Published
How wide can a driveway onto a state highway be? The state publishes a table
There is a published answer, and almost nobody looks it up
"How wide should the driveway be?" is usually settled by eye, by the width of the machine, or by whatever the last one was. On a county road that is mostly fine. On US-20 it is not, because the state publishes minimum and maximum widths for every approach type, and a permit is written against them.
The figures live in IDAPA 39.03.42, the state rule covering encroachments on state rights-of-way. The approach permit page covers who issues that permit and what else it demands; this page is just the geometry.
Two things about the table surprise people. The first is that there is a minimum at all — a driveway can be too narrow to permit. The second is that the width band changes with the posted speed of the highway, not with the size of your house.
The width table, as published
Widths and radii are in feet. The two speed columns are the posted speed of the highway your approach meets, not the speed on your driveway.
| Approach use | Under 35 mph min – max | 35 mph and over min – max | Radii min – max |
|---|---|---|---|
| Single residential, farmyard, field | 12 – 40 | 20 – 40 | 20 – 30 |
| Multiple residential | 28 – 40 | 28 – 40 | 20 – 30 |
| Commercial (one-way) | 15 – 30 | 20 – 30 | 30 – 40 |
| Commercial (two-way) | 25 – 40 | 25 – 40 | 30 – 40 |
| Boulevard approach | 84 | 84 | Contact Department |
| Joint-use residential / farm | 25 – 40 | 25 – 40 | 20 – 30 |
| Joint-use commercial | 12 – 40 | 20 – 40 | 30 – 40 |
Location, design, construction and operations of all approaches will comply with current Department geometric standards and design principles.
— IDAPA 39.03.42.400.01 (adminrules.idaho.gov, retrieved 2026-08-17; the subsection carries a 7-1-24 effective marker)
That sentence sits above the table and does real work: the table is the published band, and the Department's current geometric standards sit behind it. The table is where to start a conversation, not where to end one.
The three rows most Island Park lots land in
- Single residential, farmyard, field — one cabin, one approach. The widest band on the table and the lowest minimum. Note that a field approach and a house approach share a row, which is why an old field access is often geometrically fine and fails on something else entirely.
- Joint-use residential / farm — one approach serving two properties. The minimum jumps from 12 feet to 25. If two neighbours are sharing an access to avoid the spacing rule, they are also buying a wider approach.
- Multiple residential — a shared access serving several dwellings. Minimum 28 feet at any speed, and unlike the single residential row it does not relax below 35 mph.
Read the joint-use row before you agree to share a driveway. Sharing is a common and often sensible answer to the state's access-spacing minimums, but it more than doubles the required width, and the extra surface is yours to build and maintain.
Radii are the part people forget
The last column is the turning radius where the approach meets the highway — the curve, not the straight width. It runs 20 to 30 feet for residential and farm approaches and 30 to 40 feet for commercial ones, and it is measured in the same table that sets the width.
This matters on a corridor lot for a reason that has nothing to do with cars. The vehicles that struggle at a tight radius are the ones you need most during a build and after it: a concrete truck, a lowboy with a machine on it, a fuel delivery, a septic pumper, a fire apparatus. A 12-foot approach built to the minimum with a tight curve satisfies a table and defeats a mixer.
| Decision | Cheap now | Expensive later |
|---|---|---|
| Width at the minimum | Less surface, less gravel, less clearing | Two vehicles cannot pass; deliveries reverse onto a highway |
| Radius at the minimum | Smaller footprint in the right-of-way | Long vehicles cut the shoulder, and the shoulder is the state's |
| Joint use to solve spacing | An approach that can actually be permitted | 25-foot minimum width, plus a shared-maintenance agreement you will want in writing |
| Building to the maximum | Room for everything | More surface to plow, grade and re-gravel every year |
How to use the table properly
- Find the posted speed on your frontage, not the speed you drive. It selects the column, and on US-20 it changes more than once between Last Chance and the Montana line.
- Pick the row honestly. An approach serving two cabins is joint-use even if one is not built yet, and a residence taking paying guests may not be a residential row at all.
- Design between the minimum and the maximum, then check the shape against the largest vehicle that must use it — usually a concrete truck or a lowboy, not a car.
- Confirm the geometry with the Rigby district office before you build, because the rule puts current Department standards above the table.
- Get the width and radii onto the permit drawing, so the approach that is inspected is the approach that was approved.
- Settle the surface separately — the table is geometry only, and what you put on it is covered on the driveway gravel page.
Re-verify before relying on any figure here. This page was written from the current published text of IDAPA 39.03.42 retrieved on 2026-08-17. Administrative rules are amended, the rule is free to read at adminrules.idaho.gov, and the district office in Rigby answers the telephone.
Common questions
How wide does a driveway onto US-20 have to be?
For a single residential, farmyard or field approach the published band is 12 to 40 feet where the posted speed is under 35 mph, and 20 to 40 feet at 35 mph and over. Those are the state's figures for approaches onto state highways, not county roads.
Is there really a minimum width?
Yes. Every approach type in the table has a minimum as well as a maximum, so a driveway can be too narrow to permit. The lowest minimum on the table is 12 feet, for a single residential, farmyard or field approach under 35 mph.
Why does the posted speed change the width?
The table bands widths by whether the highway is posted under 35 mph or at 35 mph and over. For a single residential approach the minimum rises from 12 feet to 20 feet in the faster band, while the 40-foot maximum is unchanged.
What is a joint-use approach and why is it wider?
It is one approach serving more than one property. The published minimum is 25 feet for joint-use residential or farm, against 12 feet for a single residential approach. Sharing an access can solve a spacing problem, but it buys a wider approach.
What are radii and why do they matter?
The radii columns set the curve where the approach meets the highway: 20 to 30 feet for residential and farm approaches, 30 to 40 feet for commercial. A tight radius is what stops a concrete truck or a lowboy, not the straight width.
Does the table apply to county roads too?
No. IDAPA 39.03.42 governs encroachments on state rights-of-way. An access onto a Fremont County road is a county matter with its own requirements.
Can I build wider than the maximum?
The table publishes a maximum for each approach type, and the rule requires compliance with current Department geometric standards and design principles. Anything outside the band is a conversation with the district office, not a default.
What is a boulevard approach?
It is a two-way approach intended for high volumes of large commercial vehicles, published at 84 feet, with opposing traffic separated by a raised non-traversible median. The radii entry says to contact the Department. It is not a residential option.
Where do I check the current figures?
IDAPA 39.03.42 is published free at adminrules.idaho.gov. This page was written from the text retrieved 2026-08-17; rules are amended, so confirm the current version and check the geometry with the Rigby district office before building.