The most dangerous number on a truck is one the driver never sees. It is not on the dash, not in the logbook and not on the bill of lading. It is a fraction, half the width of the axles over the height of the center of gravity, and it changes every time the trailer is loaded. Before dawn on Friday an 18-wheeler went over at a Houston interchange, and whatever the cause turns out to be, that fraction is where the inquiry begins.
Friday morning, an 18-wheeler overturned on Interstate 10 westbound at the Interstate 69 interchange, the Eastex Freeway, northeast of downtown. The station reported that the crash left “the truck’s trailer resting on a guardrail,” that “The trailer was loaded with furniture,” and that “The driver exited the truck safely,” with no further word on injuries. By traffic information verified at 4:36 a.m., the right shoulder and the right and center lanes were affected, with backups expected into downtown for the morning commute (KPRC). No cause was given, and no one was named. The reports do not say where on the interchange the truck went over, how fast it was going, or what the load did. A wreck that hurts no one is still a record of how a truck behaves, and this one belongs to a family the trucking industry has studied for decades: a tall, light load, a road that bends, and a trailer that went over.
The fraction. Whether a truck tips or slides in a turn is decided by one ratio. The University of Michigan’s study of heavy-truck rollover, written for a truck maker and a university research center in 1999, defines the rigid-vehicle limit as “the ratio of the half track (T/2) to the cg height (h)”: half the distance between the tires, divided by how high the center of gravity sits. “The rollover thresholds of passenger cars are virtually always greater than 1 g,” the study says, which means a car’s tires slide before the car tips. “The rollover threshold of a loaded heavy truck often lies well below 0.5 g.” A typical five-axle tractor and van semitrailer has a threshold the study puts at perhaps as high as 0.5 g with a dense load carried low, and as low as 0.25 g with the worst-case load (UMTRI, 1999). Furniture pulls the number toward the bottom of that range. It is bulky, it fills the box to the roof, and the center of gravity climbs with it; the study’s worst case is a load that fills the trailer’s volume while also reaching legal gross weight. The commercial driver’s manual says the same thing in the words of the license test: “A high center of gravity (cargo piled up high or heavy cargo on top) means you are more likely to tip over” (CDL manual, Section 3.2.3). The ratio and its arithmetic are the subject of Threshold. What a dispatch needs is one consequence of it. “Drivers regularly maneuver vehicles at well over 0.2 g,” the study notes, and the highway design guidelines produce “lateral accelerations as high as 0.17 g at the advisory speed” on a curve. A driver who takes a bend at the speed that felt fine with a dense load can be past the line with a tall one, and the study’s description of the moment is the reason so many of these wrecks are reported with the word “suddenly”: “rollover is very much an either-or situation,” “something like walking up to a cliff with your eyes closed” (UMTRI, 1999).
The load. Federal law makes the load the driver’s responsibility before the truck moves and every few hours after. A driver may not operate a commercial motor vehicle, and a carrier may not require or permit him to, unless the cargo is “properly distributed and adequately secured” under the securement rules; the driver must inspect the cargo and its securing devices “within the first 50 miles after beginning a trip,” and again whenever he changes duty status, after three hours of driving, or after 150 miles, whichever comes first (49 CFR 392.9). The securement rules say what secured means. Cargo “must be contained, immobilized or secured in accordance with this subpart to prevent shifting upon or within the vehicle to such an extent that the vehicle’s stability or maneuverability is adversely affected” (49 CFR 393.100), and “firmly immobilized or secured on or within a vehicle by structures of adequate strength, dunnage or dunnage bags, shoring bars, tiedowns or a combination of these,” with tiedowns whose combined working load limit “must be at least one-half times the weight of the article or group of articles” (49 CFR 393.106). The manual turns that into the walk-around: “As part of your pre-trip inspection, make sure the truck is not overloaded and the cargo is balanced and secured properly,” with blocking placed “to keep it from sliding” and bracing that “goes from the upper part of the cargo to the floor and / or walls of the cargo compartment” (CDL manual, Sections 3.1 and 3.3.1). The driver’s inspection rules have two exceptions, a sealed vehicle the driver has been ordered not to open and one loaded so that inspection is impracticable, and the manual tells a driver even then to check that the weights are legal (49 CFR 392.9; CDL manual, Section 3.3.5). Inside a closed van, furniture that is not blocked and braced does what any tall, light thing does in a turn. It leans, and a leaning load moves the center of gravity toward the side the truck is already tipping toward. Shed was about cargo that leaves the truck. This is about cargo that stays in it and takes it over.
The curve. Texas does not post a speed for a loaded trailer. It posts one duty for everyone, “a speed greater than is reasonable and prudent under the circumstances then existing” is forbidden, and it names the curve: a driver shall “drive at an appropriate reduced speed if” he “is approaching and going around a curve” (Transportation Code Section 545.351). The manual says why the number on the yellow sign is not the number for a truck: “Tests have shown that trucks with a high center of gravity can roll over at the posted speed limit for a curve,” so “Slow to a safe speed before you enter a curve,” because braking inside it “is dangerous because it is easier to lock the wheels and cause a skid” (CDL manual, Section 2.6.3). An interchange ramp is a curve with a yellow sign, and what that sign is in law, and is not, is the subject of Advisory. The duty itself is the one Reasonable and Prudent explains, and for a truck with a tall load it is met or failed before the bend, not in it.
What the case asks. No one was hurt this time. The lanes this truck had closed by 4:36 in the morning are full of cars at 7:30, and the record of a wreck that hurt no one is the same record as the one that will. How fast the truck was going, which the engine’s own recorder kept (the black box). What the load was and how high it was stacked, which is on the bill of lading and in the loader’s own photographs if anyone took them. Who loaded it and who secured it, because when a shipper or a warehouse stacks a box to the roof and a carrier sends it out, Texas lets a jury put a number on each of them: the trier of fact “shall determine the percentage of responsibility, stated in whole numbers,” for each claimant, each defendant, each settling person and each responsible third party designated in the case (Civil Practice and Remedies Code Section 33.003), the allocation Comparative Fault describes. Whether the driver did the pre-trip and the 50-mile check, and what his hours were (Logbook). What the company taught him about tall loads and ramps, and what its own trucks had done on them before. Each answer lives in a record the company holds, and each can be repaired, sold or overwritten unless the first letter in the case asks that it be kept (Spoliation).
No one is named here, and nothing on this page decides what happened at the Eastex interchange. A dispatch keeps the record of what the law asks when a trailer full of light freight goes over on a city interchange: what the load weighed and where it sat, what the fraction was when the truck left the dock, what speed the bend allowed for that fraction, and who, between the loader, the carrier and the driver, was supposed to know it.
Dispatch is the Night Docket’s news strand: what happened on Texas roads and in Texas courts, reported from credible outlets and agency releases, with no names and no conclusions about open investigations, then connected to the entries that explain the science and the law. General information, not legal advice.
