Minimum Width and Turning Radius Requirements – InterNACHI Inspection Narrative Library

Scroll down for radius calculator and instructions

For DRIVEWAYS

For driveways used as fire apparatus access, the model codes do not give a single numeric minimum turning radius; they leave it to the fire code official, but common adopted standards cluster around a 25 ft inside radius and roughly 45–50 ft outside radius.

What the codes say

International Fire Code (IFC) 503.2.4: the required turning radius of a fire apparatus access road “shall be determined by the fire code official,” so the base text has no fixed number.
NFPA 1: requires that turns in fire department access roads have a radius adequate for the AHJ’s apparatus and allows the AHJ to increase minimum inside radius; many jurisdictions using NFPA 1 adopt 25 ft minimum inside radius in local policy.

Typical numeric standards used

Many jurisdictions and fire department standards that build on IFC/NFPA use the following as minimums for access roads and driveways serving buildings:

Inside turning radius: 25 ft minimum.
Outside turning radius: 45–50 ft minimum, often stated as 45 ft or 48 ft, and sometimes 50 ft for larger apparatus.
Some amendments allow slightly smaller radii on minor access, e.g., 20 ft inside / 40 ft outside, but still reserve 25 ft inside and 45+ ft outside for primary fire lanes and buildings over certain heights.

A concrete example: one fire department design guide calls for “25 ft inside and 48 ft outside radius” for fire apparatus access roads. Another specifies “25 ft inside and 45 ft outside.” Others require a 25 ft inside and 50 ft outside radius for roads serving buildings.

Practical rule of thumb for a driveway

If you are designing or evaluating a driveway that is intended to function as a fire apparatus access road:

Treat 25 ft inside turning radius as the minimum you want to see at bends, curves, or turnarounds unless your AHJ has published different apparatus templates.
Assume 45–50 ft outside radius (which corresponds to a 20–25 ft wide drive around that curve) to comfortably accommodate typical engines and many ladder trucks.
Always confirm with the local fire marshal or published local amendments, since IFC explicitly defers the exact radius to the fire code official and some Colorado jurisdictions may have apparatus with larger turning needs.

For FIRE ACCESS ROADS

For most jurisdictions using the IFC or NFPA 1 as a base, the typical minimums are:

Minimum width (base requirement): 20 ft unobstructed, exclusive of shoulders.^[1]^[2]^[3]^[4]
Wider near hydrants and aerial access: commonly 26 ft minimum where a hydrant is on the road or where aerial apparatus access is required at taller buildings.^[5]^[2]^[6]^[1]
Turning radius (general rule): The IFC base text says the required turning radius is “as determined by the fire code official,” so it is locally set.^[2]^[7]^[4]^[1]
Common local standards and NFPA guidance use about:
- Inside turning radius: 25 ft minimum.^[3]^[8]^[9]^[2]
- Outside turning radius: 45 ft minimum (inside 25 ft + 20 ft road width).^[10]^[9]^[3]

In practice, you usually state it as: an all‑weather fire apparatus access road at least 20 ft wide (26 ft at hydrants/aerial access) with a turning radius adequate for the responding apparatus, often interpreted as roughly a 25 ft inside and 45–50 ft outside radius unless the AHJ specifies otherwise.^[8]^[1]^[2]^[3]

Determining Turn Radius of Existing Driveways/Roads

In the INL Wildfire Survey template for Spectora Select:

DRIVEWAY CONDITIONS;
Driveway Tightest Turning Radius;
Select the pencil icon to the right of the title (#2 above)
Scroll down to the radius calculator and explanation

Calculating Turn Radius

You'll find the radius of a turn using a phone app called RADIUS CALCULATOR.

Measure a straight line across the radius at the tightest point. Measuring between the points at which the line touches the radius at both ends will give the span length.
In the center of the span, measure the distance between the span line and the edge of the radius. That measurement is the height.

In the example below, the larger span was 12 feet and the height 1 foot. The radius was 222" = 18'-6"

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