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A Bridge Over Troubled Pavement

Last week’s article got back to the ground floor of truck law. The basic definitions of an axle and tandem axle were discussed in depth. If there was perfect, universal understanding of this topic, these articles would not even need to be written. Unfortunately, when poor teaching is coupled with a complicated law, expensive mistakes happen. This week’s article will focus on the correct application of the Federal Bridge Formula and a common misinterpretation of it.

Guess what? In 2010 Illinois finally fell in line with the rest of the nation and became the 50th state to adopt the Federal Bridge Formula (FBF) uniformly on all roads. In the mid-1980’s, Illinois was forced by the federal government to do so on federally funded highways, like interstates and toll roads. Over time, Illinois began to designate other state highways to be compliant with the FBF, and local governments were allowed to do the same with their roads.

For clarity, when the FBF is being discussed in this article, it is not in reference to actual bridges which span water, rail or highways. In truck enforcement world, those are more appropriately referred to as “elevated structures”.

The FBF is a relatively simple concept to understand. Think of it like this: if you step in the mud with your boot, you will sink. If you lay a piece of plywood over the mud, it spreads your weight out and you do not sink (or at least not as quickly!). Axle are like the plywood…the more axles over a greater distance spreads out the weight and decreases pavement fatigue.

The formula to calculate the FBF is tricky, therefore there are multiple charts and calculators available with the math already done. The FBF is calculated by taking a measurement between the centers of axles to determine legal weight.

Many times when measurements are taken between the extreme axles, it’s referred to as “outer bridging”. When it’s between the internal axles, it has been called “inner bridging”. These are not legal terms and do not exist in the Illinois Vehicle Code or the federal regulations.

Here’s an example of how the FBF works: a 3-axle dump truck hauling stone is stopped. The truck has a steer axle and a tandem drive axles. As was learned last week, the single steer axle is allowed 20,000 pounds. The tandem drive axle is allowed 34,000 pounds and no single axle within the tandem may exceed 20,000 pounds.

The FBF says enforcement may take a measurement from the center of the steer axle to the center of the last axle to obtain the gross weight. Let’s say the distance is 21’7”. The officer would need to round up to 22’ with a maximum gross weight of 52,500 pounds on those three axles.

Simple enough, right? Yes, however poor instruction leads to poor enforcement. To understand how this happens, let’s look at the definition of the FBF in 625 ILCS 5/15-111(a): “or a total weight on a group of 2 or more consecutive axles in excess of that weight produced by the application of the following formula:”

The key words here are “2 or more consecutive axles”. This series of one number and four words has caused decades of confusion in truck enforcement due to people reading way too far into it.

In many courses offered to local law enforcement, the instructor will teach this 6-axle combination has only five possible FBF measurements to take. By referring to the steer axle as “axle 1”, this type of instruction will say only five measurements can be taken within “groups” of axles. This is incomplete and not in the law.


The law actually says “2 or more consecutive axles”. This complete method means twelve measurements can lawfully be taken between axles. Further, none of the six axles can individually exceed 20,000 pounds, and none of the three tandems (2-3), (3-4), (5-6) can exceed 34,000 pounds.


Normally, all these measurements would not produce overweights on a properly loaded truck. However, when one axle in the group is an adjustable tag or pusher axle, a vehicle loaded to the proper overall gross weight may easily exceed a bridge measurement. This is because many times an adjustable axle does not carry an equalized weight when compared to fixed axles.

A glaring benefit to the truckers exists with the incorrect application of the FBF. If there are less ways to calculate an overweight, there are less ways to receive citation.

That logic is tragically flawed because it provides a false sense of security. If the truckers do not load legal based on faulty advice from a misinformed police officer, what happens when they are stopped and weighed by a police officer who knows what he is doing?

That’s right. Costly overweights. It’s safer to have the whole story instead of parts of it.

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