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# Idaho Axle Weight Limits

### Axle spacings and weight calculator

Max Legal Weight for GVW under 80,000 lbs and Max Permitted Weight for GVW over 80,000 lbs

### Maximum Legal Weight Allowed

#### Weights Allowed on Interstate

The Federal Highway Amendment Act of 1974 established allowable legal weight limits on Interstate System Highways at 20,000 pounds on single axles, 34,000 pounds on tandems, and total gross loads not exceeding 80,000 pounds.

#### Weights Allowed on Non-Interstate Highways

Allowable legal weight limits on non-interstate highways are set at 20,000 pounds on single axles, 37,800 pounds on tandems, and total gross loads not exceeding 80,000 pounds.

#### Permit Types to 80,000 Pounds Gross Weight

Permits will be issued for vehicle combinations operating on Interstate and non-interstate highways with total gross loads exceeding 80,000 pounds but not to exceed 20,000 per single axle, 34,000 pounds per tandem, and not to exceed the weight limit for any group of 2 or more consecutive axles established by Section 49-1001, Idaho Code.

### Maximum Permit Weight Allowed

#### Allowable Gross Vehicle Weight

The gross vehicle weight allowable by overweight permit is subject to the seasonal stability of the roadway and the capacity of the structures on the route of travel. For the purpose of issuing overlegal permits, 7 levels of overweight are established, based on the weight formula of

#### W = 500((LN/N-1) + 12N + 36)

and routes for carrying the various levels of overweight are designated by color coding.

The Weight Formula (“W”) is the maximum weight in pounds (to the nearest 500 pounds) carried on any group of 2 or more consecutive axles. “L” is the distance in feet between the extremes of any group of 2 or more consecutive axles, “N” is the number of axles under consideration. The load factor based on the most critical bridge on the highway route will also be used in determining allowable weights.

Annual permits involving overweight loadings will be available at the following levels:

1. Red Routes -- The red routes contain posted bridges and require approval or analysis from the Department. A vehicle configuration may be issued an annual overweight/oversize permit for travel on red routes only, upon completion of an analysis verifying the requested weights are acceptable. The annual permit will be issued for a specific vehicle configuration, operating on a specific route, at specific weights. All information will be listed on the annual permit and will be subject to revocation at such time the vehicle configuration changes (such as axle spacings), the approved weights change, or a bridge rating changes. Annual permits issued for red routes will be in addition to the annual permit required for other routes.
2. Yellow Routes -- The yellow overweight level is based on a single axle loading of 22,500 pounds, a tandem axle loading of 38,000 pounds, and a tridem axle loading of 48,000 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 560 ((LN/N-1) + 12N + 36).
3. Orange Routes -- Orange overweight level is based on a single axle loading of 24,000 pounds, a tandem axle loading of 41,000 pounds, and a tridem axle loading of 51,500 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 600 ((LN/N-1) + 12N + 36).
4. Green Routes -- The green overweight level is based on a single axle loading of 25,500 pounds, a tandem axle loading of 43,500 pounds, and a tridem axle loading of 54,500 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 640 ((LN/N-1) + 12N +36).
5. Blue Routes -- Blue overweight level is based on a single axle loading of 27,000 pounds, a tandem axle loading of 46,000 pounds, and a tridem axle loading of 57,500 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 675 ((LN/N-1) + 12N + 36).
6. Purple Routes -- The purple overweight level is based on a single axle loading of 30,000 pounds, a tandem axle loading of 51,500 pounds, and a tridem axle loading of 64,500 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 755 ((LN/N-1) + 12N + 36).
7. Black Routes -- The black overweight level is based on a single axle loading of 33,000, pounds a tandem axle loading of 56,000 pounds, and a tridem axle loading of 70,500 pounds or the equivalent loading as determined by spacings and number of axles and computed by applying the formula W = 825 ((LN/N-1) + 12N + 36).

#### Maximum Tire Weights

The maximum overweight levels shall not exceed 800 pounds per inch width of tire.

#### Extra Length/Excess Weight Permit

Gross weight limited to 105,500 pounds on interstate, non-interstate and local highways and length limited to those specified in IDAPA 39.03.22, “Rules Governing Overlegal Permits for Extra Length, Excess Weight and Up to 129,000 Pound Vehicle Combinations.”

#### Annual Overlegal Permits

Annual permits may not be issued for gross weights in excess of 200,000 pounds for any colored route. Gross weights in excess of 200,000 pounds must operate by single trip permit.