Curb Radius Design Parking Lot: 15, 25, and 50 Foot Specs
Direct Answer (60 words): Standard parking-lot curb radii are 15 feet inside for passenger-only access, 25 feet inside for SU-30 single-unit trucks, and 50 feet inside for WB-50 tractor-trailer access. The numbers come from AASHTO design-vehicle turning templates. Choosing the wrong radius forces over-tracking onto landscape, opposing curb damage, and ADA route conflicts. Always design to the largest vehicle expected on site.
Curb radius is the single most consequential geometric choice in a parking-lot design. It dictates which trucks can reach the dock, which fire apparatus can swing the corner, and how far striping crews can place ADA ramps without conflict. This guide walks the standard design vehicles, the radius each one needs, and the framework we use to spec radii on commercial site work.
Where do curb radii come from?
The American Association of State Highway and Transportation Officials publishes design-vehicle turning templates in "A Policy on Geometric Design of Highways and Streets" (the Green Book). The U.S. Federal Highway Administration incorporates the same templates (FHWA Pavement Design). State DOTs including Oregon DOT in Standard Specification 00759 reference the Green Book templates for parking-lot and access-road geometry (ODOT Standard Specifications).
Each design vehicle has a minimum inside-curb radius below which the rear axle tracks across the curb.
What are the standard design vehicles?
| Vehicle Class | AASHTO Designation | Inside Radius Minimum |
|---|---|---|
| Passenger car | P | 15 ft |
| Single-unit truck | SU-30 | 25 ft |
| Single-unit truck (longer) | SU-40 | 32 ft |
| Bus | BUS-40 | 35 ft |
| Tractor-trailer (28-ft trailer) | WB-40 | 40 ft |
| Tractor-trailer (53-ft trailer) | WB-67 | 60 ft |
| Fire apparatus | (varies, typically 35 ft) | 35 ft per NFPA 1 |
When does a 15-foot radius work?
A 15-foot inside radius is the minimum for passenger-only parking-lot access. Suitable for:
- Office park staff lots
- Residential apartment lots
- Restaurant patio-side curbs
- Medical office reception drop-offs (small vehicles only)
A 15-foot radius will not accommodate a UPS or USPS delivery van swinging into a tight corner. If the site sees daily delivery traffic, even passenger-only lots should bump to 25 feet at the entrance.
When does a 25-foot radius become the standard?
The 25-foot inside radius is the workhorse number for commercial sites. It accommodates:
- SU-30 single-unit delivery trucks (UPS, FedEx ground, food service)
- Smaller fire apparatus on inside-property turns
- Standard moving trucks and light freight
- Most retail loading
The 25-foot radius is the default for shopping center parking-lot drive aisles unless the site has a specific larger-vehicle requirement.
When do you need a 50-foot or larger radius?
A 50-foot inside radius accommodates a WB-50 tractor-trailer and is required at:
- Distribution-center entrances
- Warehouse loading dock approaches
- Big-box retail truck-court access
- Industrial yard back-of-property circulation
- Truck-friendly drive-thru pickup lanes
A radius at 60 feet or more accommodates the WB-67 (53-foot trailer) common on Oregon I-5 freight routes.
What does NFPA say about fire apparatus radii?
NFPA 1, the Fire Code, references a 35-foot inside radius for fire apparatus access in most jurisdictions (NFPA 1 Fire Code). Property-specific fire-apparatus access requires 26 to 28 feet of clear road width and a 35-foot turning inside radius. Fire-marshal sign-off in Oregon typically references the local jurisdiction's amendment to NFPA 1.
What happens if a radius is too tight?
A radius below the design-vehicle minimum produces:
- Off-tracking of the rear axle across the curb
- Curb damage from trailer wheels riding the face
- Pavement rutting at the curb line where wheels track repeatedly
- ADA ramp displacement when crews relocate a ramp away from the wheel track
- Insurance claims from low-clearance vehicle floor damage
How do you measure inside vs outside radius?
Curb-radius callouts on construction drawings reference the inside (concave) curve. The outside curve is automatically larger by the lane width. A drive aisle with a 25-foot inside radius and a 24-foot lane width has a 49-foot outside radius.
Curb radius and slipform paving
A slipform paver cannot negotiate inside radii under 25 feet. On a 15-foot radius parking lot turn, the curb is hand-formed rather than slipformed. The American Concrete Pavement Association documents 25 feet as the practical lower bound for slipform machine production (ACPA).
Industry Baseline Range for radius work
| Component | Range |
|---|---|
| Hand-formed curb on tight radius (under 25 ft) | $14 to $24 per linear foot |
| Slipform curb on tangent or large radius | $10 to $16 per linear foot |
| Curb-radius rebuild after wheel-track damage | $50 to $90 per linear foot |
| Survey and template layout | $400 to $1,200 per radius |
Current Market Reality
Tight-radius hand-formed curb pricing has climbed faster than tangent slipform pricing in 2025 to 2026. Lumber-form costs, the labor-hour load of stake setting, and the higher rejection rate on inspection have all pushed Oregon hand-formed pricing 25 to 40 percent above slipform pricing on a per-foot basis.
Real install reference
In April 2026 we formed and poured 21 corner radii at a Hood River industrial-park curb run. Six radii served the SU-30 single-unit truck route at 25-foot inside radii. Three radii at the dock-approach drive aisle ran 50 feet to clear WB-50 trailer turns. The remaining 12 radii at passenger access ran 15 feet inside.
Frequently Asked Questions
What is the standard inside radius for a parking lot drive aisle? The standard inside radius for a commercial parking-lot drive aisle is 25 feet. This accommodates the AASHTO SU-30 single-unit truck and most light commercial vehicles.
What inside radius does a fire truck need? NFPA 1 references a 35-foot inside radius minimum for fire apparatus access. Local jurisdictions may write tighter or looser amendments to NFPA 1.
What radius does a 53-foot tractor-trailer need? The AASHTO WB-67 (53-foot trailer) requires a 60-foot inside radius minimum. Distribution centers and big-box truck courts plan to this number.
Can a slipform paver handle a 15-foot radius? No. Slipform pavers cannot negotiate inside radii under 25 feet. Tight radii are hand-formed traditionally and tied into the slipform tangent run.
Does the inside or outside radius go on the construction drawing? The inside (concave) curve is the radius callout on construction drawings. The outside (convex) curve is computed by adding the lane width.
We design and pour commercial curb radii across Oregon. To plan your site, start with our concrete curb guide, the best curb for drive-thru lane reference, or get a quote on curbing in Hillsboro Oregon.