Myth: One truck does more damage than 9,600 automobiles

December 2018/January 2019

Todd Spencer

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In order to justify tolling heavy-duty trucks, proponents often cite an obscure statistic from a GAO study that a single “tractor-trailer has the same impact on an interstate highway as at least 9,600 automobiles.”

It is important to understand the origins of this myth, as the data behind the phrase originates from the highly regarded American Association of State Highway Officials (AASHO) Road Test, what is now the American Association of State Highway and Transportation Officials (AASHTO). The Road Test was “to study the performance of pavement structures of known thickness under moving loads of known magnitude and frequency.”

Construction for the Road Test began in Ottawa, Ill., in August 1956. The project consisted of 7 miles of two-lane pavement. The 836 test sections used a wide range of surface, base and subbase thicknesses, and included 16 short-span bridges.

Test traffic began on Oct. 15, 1958, with the Department of Defense providing heavy vehicles and drivers. The AASHO Road Test ended Nov. 30, 1960. The majority of test sections were deliberately underdesigned in order to achieve pavement failure, while the other sections, built according to the modern standards of the day, held up under thousands of axle loads.

The test data demonstrated a geometric relationship between axle loads and pavement effects, so that even small increases in axle loads could have a large effect on pavement wear. However, the data also determined that a small increase in pavement strength and thickness would accommodate a large increase in axle loading, which researchers rarely mention.

The intent of the study was to help engineers determine load-related pavement requirements with the test data establishing the relationships for pavement structural designs based on expected demand over the life of a pavement. The data from the Road Test helped to create a standard value for axle weights, called “equivalent single axle loads,” or ESAL. An ESAL represents an 18,000-pound single-axle load, which is assigned a value of 1.00.

To get down to the nitty gritty of how they arrived at the 9,600 number, you need to do the math.

First the truck. A standard five-axle tractor-trailer weighing 80,000 pounds has an ESAL value of 3.83. This is determined by examining the various axles, i.e., the steer axle (12,000 pounds) and the tandem axles (34,000 pounds), and adding their ESAL values together (0.19 for the steer axle plus 1.82 for one tandem plus 1.82 for the second tandem adds up to a total 3.83 ESAL).

In comparison, a two-axle passenger car weighing 4,000 pounds has an ESAL value of 0.0002 for both the front axle and rear axles, equating to a total ESAL value of 0.0004. Thus, a single tractor-trailer has the same impact as 9,600 passenger cars. They get that by dividing the truck’s total ESAL (3.83) by the car’s total ESAL (0.0004).

However, it is not that simple as there are several other factors to consider other than ESAL equivalency. First, we must consider the two types of pavement, flexible or rigid. Modern designs use flexible pavement, which lowers the ratio to approximately 800 to 1,000 cars per single truck. Moreover, if engineers design the roadway properly, considering both the depth and type of concrete, the pavement will be more durable as one additional inch of concrete slab would allow for twice the demand without lowering the life of the highway.

The Transportation Research Board stated, “When a highway is properly designed … it will not be damaged by the traffic it is designed to support.” Thus, the pavement damage is not because of heavy-trucks but because the state or federal builders did not design the highways correctly. The reason for the excessive damage is more likely because construction companies use less concrete and less flexible concrete slabs than the infrastructure needs.

Other factors to consider for pavement wear seldom mentioned are:

Underlying soil.

Roadbed design.

Pavement type.

Pavement thickness.

Weather effects.

Deferred maintenance.

While it may cost more upfront, if you build it right it’s less expensive in the long run. LL