Key Legal Limits in Kentucky
Some of the most important weight limits:
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A single axle (or two axles less than 42 inches apart) may carry up to 20,000 lb on interstate (Class “AAA”) highways.
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A tandem axle (two axles spaced ≥ 42″ and ≤ 96″) may carry up to 34,000 lb.
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A tridem axle group (three axles spaced ≥ 42″ and < 120″) may carry up to 48,000 lb.
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The maximum gross vehicle weight (GVW) on interstate/“AAA” highways is 80,000 lb.
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On other non‐interstate roads (Class “AA” and “A” highways) the limits are lower: for example, on Class “AA” highways the GVW limit is 62,000 lb; on Class “A” it’s 44,000 lb.
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There’s also a tire width limit: No single axle can exceed 700 lb per inch of tire width (based on the aggregate width of all tires on that axle).
Bridge Formula (Federal/State) Application
Beyond those fixed axle‐group limits, Kentucky uses a bridge formula to determine permissible weight when you have multiple axles/spacings that don't neatly fit the standard single/tandem/tridem groups. From Kentucky’s regulation (603 KAR 5:066) for Class “AAA” highways:
“The maximum gross weight allowed on a vehicle with any other axle configuration shall be established by the bridge weight formula:
W=500(L NN−1+12N+36)W = 500 \Big(\frac{L\,N}{N-1} +12N +36\Big)W=500(N−1LN+12N+36)
Where W = gross weight, L = distance in feet between extreme axles of the group, N = number of axles in the group.”
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And note: For Kentucky’s interstate (“AAA”) system, that formula applies in addition to the fixed axle limits (single axle ≤20,000 lb; tandem ≤34,000 lb; etc.).
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The objective of the bridge formula: to ensure that weight is appropriately distributed across axles/spacing so as not to overstress bridges/roadways.
What This Means in Practice for Trucking in Kentucky
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If your vehicle/trailer combination has standard axle grouping (e.g., single, tandem, tridem) and you accommodate the standard spacings, you can rely on the fixed limits (20 k, 34 k, 48 k per axle group) and GVW of 80,000 lb (on interstate).
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But if you have non‐standard axle configurations (e.g., many axles, different spacing), you must check the bridge formula to make sure you’re under the maximum allowed weight for that grouping.
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Even if your GVW is ≤ 80,000 lb, you can still violate the bridge formula if axle spacing/number of axles is unfavorable.
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When travelling on non‐interstate (lower class) roads, the gross weight cap is much lower (e.g., 62,000 lb on Class AA, 44,000 lb on Class A). So routing matters a lot.
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The tire‐width rule means even if axle weight is under the cap, you still must check tire‐width weight transmission — e.g., if you have very wide tires you may hit that limit.
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Overweight or oversized loads (loads requiring permits) will require special permits and may demand route approvals, especially for bridges. In Kentucky, if a structure has a posted load limit less than the default limit, you must not exceed the posted limit.
Tips & Considerations for Compliance
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Check your axle spacing carefully — whether your axles qualify as single, tandem, tridem, or “other configuration”.
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Calculate using the bridge formula if you have more than 3 axles or spacing doesn’t fit standard definitions.
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Plan routing: Use interstate/“AAA” highways when possible to take advantage of higher 80,000 lb GVW limit; avoid roads with lower class if you want to haul heavier loads.
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Check for posted bridge weight limits or signs – if a bridge posts a lower weight than the default statutory limit, you must adhere to the posted limit.
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Document everything: axle weights (via scale tickets), tire widths, spacings — useful if weighed/enforced.
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Over‐weight/oversize loads: Keep in mind the permit process and that a permit doesn’t always override axle/spacing/bridge formula rules.
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Stay current: Regulations may change — always check the latest version of Kentucky Administrative Regulations (e.g., 603 KAR 5:066) and KYTC’s web resources.