is that the wheels should be angled to prevent the TMA from being pushed into workers in case of an impact. This is not the preferred method and not what ATSSA teaches.

Angling the front wheels may push the TMA into an open lane, possibly increasing the number of vehicles involved in the crash.

Instead, the preferred method is to point the wheels straight ahead not turned left or right and allow for the TMA’s roll-ahead distance.

The roll-ahead distance is the distance a TMA would roll forward if struck. Determining this distance depends on several factors such as speed, the weight of the impacting vehicles and the weight of the TMA’s host vehicle.

It is important to consider the roll-ahead distance when selecting the safe separation distances between the TMA and roadway workers on foot.

Crash tests of TMAs are performed with the front wheels steered straight so the anticipated roll-ahead distance is based on the wheels being oriented in that direction.

Pointing the TMA host vehicle’s wheels straight forward should be the default position unless there is a “sound and reasonable” decision to deviate. An example of a sound reason would be if the attenuator host vehicle was positioned on a curved road where having the wheels pointed straight could create a dangerous condition if the attenuator were struck and pushed forward. In this case, angling the steering wheels could be a safer alternative.

The decision to deviate from having the front wheels oriented straight forward must be made on a case-by-case basis. The decision should be made a by a responsible agent such as a project engineer who understands the conditions at the site.

It is critical that TMAs are used in accordance with the manufacturer’s instructions and per the Manual on Uniform Traffic Control Devices MUTCD. You may also refer to ATSSA’s “Field Guide for the Placement of Shadow Vehicles in Work Zones,” which is available here under “Positive Protection.” This guide provides additional information, including sample roll-ahead distances.

Juan Morales is a professional engineer P.E. and a chief instructor for ATSSA training.