A lot of people complain that driving across the United States is boring. Having done the coast-to-coast trip seven times now, I can’t agree. Sure, the stretches through the Corn Belt get a little monotonous, but for someone like me who wants to know how everything works, even endless agriculture is fascinating; I love me some center-pivot irrigation.
One thing that has always attracted my attention while on these long road trips is the weigh stations that pop up along the way, particularly when you transition from one state to another. Maybe it’s just getting a chance to look at something other than wheat, but weigh stations are interesting in their own right because of everything that’s going on in these massive roadside plazas. Gone are the days of a simple pull-off with a mechanical scale that was closed far more often than it was open. Today’s weigh stations are critical infrastructure installations that are bristling with sensors to provide a multi-modal insight into the state of the trucks — and drivers — plying our increasingly crowded highways.
All About the Axles
Before diving into the nuts and bolts of weigh stations, it might be helpful to discuss the rationale behind infrastructure whose main function, at least to the casual observer, seems to be making the truck driver’s job even more challenging, not to mention less profitable. We’ve all probably sped by long lines of semi trucks queued up for the scales alongside a highway, pitying the poor drivers and wondering if the whole endeavor is worth the diesel being wasted.
The answer to that question boils down to one word: axles. In the United States, the maximum legal gross vehicle weight (GVW) for a fully loaded semi truck is typically 40 tons, although permits are issued for overweight vehicles. The typical “18-wheeler” will distribute that load over five axles, which means each axle transmits 16,000 pounds of force into the pavement, assuming an even distribution of weight across the length of the vehicle. Studies conducted in the early 1960s revealed that heavier trucks caused more damage to roadways than lighter passenger vehicles, and that the increase in damage is proportional to the fourth power of axle weight. So, keeping a close eye on truck weights is critical to protecting the highways.
Just how much damage trucks can cause to pavement is pretty alarming. Each axle of a truck creates a compression wave as it rolls along the pavement, as much as a few millimeters deep, depending on road construction and loads. The relentless cycle of compression and expansion results in pavement fatigue and cracks, which let water into the interior of the roadway. In cold weather, freeze-thaw cycles exert tremendous forces on the pavement that can tear it apart in short order. The greater the load on the truck, the more stress it puts on the roadway and the faster it wears out.
The other, perhaps more obvious reason to monitor axles passing over a highway is that they’re critical to truck safety. A truck’s axles have to support huge loads in a dynamic environment, and every component mounted to each axle, including springs, brakes, and wheels, is subject to huge forces that can lead to wear and catastrophic failure. Complete failure of an axle isn’t uncommon, and a driver can be completely unaware that a wheel has detached from a trailer and become an unguided missile bouncing down the highway. Regular inspections of the running gear on trucks and trailers are critical to avoiding these potentially catastrophic occurrences.
Ways to Weigh
The first thing you’ll likely notice when driving past one of the approximately 700 official weigh stations lining the US Interstate highway system is how much space they take up. In contrast to the relatively modest weigh stations of the past, modern weigh stations take up a lot of real estate. Most weigh stations are optimized to get the greatest number of trucks processed as quickly as possible, which means constructing multiple lanes of approach to the scale house, along with lanes that can be used by exempt vehicles to bypass inspection, and turnout lanes and parking areas for closer inspection of select vehicles.
In addition to the physical footprint of the weigh station proper, supporting infrastructure can often be seen miles in advance. Fixed signs are usually the first indication that you’re getting near a weigh station, along with electronic signboards that can be changed remotely to indicate if the weigh station is open or closed. Signs give drivers time to figure out if they need to stop at the weigh station, and to begin the process of getting into the proper lane to negotiate the exit. Most weigh stations also have a net of sensors and cameras mounted to poles and overhead structures well before the weigh station exit. These are monitored by officers in the station to spot any trucks that are trying to avoid inspections.
Most weigh stations in the US are located off the right side of the highway, as left-hand exit ramps are generally more dangerous than right exits. Still, a single weigh station located in the median of the highway can serve traffic from both directions, so the extra risk of accidents from exiting the highway to the left is often outweighed by the savings of not having to build two separate facilities. Either way, the main feature of a weigh station is the scale house, a building with large windows that offer a commanding view of the entire plaza as well as an up-close look at the trucks passing over the scales embedded in the pavement directly adjacent to the structure.
Scales at a weigh station are generally of two types: static scales, and weigh-in-motion (WIM) systems. A static scale is a large platform, called a weighbridge, set into a pit in the inspection lane, with the surface flush with the roadway. The platform floats within the pit, supported by a set of cantilevers that transmit the force exerted by the truck to electronic load cells. The signal from the load cells is cleaned up by signal conditioners before going to analog-to-digital converters and being summed and dampened by a scale controller in the scale house.
The weighbridge on a static scale is usually long enough to accommodate an entire semi tractor and trailer, which accurately weighs the entire vehicle in one measurement. The disadvantage is that the entire truck has to come to a complete stop on the weighbridge to take a measurement. Add in the time it takes for the induced motion of the weighbridge to settle, along with the time needed for the driver to make a slow approach to the scale, and each measurement can add up to significant delays for truckers.
To avoid these issues, weigh-in-motion systems are often used. WIM systems use much the same equipment as the weighbridge on a static scale, although they tend to use piezoelectric sensors rather than traditional strain-gauge load cells, and usually have a platform that’s only big enough to have one axle bear on it at a time. A truck using a WIM scale remains in motion while the force exerted by each axle is measured, allowing the controller to come up with a final GVW as well as weights for each axle. While some WIM systems can measure the weight of a vehicle at highway speed, most weigh stations require trucks to keep their speed pretty slow, under five miles per hour. This is obviously for everyone’s safety, and even though the somewhat stately procession of trucks through a WIM can still plug traffic up, keeping trucks from having to come to a complete stop and set their brakes greatly increases weigh station throughput.
Another advantage of WIM systems is that the spacing between axles can be measured. The speed of the truck through the scale can be measured, usually using a pair of inductive loops embedded in the roadway around the WIM sensors. Knowing the vehicle’s speed through the scale allows the scale controller to calculate the distance between axles. Some states strictly regulate the distance between a trailer’s kingpin, which is where it attaches to the tractor, and the trailer’s first axle. Trailers that are not in compliance can be flagged and directed to a parking area to await a service truck to come by to adjust the spacing of the trailer bogie.
Keep It Moving, Buddy
Despite the increased throughput of WIM scales, there are often too many trucks trying to use a weigh station at peak times. To reduce congestion further, some states participate in automatic bypass systems. These systems, generically known as PrePass for the specific brand with the greatest market penetration, use in-cab transponders that are interrogated by transmitters mounted over the roadway well in advance of the weigh station. The transponder code is sent to PrePass for authentication, and if the truck ID comes back to a company that has gone through the PrePass certification process, a signal is sent to the transponder telling the driver to bypass the weigh station. The transponder lights a green LED in this case, which stays lit for about 15 minutes, just in case the driver gets stopped by an overzealous trooper who mistakes the truck for a scofflaw.
PrePass transponders are just one aspect of an entire suite of automatic vehicle identification (AVI) systems used in the typical modern weigh station. Most weigh stations are positively bristling with cameras, some of which are dedicated to automatic license plate recognition. These are integrated into the scale controller system and serve to associate WIM data with a specific truck, so violations can be flagged. They also help with the enforcement of traffic laws, as well as locating human traffickers, an increasingly common problem. Weigh stations also often have laser scanners mounted on bridges over the approach lanes to detect unpermitted oversized loads. Image analysis systems are also used to verify the presence and proper operation of required equipment, such a mirrors, lights, and mudflaps. Some weigh stations also have systems that can interrogate the electronic logging device inside the cab to verify that the driver isn’t in violation of hours of service laws, which dictate how long a driver can be on the road before taking breaks.
Sensors Galore
Another set of sensors often found in the outer reaches of the weigh station plaza is related to the mechanical status of the truck. Infrared cameras are often used to scan for excessive heat being emitted by an axle, often a sign of worn or damaged brakes. The status of a truck’s tires can also be monitored thanks to Tire Anomaly and Classification Systems (TACS), which use in-road sensors that can analyze the contact patch of each tire while the vehicle is in motion. TACS can detect flat tires, over- and under-inflated tires, tires that are completely missing from an axle, or even mismatched tires. Any of these anomalies can cause a tire to quickly wear out and potentially self-destruct at highway speeds, resulting in catastrophic damage to surrounding traffic.
Trucks with problems are diverted by overhead signboards and direction arrows to inspection lanes. There, trained truck inspectors will closely examine the flagged problem and verify the violation. If the problem is relatively minor, like a tire inflation problem, the driver might be able to fix the issue and get back on the road quickly. Trucks that can’t be made safe immediately might have to wait for mobile service units to come fix the problem, or possibly even be taken off the road completely. Only after the vehicle is rendered road-worthy again can you keep on trucking.
Featured image: “WeighStationSign” by [Wasted Time R]
Is that a common distance between signs in the US?
Depends on the State and how recent the sign.
Appears to be shot at a high zoom, making everything look closer together.
The wooden posts on the front one are characteristic of a temporary sign, but I have no idea why it’s there. Possibly because it’s holding the electric “open” sign?
The metal posts are cool – they have breakaway joints at the bottom so they don’t bisect your car if you run off the road and hit one. Or, they don’t bisect it as much…
Typically for most non weight permitted trucks,the weight distribution is as follows.
Front axle= 12,000 lb.max
Drive axles= 34,000 lb.max
Rear tandem axles= 34,000 lb.max
You had mentioned in your article the space between kingpin and the front trailer axle. This is
what gets a lot of inexperienced drivers in “Hot water”.This system is called the “Bridge Law”,and this law can vary from state to state.
Most,not all trucks/trailers,can slide the 5th wheel (where the kingpin is located)as well as the trailer tandems to ensure the safest weight distribution possible
probibly because someone neglected to add “remove the temporary sign” to the work order.
im more disturbed by how they just didnt install permanent footings for the temp sign. your tax dollars at work. road signs are not cheep.
When a politician’s niece/nephew owns the sign company it is.
Same as the rest of the world.
Interesting. Thanks.
The pre-pass also depends on the company’s safety standing with the FMCSA. If a company has got caught with violations to many times, pre-pass means nothing
need one for flock cameras
If the truck is crossing several state with a load do you *really * need to weigh it at every state border?
I poured concrete pads for diesel service pumps for Dad. It was routine for logging trucks to get stopped running 125,000 lbs. Dad was a NY CE PE. He designed the slab to take a >125,000 lb rolling load.
5/8” bar on 8” x 12” spacing in 8” of concrete. It did not crack.
All ‘crete cracks.
They cut grooves into slabs, hoping to control where the cracks form.
Your dad knows this.
As you note, there are obvious reasons for haul companies to cheat.
Truck brakes suck hard enough at rated load.
They need to check several times in each state.
Open and close at random, the weigh stations are being monitored at all times.
If your really really want to f w a trucker…
Put a gallon of red ag diesel into one of his tanks.
Don’t worry, if he catches you, you’ll have good laugh together…Swap prank stories.
Best not to f w truckers.
‘Getting away with it’ is how they feed their kids and/or pay for their meth.
This is an actual real piece of journalism. Not like the usual empty click-bait you serve up. Thanks!😎
Not only informative but also interesting to read. Well done!
Article needs a correction. Weight is not spread equally across all 5 axles on a semi. Steer axle is allowed 12,000 pounds and each axle after that is allowed 17,000 pounds.
Second correction; most scales use a floating pneumatic load system, not cantilevered electronic load cells. I’m sure there are some that use electronic load cells, but most of them just just air bags and a pressure monitoring system. It’s why the scale seems to “float” when you drive over it.
See my reply below; this is not how practical scales work. Large commercial scales do indeed use strain gages both in shear beams and, at smaller form factor, in machined blocks which keep the platform level and corners correct. Cheap bathroom scales often use half-bridge beams, known in the trade by the precise technical term “garbage.” Tabletop scales sometimes use other tehnologies like vibrating wires or cantilevered plates which form a variable capacitor, but at commercial scale the state looks dimly upon unproven technology. Hydraulic, not pneumatic, scales are a thing but they are a very expensive thing, and tend to seem rigid rather than floating for their own reasons.
I’m curious where these “open” weigh stations are.
I drive mostly the east coast USA corridor. 5-6 times a year. I also drive NY to CA at least once a year.
In the last… 20ish years, I can count the number of open weigh stations I have seen on one hand.
I know they MUST exist.
I have just never seen most of them.
In my life, I think I have seen fewer than 10 open weigh stations. And I’m retired. Have driven mostly in the Northeast.
Go someplace where roads deteriorate rapidly because of soil conditions, such as the South. Weigh stations on I-10 and I-20 tend to do a booming business and are usually open if it’s not a Federal holiday. Down here it’s the idle empty weigh station which is a sightseeing destination. This is even more so because weigh stations are responsible for writing permits for oversize loads, and a lot of heavy industrial equipment moves here like giant chemical reactors and engines which have to be transported in one piece.
On weight distribution, it is best to think not of axles, but of tires. 80,000 lb is supported by 18 tires, 2 tandem = 4 axles of 4 tires each = 16, plus 2 tires on the steering axle. If it were all even this would give around 4,500 lb per tire. Most of the trucks I’ve seen run about 130 PSI tire pressure, so the whole truck is held up by about 600 sq in of contact with the road, or 35 sq in per tire. In practice trucks that are close to the limit tend to run wider tires on the front axle because only two tires are up there carrying the engine.
Static scales almost always use strain gage weight sensors which will generally give a legal for trade accuracy of 20 lb on a capacity of 120,000 lb. Only static scales are legal for trade (including issuing tickets with fines) so all weigh stations will have at least one.
In-motion systems can use strain gages or piezo elements. If you see a WIM sensor that consists of plates bolted down over a shallow pit and requires trucks to come off the main road and slow down, it’s most likely strain gage style. If it’s embedded in the roadway and subject to highway traffic, it’s piezo and really crappy as a scale, although it will be good enough to send a light or unloaded truck on its way without stopping.
You see both kinds of WIM sensor and sometimes both at once depending on how busy the road is, how much money the state had to spend, and how much room they have; in some cases there just isn’t enough real estate for the installation the state would like.
On back roads you still see the opposite end of the spectrum, popup weigh stations with no infrastructure but a wide shoulder pulloff. Suspect trucks are weighed with hand carried portable wheel weighers, usually a set of 4 to get one axle at a time. These are not very accurate but those stations are usually looking for scofflaws like lumber trucks evading the main highway scales, so they won’t just be a little over. Although axle weights also aren’t supposed to be legal for trade there are accommodations. Often they are undercalibrated by 10% or so to give you the benefit of that doubt.
And of course once upon a time all those static scales used to be mechanical with beams and levers, and you’d balance a massive six foot long team before inserting a little tag to punch it with the weight. During the 1980’s and early 1990’s a lot of those were still in service with a single strain gage load cell taking the place of the beam (which was often still there, just locked down, with the new load cell mounted to the stilyard rod that transmits the reduced weight force from the platform noseiron to the beam). In those days WIM systems didn’t exist because not only the WIM scale platforms but the automation to link everything together wasn’t practical until the mid 1990’s.
I posted this above: most scales use a floating pneumatic load system, not cantilevered electronic load cells. It’s why the scale seems to “float” when you drive over it.
Article needs a correction. I had never heard of a cantilevered electronic scale before, but it makes sense so they probably do exist.
No, they don’t. Load cells in large scales like truck and tank scales generally use single or double ended shear beam strain gage load cells (which in turn are blocks of metal with strain gages expoxy bonded to them, in a wheatstone bridge format which results in 2 to 3 millivolts output per volt of excitation maximum at full capacity).
Shear beam cells are mounted in self-checking assemblies, some of which have the platform sitting on a big ball bearing which rides in a cup on the top of the load cell, making them “float.” There are also pillar cells like DigiTol which are checked in some other way and can rock slightly. And of course mechanical scales and retrofits “float” because the entire lever system is actually suspended from the main bearings.
All scales have to move a bit to work right, but movement in a direct strain gage system may be limited to thousandths of an inch. That actually creates service difficulty because even when it is working the scale can seem rigid. Techs do prefer systems that rock or float when they aren’t jammed in a bind, but sometimes this movement creates problems. I have never seen an electronic multi-platform truck scale which permitted much movement, because of interference between the modules as they shift.
There are hydraulic scales which use fluid in pistons to transfer the load force to a strain gage pressure sensor, but these are very expensive by today’s standards and rare; you usually see them in explosion proof or marine applications. No commercial scale has ever used pneumatics (gas pressure) to support the load and if someone built one, it would not conform to Handbook 44 and would not be legal for trade.
I have worked in the scale industry for 40 years, and if there ever was such a thing trust me, I would know about it.
“popup weigh stations”
I knew a logger who was put out of business by one.
All 3 of his trucks(in a line) with “Ag” fuel AND over weight.
(And MANY other defects…bad brakes, lights, etc)
He was not a “nice” guy. But it was ALL his fault.
Play with fire……..,,,,,,,,
” Any of these anomalies can cause a tire to quickly wear out and potentially self-destruct at highway speeds, resulting in catastrophic damage to surrounding traffic.”
And an underwear change.
For the surrounding traffic, if it’s not a steer tire there’s a good chance the trucker won’t notice until his next stop.
A friend who maintained static scales once told me of a particularly odd situation he encountered. It seems that when ever it rained, the scale would be off by some random amount. It turns out a critter was in the pit and would hop up on one of the cantilever arms when it would fill with rainwater.
Never seen similar in Europe. The police have mobile scales here (just a set of plates that fit in any police van) and either set up temporary weighing stations for all trucks or weigh any suspect overweight.
There are a few but they’re much less common, I can picture a couple in the UK (usually signposted VOSA Weighbridge). Outfits like aggregate depots and grain stores are more likely to have them if you need to weigh a vehicle.
There used to be a paper factory a few miles from my home. Across the road from the factory they had their own weighbridge. That’s the only one I’ve ever seen. Both factory and weighbridge closed down a long time ago now.