Swept-Path Width (Turn Lane Occupancy)
The total roadway width a turning vehicle covers, the number a turn lane or intersection is sized on.
Example
You enter
- Turn radius R (ft) 50
- Tractor / unit wheelbase (ft) 20
- Trailer kingpin-to-axle (ft; 0 if single unit) 40
- Vehicle width (ft; ~8.5 legal max) 8.5
You get
- Swept-path width 36.14 ft
- Off-tracking (rear inside front) 27.64 ft
Checked against published worked examples from: AASHTO Green Book (single-unit cross-check).
Details, formula, and sources
ScopeSPW = vehicle width + low-speed off-tracking (+ front swing-out). The off-tracking OT = R - sqrt(R^2 - sum(L^2)) is how far the rear axle tracks inside the front's turn radius; adding the 8.5 ft body width gives the pavement band. A single unit (20 ft wheelbase) on a 50 ft turn sweeps 12.7 ft; a tractor-trailer (20 + 40 ft) sweeps 36.1 ft -- why a long combination needs a wide turn lane. Front swing-out from the turning template is added if entered. High-speed off-tracking is separate; per the AASHTO Green Book, the design vehicle and agency govern.
Formula or tableOT = R - sqrt(R^2 - sum(L_i^2)); SPW = vehicle_width + OT + front_swingout. R the turn radius, L_i each unit's wheelbase.
Source and editionAASHTO Green Book (Policy on Geometric Design of Highways and Streets) swept-path width = off-tracking plus vehicle width; standard turning geometry.
Free accesstransportation.org for TOC; the swept-path relation (off-tracking plus width) is public. AASHTO design-vehicle turning templates are licensed.
What governsMath aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
API field namesturn_radius_ft, wheelbase1_ft, wheelbase2_ft, vehicle_width_ft, swept_path_width_ft, off_tracking_ft
- Swept-path width SPW = vehicle width + OT + front swing-outAASHTO Green Book
- Off-tracking OT = R - sqrt(R^2 - sum(L_i^2)), rear tracks inside the frontAASHTO Green Book / truck-off-tracking tile
- Scope low-speed steady-state; high-speed off-tracking (rear swings out) is separateanalysis scope