Offtracking. The engineer’s word for why the back of a truck does not go where the front went. A driver who understands it starts his turn from a different place; a driver who does not fills a lane he thought was empty.
The word. “Offtracking is said to occur when a vehicle makes a turn” and its rear wheels “do not follow the same path as its front wheels.” It comes in two kinds. Low-speed offtracking happens in a slow turn, when “the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle,” which is the one that fills the curb lane on a right turn. High-speed offtracking “results from the tendency of the rear of the truck to move outward” in a curve taken fast, the one that puts a trailer’s wheels over the line on a ramp (FHWA). The longer the distance from the kingpin, where the trailer rides on the tractor, to the trailer’s rear axle, the more the trailer cuts inside.
The measure. A Federal Highway Administration truck size and weight analysis put numbers on it for a standard 90-degree right turn on a 42-foot radius, taken at 5 kilometers an hour: a tractor pulling a 53-foot semitrailer offtracks 16.12 feet, with a swept path 24.12 feet wide, and a tractor pulling the familiar double of two 28-foot trailers offtracks 13.52 feet, with a swept path of 21.52 (FHWA). A single lane is not that wide. The trailer on a square right turn is going to occupy the curb lane, the corner and part of what is beyond them, and the question on the street is only whether anything was there.
The driver’s rules. The federal model manual that commercial driver tests are built on teaches the sweep as a hazard with a procedure. “Because of wide turning and offtracking, large vehicles can hit other vehicles or objects during turns.” “Turn slowly to give yourself and others more time to avoid problems.” If the right turn cannot be made without swinging into another lane, “turn wide as you complete the turn,” and “[k]eep the rear of your vehicle close to the curb.” “Don’t turn wide to the left as you start the turn,” because “[a] following driver may think you are turning left and try to pass you on the right” (Model Commercial Driver’s License Manual). The procedure is the duty in miniature: the swing comes at the end of the turn, not the beginning, the rear stays close to the curb, and the mirror on the right is checked before the trailer takes the space.
The law. Texas writes the same turn into the Transportation Code. An operator turning right “shall make both the approach and the turn as closely as practicable to the right-hand curb or edge of the roadway” (Section 545.101), and shall “signal continuously for not less than the last 100 feet of movement of the vehicle before the turn” (Section 545.104). The driver behind has a rule too: he “may pass to the right of another vehicle only if conditions permit safely passing to the right” and the vehicle ahead “is making or about to make a left turn,” and only on a roadway wide enough for two or more lines of moving vehicles in each direction (Section 545.057). “As closely as practicable” is the phrase the whole case lives in, because a 53-foot trailer cannot hug a curb through a square corner and the law does not pretend it can; what it asks is the manual’s turn, slow, signaled, swung late and watched in the mirror. A car or a rider that came up the inside of a signaling truck has the passing statute to answer; a truck that swung left first without a signal, or never looked right, has the turning statute. Texas divides the fault between them under proportionate responsibility rather than ending it at whoever touched the trailer, which is Comparative Fault, and what the trailer’s open side then does to a car or a rider is Guard, Underride and The Cage.
The move. A swept path can be reconstructed after the fact, and the proof is mostly the carrier’s. The tractor’s forward and side cameras, which show the signal, the mirror check and where the cab was when the turn began. The engine and position data, which fix the speed through the turn. The point of contact on the trailer, measured from the landing gear and the rear axle, which places the other vehicle beside the truck or behind it. The tire marks of the trailer’s rear axle on the pavement and the curb, which are the swept path drawn on the ground. The intersection’s own geometry, the corner radius and the lane widths, against which an engineer runs the turning template. And the carrier’s training file on turns, because the model manual’s procedure is the floor the licensing test is built on, and a driver taught less than that was taught by the company. The preservation letter goes out the first week, because the cameras overwrite, which is Spoliation.
Offtracking. The front of the truck turns the corner. The back of the truck turns through it, and the law asks the driver to know the difference before the space beside him is gone.
The term and the measure: FHWA, Western Uniformity Scenario Analysis, Chapter VI, Roadway Geometry (Offtracking; Table VI-2, Offtracking Characteristics) (low-speed and high-speed offtracking defined; a standard 90-degree right turn with a 42-foot radius at 5 kilometers per hour; 53-foot single semitrailer, 3-S2: 16.12 feet of low-speed offtracking, 24.12-foot swept path; STAA double, two 28-foot trailers: 13.52 and 21.52). The procedure: AAMVA, 2005 Model Commercial Driver’s License Manual (July 2010), published by FMCSA, Section 2.7.6, Space for Turns. The law: Tex. Transp. Code § 545.101, § 545.104 and § 545.057. General information, not legal advice.
