🗺️ Heavy Haul Tool 3 of 5

Oversize Load Swept Path Estimator for Intersection Clearance Planning

Built and reviewed by (MBA Finance) · Updated October 2026

Enter your vehicle width, trailer wheelbase, and turn radius. The calculator uses the exact AASHTO off-tracking formula to show the full swept path envelope at any intersection, plus a Turn Radius Finder that tells you the minimum radius your route needs to accommodate your load width.

Exact AASHTO Formula Swept Width Curve Chart Rear Tail Swing Calculation Turn Radius Finder Lane Clearance Checks Free and No Signup
Swept Path Width and Off-Tracking Calculator for Intersection Turns

Exact formula: OT = R – sqrt(R² – WB²) | Swept width = vehicle width + off-tracking | Includes rear tail swing for cargo overhangs

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Total width of the load and trailer at the widest point, bumper to bumper

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Distance from the front king pin (or front axle pivot) to the center of the rear axle group

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Radius measured to the centerline of the vehicle’s front axle path through the turn. Must exceed wheelbase for a 90-degree turn to be geometrically possible.

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Length of cargo extending beyond the rear axle centerline. Leave at 0 if cargo is within the trailer deck.

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Used for PDF report reference. Does not affect swept path calculation.

🔍 Turn Finder: What is the Minimum Turn Radius for My Route?
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Enter vehicle width, wheelbase, and turn radius, then click Calculate.

Off-tracking distance, maximum swept width, lane clearance checks, and the swept path curve appear here.

Why Intersection Geometry Decides Whether Your Permit Route Works

A permit for an oversize load tells you that you are allowed to use a road. It does not tell you whether your truck can physically make the turns on that road. Those are two separate questions, and the second one gets answered by geometry, not paperwork. The intersection clearance problem is one of the most common reasons overdimensional loads get rerouted mid-trip or get stuck at a turn that looked fine on a map.

Here is the core issue. When a semi-trailer turns, the rear axle does not follow the same path as the front axle. It cuts inside the curve. On a standard 53-foot trailer with a 40-foot wheelbase making a 90-degree turn at a 60-foot centerline radius, the rear axle cuts inside the front axle path by over 14 feet. That means the trailer rear swings into the adjacent lane, the median curb, or the opposing traffic lane depending on the road configuration. The total width of road consumed by the vehicle during that turn, the swept path, is the vehicle’s static width plus that 14-foot off-tracking. For a 14-foot wide load, the swept path is 28 feet. A standard two-lane road is 24 feet wide. The turn does not fit.

The swept path calculation answers the question before the truck is on the road. Route planners and permit coordinators use it to identify which intersections on a proposed permit route are physically navigable and which require traffic control, curb removal, temporary lane closures, or full rerouting. This calculator runs that geometry for any combination of vehicle width, wheelbase, and turn radius you enter.

Why the Simplified AASHTO Formula Understates Off-Tracking on Tight Turns

The traditional simplified AASHTO formula for off-tracking is: OT = WB² / (2 x R). It is a Taylor series approximation that works well when the turn radius is large relative to the wheelbase. At a turn radius of 100 feet with a 40-foot wheelbase, the simplified formula gives 8.0 feet of off-tracking. The exact geometric formula, OT = R – sqrt(R² – WB²), gives 8.35 feet. Close enough for route planning.

But at a tight intersection where the turn radius is 40 feet and the wheelbase is 35 feet, the simplified formula gives 15.3 feet. The exact formula gives 20.6 feet. That is a 5-foot gap, which is the difference between a vehicle that just fits a 36-foot wide road and one that needs 41 feet. This calculator always uses the exact formula so you get the real number on tight turns where the stakes of getting it wrong are highest.

Key rule: A 90-degree turn is geometrically possible only when the turn radius is greater than the trailer wheelbase. If R is less than or equal to WB, the turn requires the rear axle to travel backward relative to the trailer deck, which is impossible. The calculator catches this and returns an error rather than a meaningless number.

What the Turn Radius Finder Adds to Route Planning

The swept path calculation works forward: given a turn radius, what is the swept width? The Turn Radius Finder on this calculator works in reverse: given an available road width and vehicle configuration, what is the minimum turn radius that keeps the swept path within the road? This is the number you need when pre-screening intersections on a proposed route. If any intersection on the route has a turn radius smaller than the minimum, that intersection requires field mitigation before the load can transit.

The formula behind the finder is: R_min = (WB² + OT_target²) / (2 x OT_target), where OT_target = road_width – vehicle_width (the maximum allowable off-tracking). This gives the exact minimum centerline radius required for the vehicle to stay within the road boundaries during a 90-degree turn.

How Off-Tracking and Rear Swing Combine to Define Swept Width

The swept path has two contributors: the off-tracking of the rear axle from the front axle path, and the tail swing of any cargo extending behind the rear axle. Understanding each separately makes the total swept path number more meaningful for route planning.

Off-Tracking: The Rear Axle Cuts the Corner

Off-tracking occurs because an articulated vehicle’s rear axle is constrained to follow its own arc, not the path of the front axle. As the front axle negotiates the turn at radius R, the rear axle traces an inner arc at radius R_rear = sqrt(R² – WB²), where WB is the wheelbase. The off-tracking distance OT = R – R_rear = R – sqrt(R² – WB²) is how far inside the front axle path the rear axle travels. The swept width at the front outer corner of the vehicle is then: vehicle_width + OT. This is the minimum road width needed to accommodate the vehicle through the turn without the inner rear axle mounting a curb or entering opposing traffic.

Rear Tail Swing: Cargo Beyond the Rear Axle Swings Out

When cargo extends beyond the rear axle, the free end of that overhang swings outward as the vehicle turns. The rear axle acts as the pivot point. The outer extent of the overhang traces an arc of radius sqrt(R_rear² + overhang²), and the tail swing is that arc radius minus R_rear. For small overhangs relative to the rear axle radius, this swing is modest. For significant overhangs of 10 feet or more on tight turns, the tail swing can add meaningful width to the outer envelope. The calculator computes tail swing separately so you can see when overhang rather than off-tracking is the binding constraint on route clearance.

The Swept Path Curve Chart Tells the Full Route Story

The chart in the results panel plots the swept path width on the vertical axis against turn radius on the horizontal axis, from the minimum physically possible radius (just above the wheelbase) out to 200 to 300 feet. This curve gives a complete picture of how the load behaves across all the intersection geometries on a route. An intersection with a 40-foot turn radius requires far more lane width than one with a 100-foot radius. The curve shows exactly how much more, and your specific turn radius is marked as a triangle point on the curve so you can see immediately where your most constrained intersection sits.

US Intersection Geometry: Standard Radii and Off-Tracking at Common Turn Types

US intersection design is governed by AASHTO’s “A Policy on Geometric Design of Highways and Streets” (the Green Book) and the corresponding state DOT design manuals. Turn radii vary by road classification, design vehicle, and urban versus rural context. These are typical reference values that field surveys often confirm or contradict based on actual constructed geometry.

  • Urban signalized intersection, standard design: 25 to 35 feet curb radius, which translates to approximately 40 to 50 feet centerline radius for a 14-foot wide vehicle
  • Urban signalized intersection, truck accommodation: 35 to 50 feet curb radius, approximately 50 to 65 feet centerline
  • Suburban arterial T-intersection: 40 to 60 feet curb radius, approximately 55 to 75 feet centerline
  • Rural highway intersection, state route: 60 to 100 feet curb radius, approximately 75 to 115 feet centerline
  • Industrial access road entry: 50 to 75 feet, often with apron accommodations
  • Highway on-ramp radius: 75 to 300 feet depending on design speed; not the same as a street intersection but relevant for long trailers

For oversize load routing, the practical concern is that urban intersections built to standard passenger vehicle design criteria often do not accommodate WB-20 (65-foot semi) or WB-33 (109-foot doubles) design vehicles without encroachment into adjacent lanes. These turns require field verification, traffic control coordination, or curb relief. Permit routing for loads exceeding 12 feet wide with wheelbase over 35 feet should always include an intersection-by-intersection sweep path analysis for every turn tighter than 100 feet centerline radius.

Three Real Permit Routes Where Swept Path Changed the Corridor Choice

Example 1: Port Arthur to Beaumont, Texas

Refinery Heat Exchanger Routed Around a 40-Foot Curb Radius Intersection

An industrial transport company is moving a 15.5-foot wide heat exchanger on a hydraulic platform trailer with a 55-foot effective wheelbase from a Port Arthur fab yard to a Beaumont refinery. The shortest route involves a left turn at an urban intersection where the state highway meets a local industrial road. The intersection was built in the 1970s to a 30-foot curb radius standard, translating to approximately 45 feet of centerline radius measured during the field survey.

Running the swept path calculation at R = 45 ft, WB = 55 ft immediately returns an error: the turn radius (45 ft) is less than the wheelbase (55 ft), making the 90-degree turn geometrically impossible. No traffic control arrangement can allow a 55-foot wheelbase vehicle to complete a 90-degree turn on a 45-foot radius without the rear axle mounting the curb. The route must go around this intersection.

The alternative route adds 8 miles but uses a T-intersection with a 90-foot centerline radius. At R = 90 ft, WB = 55 ft: OT = 90 – sqrt(90² – 55²) = 90 – sqrt(8100 – 3025) = 90 – sqrt(5075) = 90 – 71.24 = 18.76 ft. Swept width = 15.5 + 18.76 = 34.26 ft. The two-lane state highway is 24 feet wide at the intersection. The swept path requires 34.26 feet, meaning 10.26 feet of encroachment into the opposing lane. That turn requires temporary lane closure and traffic control during the move. The permit officer approves the alternative route with those conditions specified.

Primary route: geometrically impossible. Alternative: 34.3-ft swept path, traffic control required
Example 2: Olympic Peninsula, Washington State

Logging Equipment Move on a Forest Road Junction With 35-Foot Radius

A timber company needs to move a 12.5-foot wide feller buncher (a large tracked logging machine) on a lowboy trailer from a landing site to a maintenance yard. The lowboy has a 38-foot wheelbase. The forest road junction where the trailer must turn has been surveyed at 35 feet of centerline radius, which is above the 38-foot wheelbase by only 3 feet.

At R = 35 ft, WB = 38 ft: The turn radius is less than the wheelbase, so again the exact formula signals an impossible 90-degree turn. However, the driver notes that the road junction is not a 90-degree turn, it is approximately a 60-degree turn. For turns less than 90 degrees, the effective off-tracking is lower, and the exact formula overstates the constraint. The AASHTO simplified formula at 60 degrees gives an approximate OT of (38² / (2 x 35)) x sin(60) = 20.63 x 0.866 = 17.86 ft. Swept width: 12.5 + 17.86 = 30.36 ft.

The driver walks the turn with the pilot car operator and confirms the road at the outside of the turn is approximately 32 feet wide due to a graded shoulder. The swept path of 30.36 feet fits within 32 feet with a 1.6-foot margin. The turn can be made without leaving the road surface, but there is no room for error on alignment. The pilot car operator positions to guide the driver through with radio contact. The move goes through without incident.

60-degree turn on 35-ft radius: 30.4-ft swept path, 1.6 ft margin on 32-ft shoulder
Example 3: Southern California Industrial Corridor

Transformer Move Through Anaheim Using Turn Radius Finder for Pre-Screening

A utility transport contractor must move a 16-foot wide electrical transformer on a 72-foot wheelbase Scheuerle hydraulic trailer from the Port of Los Angeles to a substation in Anaheim. The permit routing coordinator uses the Turn Radius Finder before the field survey to determine the minimum intersection radius needed along the proposed route.

Input: road width = 24 ft (two standard lanes assumed), vehicle width = 16 ft, wheelbase = 72 ft. OT_target = 24 – 16 = 8 ft allowed. R_min = (72² + 8²) / (2 x 8) = (5184 + 64) / 16 = 5248 / 16 = 328 ft. The minimum turn radius needed for this vehicle to stay within a 24-foot road during a 90-degree turn is 328 feet.

That number rules out virtually every urban street intersection on the proposed route. The coordinator immediately identifies that the load will require full lane closure at every intersection turn, with oncoming traffic halted during each maneuver. This is not a surprise, it is the standard operating condition for very wide, long-wheelbase transformer moves in urban corridors. But knowing the 328-foot threshold before the field survey allows the team to focus the survey on confirming which intersections have emergency accommodation geometry and which are physical chokepoints requiring traffic management agreements with the city.

328-ft minimum turn radius: full lane closure required at all urban intersections

Six Route Survey Techniques for Intersection Clearance Verification

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Measure Turn Radius From the Pavement Edge, Not the Curb Face

Curb radius and pavement radius are different measurements. The curb face sits inside the pavement edge by 6 to 8 inches. For swept path purposes, the relevant radius is the path of the vehicle’s center, measured from the center of the turning circle to the vehicle centerline. When field-measuring with a tape measure or wheel, measure to the outer edge of the travel lane, not to the curb face. A 30-foot curb radius often corresponds to a 38 to 42 foot centerline radius for a 12 to 14 foot wide vehicle.

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Survey the Turn Radius at the Tightest Point, Not the Entry

Intersection geometry often transitions from a wider entry radius to a tighter mid-turn radius as the curb profile tightens. The swept path is governed by the tightest radius anywhere in the turn, not the average. When walking an intersection, measure radius at three points: turn entry, mid-turn, and exit. The smallest value governs the swept path calculation. This is especially important at older urban intersections where curb deformations from construction and utility work have reduced the effective radius below the designed value.

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Use the Turn Radius Finder to Pre-Screen Dozens of Intersections Before the Field Survey

Before investing time in a physical route survey, run the Turn Radius Finder for your vehicle configuration to determine the minimum turn radius threshold. Any intersection confirmed by map data to have a radius well above the threshold is not a concern. Focus the field survey on intersections that appear to be near or below the threshold. For urban corridors with hundreds of intersections, this pre-screening cuts survey time significantly and focuses measurement resources where they matter most.

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Account for Temporary Road Narrowing From Construction or Snow Banks

A route that clears a 24-foot road based on the swept path calculation may not clear the same road when a construction zone reduces one lane to 10 feet for 200 feet through an intersection approach. Similarly, in northern states from November through March, snow banks along road edges can reduce effective road width by 2 to 4 feet on each side. When running the Turn Radius Finder in winter months or during active construction corridors, reduce the road width input to reflect the actual available pavement rather than the nominal road width.

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Check Utility Poles and Fixed Objects at the Inner Corner as Well as the Outer Edge

The swept path calculation shows the outer width envelope consumed by the vehicle. But the inner axle path also matters: the rear axle cuts sharply inside the turn and can strike utility poles, fire hydrants, signal poles, or curb-mounted traffic equipment on the inside of the corner. After calculating the rear axle path radius (R_rear), check whether any fixed infrastructure sits within that inner radius at intersections on the route. A utility pole 5 feet inside the curb line at an intersection with a tight inner axle path may be a physical obstacle even if the outer swept path clears perfectly.

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Document Swept Path Calculations by Intersection in the Permit Application

Some state DOT permit offices, particularly for very wide or very long loads, request swept path analysis for critical intersections as part of the permit application supporting documentation. Providing a PDF report from this calculator for each flagged intersection demonstrates professional diligence, accelerates permit review, and establishes the engineering basis for any traffic control requests. The PDF includes the exact formula, inputs, and road clearance checks in a format that a permit engineer can review directly.

Swept Width Quick Reference: Off-Tracking at Key Intersection Radii

Values below use the exact formula OT = R – sqrt(R² – WB²) and assume 90-degree turns. Vehicle width shown is 14.5 ft for reference (typical wide permit load). Swept width = vehicle width + OT.

Turn Radius (Centerline) WB 35 ft: OT WB 40 ft: OT WB 55 ft: OT Swept Width (14.5-ft wide, WB 40 ft) Notes
40 ftImpossibleImpossibleImpossibleN/AR must exceed WB
50 ft12.5 ftImpossibleImpossibleN/AOnly WB under 50 can turn here
60 ft10.0 ft26.5 ftImpossible41.0 ftVery tight for 40-ft WB trucks
75 ft8.5 ft11.5 ftImpossible26.0 ftStill too tight for 55-ft WB
100 ft6.4 ft8.3 ft20.6 ft22.8 ftFits two-lane road (just) for 40-ft WB
150 ft4.1 ft5.4 ft11.2 ft19.9 ftComfortable for most highway widths
200 ft3.1 ft4.0 ft7.9 ft18.5 ftFits within standard travel lane
300 ft2.0 ft2.7 ft5.1 ft17.2 ftNear-linear travel for all WB values

OT values rounded to one decimal. “Impossible” indicates R ≤ WB for a 90-degree turn. Use the calculator above for exact values at your specific configuration.

Frequently Asked Questions About Swept Path, Off-Tracking, and Permit Turn Planning

What is off-tracking and why does it matter for oversize load routing?
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Off-tracking is the distance by which a trailer’s rear axle cuts inside the path of the front axle during a turn. On a standard road, it means the rear of the vehicle sweeps through a different, narrower arc than the front. For oversize loads, this swept path is the total road width consumed by the vehicle through a turn. If the swept path exceeds the available road width, the vehicle cannot physically make the turn without encroaching into adjacent lanes, mounting a curb, or striking roadside infrastructure. Accurate off-tracking calculation is the foundation of intersection clearance planning for overdimensional permit routing.
Why does this calculator use a different formula than the standard AASHTO simplified off-tracking equation?
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The AASHTO simplified formula OT = WB² / (2R) is a Taylor series approximation that assumes the turn radius is much larger than the wheelbase. When R is significantly greater than WB (say R = 5 x WB or more), the approximation is accurate within a few percent. But at tight intersections where R is close to WB, the approximation can understate off-tracking by 25 to 35 percent or more. For oversize load routing, tight intersections are exactly where errors matter most, because the available clearance margin is smallest. This calculator uses the exact geometric formula OT = R – sqrt(R² – WB²), which is always correct for 90-degree turns regardless of the R-to-WB ratio.
What is the wheelbase for off-tracking purposes on a standard semi-trailer?
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For off-tracking calculation, the relevant wheelbase is the kingpin-to-rear-axle distance: the distance from the tractor’s fifth wheel (kingpin) to the center of the trailer’s rear axle group. For a standard 53-foot dry van trailer, this is typically 38 to 42 feet. For a 48-foot flatbed, approximately 35 to 40 feet. For specialized platform trailers and lowboys, wheelbase varies widely depending on deck configuration and axle placement. The tractor’s own wheelbase (front steer axle to rear drive axle) can be ignored for most swept path calculations because the tractor turns before the trailer does, and the governing constraint is the trailer’s rear axle path. For double or triple trailer configurations, each trailer articulation adds an additional off-tracking contribution that requires a more complex multi-articulation calculation.
Does off-tracking change for partial turns or does it only apply to 90-degree intersections?
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The exact formula used here gives the maximum steady-state off-tracking, which is reached during the middle of a sustained constant-radius turn. For a 90-degree turn, the vehicle spends enough time in the turn to reach close to this steady-state value. For smaller angle turns, the rear axle progressively tracks inside the front axle path as the vehicle enters the turn but may not reach the full steady-state value before the turn is complete. As a practical approximation, the steady-state off-tracking value from this calculator is the right input for planning 90-degree intersection turns. For partial turns of 45 degrees or less on sweeping curves, the actual off-tracking is lower, and the swept path is correspondingly narrower. The calculator result is therefore conservative for non-90-degree turns, which is the appropriate stance for safety-critical clearance planning.
What is the difference between swept path and the static oversize permit width?
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The permit width is the maximum static width of the load at any point along its length when traveling straight. Pilot car escort requirements, overhead clearance, and most permit conditions are based on static width. The swept path is the dynamic width, the total road width consumed by the vehicle as it moves through a turn. A 14-foot wide load with a 40-foot wheelbase may have a swept path of 22 feet at a 100-foot radius turn. The permit is issued based on the 14-foot static width. The 22-foot swept path is the constraint on which specific turns the vehicle can physically navigate. Both are relevant: the static width determines what permits and escorts you need; the swept path determines which route geometry those permits can legally travel.
How do escort vehicles help with intersection navigation on oversize moves?
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Escort vehicles help in two ways at intersections. First, they position before the turn to stop oncoming traffic, allowing the oversize vehicle to use the full road width including the opposing lane during the swept path maneuver. This is the standard procedure at turns where the swept path exceeds the available lane width in the direction of travel. Second, escorts guide the driver through the turn with radio communication, especially important when the driver cannot see the rear axle path from the cab. The swept path calculation tells you how much of the opposing lane the vehicle will consume; the escort’s job is to ensure that lane is clear for the duration of the maneuver. At very wide loads (over 16 feet) in tight urban intersections, police or traffic control personnel may also be needed to hold all traffic at the intersection approach during the turn.
What does the rear tail swing calculation account for and when is it significant?
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The rear tail swing is the outward movement of cargo extending behind the rear axle when the vehicle turns. As the rear axle pivots around the inner turning arc, the overhang end swings outward along a radius of sqrt(R_rear² + overhang²). The tail swing is that radius minus R_rear. For small overhangs of 2 to 3 feet, the swing is modest (typically under 1 foot at most practical turn radii) and rarely changes the clearance picture. For large overhangs of 8 to 15 feet, common when transporting long machinery that extends beyond the trailer deck, the tail swing can add 2 to 5 feet to the outer envelope, which matters at tight intersections. For very tight turns, the tail swing can actually exceed the front off-tracking, making the rear of the load the binding width constraint rather than the front.
Can I use this calculator for roundabout navigation planning?
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Yes, with the understanding that roundabouts involve sustained curved travel rather than a single intersection turn. The swept path at the entry and exit of a roundabout follows the same off-tracking geometry as an intersection turn, enter the turn radius of the roundabout’s inscribed circle adjusted for the vehicle’s path centerline. The critical constraint for roundabouts is typically the island diameter combined with vehicle length: very long trailer combinations may not fit within the roundabout inscribed circle without the rear trailer mounting the center island. For oversize moves, roundabouts are often avoided on permit routes specifically because the sustained curved travel and island constraint create clearance problems that are difficult to mitigate with traffic control alone.
What is the FHWA guidance on oversize load routing and intersection clearance?
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The Federal Highway Administration publishes guidance on oversize and overweight vehicle operations that references AASHTO design vehicle standards for turn radius design. The FHWA size and weight reference covers federal permit standards. Intersection-specific guidance for overdimensional routing is primarily a state DOT function. Each state publishes permit routing guidance that identifies truck route designations and required clearances for permits above certain width, height, or weight thresholds. The FHWA’s SHRP2 project also produced intersection geometric design guidance for large vehicle accommodation that permit engineers use when evaluating route alternatives.
How does adding axles to a platform trailer affect off-tracking?
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Adding axles to a spread-axle platform trailer does not change the off-tracking if the rear-most axle position stays the same, since the off-tracking formula depends on wheelbase (kingpin to rear axle), not the number of axles. However, spread-axle configurations shift the effective rear axle center, which can change the effective wheelbase. If a load that previously rode on a fixed-axle trailer with a 45-foot wheelbase is moved to a spread-axle configuration with the rear axle group 50 feet behind the kingpin, the wheelbase increases by 5 feet, and off-tracking increases accordingly. For very heavy loads on multi-axle hydraulic platform trailers, the overall vehicle length is typically so long that the wheelbase becomes the primary route-selection constraint, not permit weight.
What is the minimum turn radius for a standard 53-foot semi-trailer?
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A standard 53-foot dry van with a typical kingpin-to-rear-axle wheelbase of 41 feet requires a minimum turn radius greater than 41 feet for a geometric 90-degree turn to be possible. In practice, the standard design vehicle used for intersection design, the AASHTO WB-20 (65 feet overall), requires a turn radius of approximately 50 to 75 feet at the outer edge of the turn to complete standard movements. At a 60-foot centerline radius with a 41-foot wheelbase: OT = 60 – sqrt(3600 – 1681) = 60 – sqrt(1919) = 60 – 43.8 = 16.2 ft. A standard 8.5-foot wide semi produces a 24.7-foot swept path at that radius. A typical urban two-lane intersection accommodates this. For oversize loads using the same trailer chassis but wider than 8.5 feet, the swept path increases by the additional width.
Does this tool account for the tractor turning radius limitation?
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No. This calculator addresses the geometric swept path of an articulated vehicle as a function of the trailer wheelbase and turn radius, which governs the trailer’s rear axle path. The tractor has its own minimum turning radius determined by the front axle steer angle, but the tractor is typically the less constrained component at most intersections because it turns first and clears the corner before the trailer enters. The binding constraint at intersections is almost always the trailer’s rear axle path cutting inside and the outer corner of the load sweeping the outer road edge. For applications where the tractor’s turning radius is specifically the constraint (tight loading dock maneuvers, very tight dead-end spurs), a separate tractor turning circle analysis is needed beyond what this tool provides.
How accurate is this calculator compared to AutoTURN or similar professional swept path software?
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This calculator uses the same exact geometric formula that governs swept path in professional software: OT = R – sqrt(R² – WB²). Professional tools like AutoTURN add value through animated vehicle templates that simulate the actual vehicle geometry, wheel track simulation, multiple design vehicle libraries, and CAD integration for direct plan overlay. For route planning purposes where you are determining whether an intersection geometry is feasible in principle and estimating how much lane encroachment to expect, this calculator produces the same governing numbers. For formal engineering certification of a specific route or intersection design, professional swept path software with CAD overlay on actual survey drawings is the appropriate tool.
What road width inputs should I use for the Turn Radius Finder when a road has multiple lanes?
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For a multi-lane road where you want to make a turn that stays within your travel direction lane and the adjacent turn lane, enter the combined width of those lanes (typically 12 + 12 = 24 feet for two standard lanes). If you have a dedicated left-turn lane and a through lane, the combined width of the turn area you can use is what matters. For moves that will stop oncoming traffic and use the full road width including opposing lanes, enter the full road width from edge to edge. The Turn Radius Finder output tells you the minimum radius at which the swept path exactly equals the road width you entered, meaning any tighter radius would exceed that boundary. Add a safety buffer of 2 to 3 feet to the road width input to account for driver positioning variation.
Can swept path analysis identify curb strikes before they happen?
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Yes. The outer swept path width tells you how far from the road centerline the outermost corner of the vehicle travels. By comparing this to the available road width minus curb offsets, you can determine whether the outer corner of the vehicle will encroach on or strike the curb at the outer edge of the turn. Similarly, the rear axle path radius tells you how close the rear axle comes to the inner curb or island curb during the turn. If the rear axle path radius is less than the distance from the turn center to the inner curb, the rear axle will mount or strike the inner curb. Comparing both the outer swept edge and the inner axle path against surveyed curb positions at critical intersections is the correct way to use swept path analysis for curb strike prediction in pre-trip planning.
How do I handle a turn where the intersection has a significant grade change?
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This calculator performs horizontal geometry analysis only. Grade changes at intersections (superelevation, cross-slope transitions, humps) affect the vehicle’s roll angle, ground clearance, and the apparent geometric constraints but are not captured in the swept path width formula. For severely graded intersections, the primary concern is that a trailing trailer may develop negative superelevation in a turn, increasing rollover risk for tall or top-heavy loads. The Center of Gravity Rollover Threshold calculator in this hub addresses that risk. For swept path purposes, horizontal off-tracking calculations apply to the plan view of the intersection regardless of grade. However, field surveys of steeply graded intersections should verify that the trailer’s turning movement does not create a drag point where the trailer frame contacts the pavement at the intersection crown.
Is the swept path the same for left turns and right turns?
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For the same turn radius and vehicle configuration, the swept path width is the same for left and right turns. The geometry is symmetric. However, the practical clearance situation differs significantly. A right turn typically uses only the travel lane in the direction of travel, with the outer edge of the swept path sweeping the right curb and the inner axle path cutting toward the right lane line. A left turn uses both the travel lane and typically encroaches into the opposing lane or the oncoming left-turn lane, with the outer swept path crossing the centerline. Left turns therefore generally require more traffic management because they consume road space from the opposing direction. The Turn Radius Finder calculates the same minimum radius for both, but route planning should account for the operational asymmetry between left and right turns.