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The 36′ Rule

Have you ever looked at an optical illusion on paper, and no matter how long you stared, it became more and more confusing? The key to solving these visual riddles is to back off, relax your eyes and look at it from a distance. Optical illusions are quite simple, they just mess with your brain. Amazingly, truck law can be like that too! The article this week discusses the most complicated paragraph in all the Illinois Vehicle Code to legislate something incredibly easy.

Definitions to Understand

Federal Bridge Formula (FBF)

The FBF is federal law adopted by all 50 states. It has nothing to do with bridges which cross over waterways or railroad tracks. It’s the “bridge” measurement between two or more consecutive axles. In the simplest terms, the more axles spread over a greater distance increases the amount of weight a vehicle can carry.

What consecutive does not mean is “adjacent”. As a matter of fact, the word “adjacent” is not used once in Chapter 15 of the Illinois Vehicle Code. Many times police officers and truckers alike try to interpret what consecutive means by inserting “adjacent”, as if somehow the FBF cannot be used if the axles are not right next to each other. Consecutive simply means axles which are in sequence, regardless how far apart they are from each other.

Tandem Axle

A tandem axle means two consecutive axles, however not all sets of two axles are tandems. To be a tandem, the two axles must be a minimum 40” on-center from each other, but not more than 96” on-center.

A small landscape trailer may have two physical axles next to each other, but if the on-center measurement is 34”, then it is not a tandem. It is a single axle. A typical semi-trailer hauling steel usually has two axles at the end, but they may be spaced at 9’ (108”). Guess what? Not a tandem.

What is the 36’ Rule?

This rule is found in 625 ILCS 5/15-111(a)(5):

“Two consecutive sets of tandem axles may carry a total weight of 34,000 pounds each if the overall distance between the first and last axles of the consecutive sets of tandem axles is 36 feet or more, notwithstanding the lower limit resulting from the application of the above formula.”

In truth, the 36’ Rule is an exception to law. Legal weight for a tandem is 34,000 pounds. This rule describes when there are two consecutive sets of tandems for a total of four axles. Note that it must be two tandems…not any set of four axles.

For instance, a police officer stops a vehicle with two consecutive sets of tandems. He measures from the center of the first axle (in the first tandem) to the center of the fourth axle (last axle in the second tandem) and the total distance is 36’ exactly. He then pulls out his handy-dandy FBF chart and the total bridge formula weight for the series of 4-axles is 66,000 pounds.

What? Who did that math? So if a driver loads each individual tandem exactly to its legal weight of 34,000 pounds (for a total of 68,000 pounds), that group of four axles receives 2,000 pounds less? Correct.

Except the 36’ Rule covers this anomaly. If the police officer takes the measurement of two consecutive tandems, and it is 36’, 37’ or 38’, this group of four axles always gets 68,000 pounds on the FBF. That’s it. It is truly that simple. So what’s all the fuss about?

Two Misconceptions about the 36’ Rule

#1If the measurement for two consecutive sets of tandems is less than 36’, the tandem only gets 32,000 pounds.

This is entirely false. Once upon a time, there were two sets of weight laws in Illinois, and one of the sets only allowed for 32,000 pounds on a tandem. However, those non-designated roads were not entitled to the FBF anyhow. On designated roads, tandems always received 34,000 pounds because the FBF applied. Today, all highways in Illinois are subject to the FBF.

A police officer cannot assume just because something does not add up quite right, that somehow he is justified take away weight granted by the legislature. This theory was wrong prior to the uniform weight law of 2010, and it is wrong afterwards.

#2The maximum gross weight for two consecutive tandems is limited to 68,000 pounds.

This is an inverted interpretation of the previous misconception. Just because a series of four axles (in a very limited circumstance) receives a higher weight (68,000 pounds), should that ever be construed to mean there is a 68,000 pound ceiling for all sets of two consecutive tandems.

Technically, the FBF is an infinite weight formula. The legislature chose to cap the four axle weight at 74,000 pounds with an on-center measurement of 48’. If they had desired to cap it at a lower weight, they would have.

Sometimes the law is most understandable when it is read without bias. Adding words to it, or inferring things which do not exist, leads to misunderstandings, poor enforcement, and lost time/revenue.

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